Most electrical installations depend on earthing via earth electrodes to protect people and equipment. In these installations, regular testing of the earth resistance is essential, but most testing methods are either time consuming and inconvenient, or prone to giving inaccurate results. As Paul Swinerd of Megger explains, however, there is now a better alternative

- It’s tempting to think checking the resistance of an earth electrode should be no more complicated than finding a second earth connection, such as a nearby water pipe, and measuring the resistance between this and the electrode under test with an ordinary ohmmeter. Unfortunately, life isn’t quite that simple.
Noise currents flowing in the earth will almost certainly produce large errors in the results obtained, and there’s no way of knowing how much of the resistance is due to the secondary earth connection and how much to the earth electrode itself. In other words, some sort of result will be obtained but, for all practical purposes, it is meaningless.
For this reason, a number of alternative methods have been devised for accurately measuring earth resistance. The simplest is to carry out a direct measurement, as described earlier, but with a purpose-designed earth tester that uses an ac test current. By choosing the frequency of this current so it is not an integer multiple of the mains supply frequency, it is possible to arrange for such an instrument to provide a high degree of noise rejection.
The results are far more meaningful than those which might be obtained with an ohmmeter, but there is still no way of confirming that they are accurate or, indeed, of separating out the contribution of the secondary earth.
A much better method, and one which is very widely used, is usually known as the three-terminal or fall-of-potential method. This uses a connection to the electrode under test and two test spikes that must be driven into the ground before the test is carried out.
One of the spikes – the current spike – injects the test current, and should be placed as far away as possible from the electrode under test. The other spike – the voltage spike – is then driven into the ground at a number of locations, preferably in a straight line, between the current spike and the electrode. At each location, a voltage measurement is taken. Since the current injected by the instrument is known, each of these voltage measurements can be converted, using Ohm’s law, to a resistance value. In practice, this conversion is performed by the instrument itself.
If a graph is plotted of resistance versus the distance the voltage spike from the electrode under test, it should have a definite plateau region where the resistance hardly varies as the rod is moved. This value of resistance is the required earth resistance for the electrode under test.
This method is accurate, and any problems with the measurements are readily apparent, as the resistance graph will depart markedly from the expected shape. The only shortcomings are that the test is time consuming to carry out, it requires a reasonable amount of space, and that the earth electrode under test must be disconnected from all other circuits while the test is underway. These are rather significant shortcomings.
To provide a more convenient way of measuring earth resistance, the clamp-on or stakeless method was introduced. This uses a tester adapted to inject a test current into the earth electrode system via a clamp arrangement, and uses the same clamphead to measure the resulting current flowing in the electrode under test. No direct connections are required, and the earth electrode does not need to be disconnected from other circuits – indeed, for successful testing, it cannot be.
While this method is quick and easy, it has several limitations. It only works in applications where there are multiple parallel earth connections so that there is a return path for the test current, and it cannot, therefore, be used to test isolated electrodes. Since there’s no way of verifying the result, it is also unsuitable for checks on new installations where no previous test results are available for comparison, but it is good for trending of earth system condition.
A new solution, which is more versatile than the stakeless method and more convenient than the traditional fall-of-potential method, is provided by the Attached Rod Technique (ART). In many ways, ART is similar to fall-of-potential testing, and all of the same connections are required. There is, however, one crucial difference – there is no need to disconnect the earth electrode from other circuits while the test is being carried out.
That may seem a relatively small advantage but, apart from the physical difficulty of breaking earth connections, it's important to remember that earthing is a safety function.
There are dangers in disconnecting an earth electrode as a fault current may be flowing and disconnection could give rise to a potentially lethal situation. Furthermore, if equipment is disconnected from the earth electrode to enable a test to be carried out, that equipment may no longer be safe, and dangerous situations may result.
While it may be possible to provide a temporary earth connection, or to switch off the electricity supply during the test, such measures are likely to be both inconvenient and costly.
So how does ART testing work? The key is in a current measuring clamp (ICLAMP) that is put round the earth electrode under test. The tester is designed to ignore any system leakage and noise currents that may be flowing through the earth electrode. This means that it can accurately measure the test current, in spite of extraneous influences.
With the equipment set up, the ART test proceeds in exactly the same way as an ordinary fall-of-potential test. It is still, therefore, time consuming, but there are a number of shortcuts that can be used in appropriate circumstances.
For example, instead of taking readings with the voltage spike at various distances between the electrode and the current spike, it is sometimes sufficient to take a few readings, with the voltage spike around 62% of the distance between them. This means that ART testing provides a very good balance between convenience and accuracy of results.
For earth resistance testing, it is important to have the right equipment and to understand the limitations of the various test methods – even ART testing isn't suitable in every case, although it is very versatile.
Earth resistance testing isn’t particularly complicated, but interpreting the results from the various test methods can be, which is why Megger has developed software that will do the earth testing calculations automatically, and supply a report.
Megger also offers an 80-page publication "Getting Down to Earth" which provides detailed practical advice on all commonly used methods of earth resistance testing. Printed copies are available free of charge from the company, or the publication can be downloaded from the Megger website (www.megger.com).

If you are a user or potential user of Valve Regulated Lead Acid (VRLA) batteries this article may be of some interest to you. matt jordan of yuasa answers some of the questions often asked by installers when considering the use of a VRLA product in applications such as Telecommunications, Stand-by UPS, Fire and Security, Mobility and Leisure markets. By following these recommendations, service life and performance of the product will be enhanced

What precisely is a Valve Regulated Lead Acid Battery?
VRLA batteries have been designed to maximise the use of gas recombination technology and can be used in modern office environments, removing the need for expensive purpose built battery rooms etc. They utilise the latest cell plate technology with each cell comprising a number of positive and negative, lead calcium alloy plates, which are filled with either lead dioxide (positive) or spongy lead (negative) active materials. Unlike motor car batteries the electrolyte (Sulphuric Acid) is trapped in a gel substance or, in the case of a Yuasa product, suspended in absorbent glass matting (AGM), which is located between positive and negative plates. Each cell has a voltage of 2V. Therefore, a 2, 6 or 12V battery will comprise one, three or six cells respectively. The battery container and lid are commonly manufactured from ABS (Acrylonitrile-Butadiene-Styrene) which are welded together to form a high integrity leak-proof seal. The container lid contains a number of low-pressure valves, which are designed to release excess gas and reseal automatically in the event of the internal gas pressure rising to unacceptable levels. A VRLA battery effectively recombines 99% of the gas generated in normal use at 20°C.

How can I do a simple battery sizing?
To carry out the most basic battery sizing for a portable tool for example, you must establish: (a) DC output voltage of the tool i.e. 6, 12, 24V etc. (b) load (amps) of the tool and (c) length of time (autonomy) the tool needs to be supported by the battery i.e. battery back up time. Note. If the tool is rated in watts, simply convert to amps by dividing the watts by the nominal voltage of the equipment. Determine if there are any physical constraints that may affect your selection, such as: application; environmental; temperature or dimensional restraints. Then, using Figure 1, which denotes a series of graphs based on time against current (amps) for a selection of Yuasa NP batteries, a battery selection can be made. NB. Battery sizes are normally calculated in the UK based on an ambient temperature of 20 to 25°C. At higher temperatures, the capacity of a battery increases while life expectancy decreases and, conversely, at lower temperatures, the capacity decreases.

Is it possible to increase the DC voltage and capacity (Ah) by connecting VRLA batteries together?
Voltage can be increased by connecting the positive terminal of one battery to the negative terminal of an adjacent battery. Therefore, connecting four 12V 7Ah batteries together in this manner will effectively raise the voltage across the battery from 12 to 48V. Battery capacity will remain at 7Ah. This is commonly termed ‘series’ connection. Capacity can be increased by connecting the positive terminal of one battery to the positive battery of the next. Negative terminals could be connected together in a similar manner. Therefore, connecting four 12V 7Ah batteries will effectively raise the capacity available across the batteries from 7 to 28Ah. Battery voltage will remain at 12V. This is commonly termed ‘parallel’ connection.

The connection of the batteries in series and parallel will increase both capacity and voltage. Note. It is recommended that consultation with the battery manufacturer be made before connecting “mixed” product types in series or parallel.

What does the term 20-hour and 10-hour mean?
Battery manufacturers rate capacities (Ah) against specific times (hours) to a specified end of discharge voltage. For example, a Yuasa NP product is rated at the 20-hour rate. Therefore, the NP7-12 VRLA battery will support a constant load of approximately 350mA (7/20) for 20 hours to an end voltage of 1.75V per cell (VPC). It will not support a constant load of 7A per hour for 20 hours. Discharging the battery over one hour will reduce the efficiency of the battery to approximately 60% of the battery’s rated capacity (4.2Ah in the case of a 7Ah battery) and will therefore support a constant load of 4200mA for this period.

What do the terms eg. ‘2C’ or ‘2CA’ mean in relation to a battery manufacturer’s published documentation?
Battery manufacturers often publish data that refers to the ‘C’ or ‘CA’ rates. Discharge curves being a typical example. Both terms refer to current (Amps) in relation to the capacity (C) of the battery. Therefore, for a Yuasa NP7-12 battery ‘2C’ equates to a current of 14A (2 x 7) and ‘1C’ or ‘C’ for the same battery relates to 7 Amps etc.

What do the terms ‘Standby’ and ‘Cyclic’ applications mean?
Standby refers to battery applications designed to support AC mains failure, such as emergency back up for alarm panels, emergency lighting, UPS systems etc.

Cyclic refers to battery applications where the battery is repeatedly charged then discharged, independently, from the charging source. Typical applications are portable equipment, wheel chairs and electric golf trolleys etc.

Can I use a standard motor car charger on my VRLA battery?
We do not recommend permanent use of car battery chargers, unless the charger has a VRLA setting. To maximise the service life and performance of a VRLA battery, it is essential the correct charging regime based on the specific type of application be used. Car battery chargers charge at a higher voltage than we would normally recommend for standby applications and, if these higher voltages are left continuously connected to a VRLA battery, it will cause irreversible damage. It is also recommended VRLA batteries be recharged after use. Never leave a VRLA battery in discharged state.

What is the correct charging regime?
This depends on the type of application ie. ‘standby’ or ‘cyclic’. In both types of applications, constant voltage charging is recommended. For standby applications, using the Yuasa NP range of batteries, the voltage of the charger should be set to give a constant voltage of 2.275V per cell at 20°C ambient. This relates to a value of 13.65V for a 12V block (6 x 2.275V).

For cyclic applications a higher constant charging voltage is used but should NEVER be left on indefinitely since it will overcharge and eventually destroy the battery. The voltage of the charger should be set to a level of between 2.4 to 2.5V per cell at 20°C ambient. This relates to a voltage of between 14.4 to 15V for a 12V block.
For standby or float we recommend that the current be limited to a maximum of 0.25CA i.e. 25% of the capacity of the battery in Amps. Typically for an NP7-12 this value would be 1.75A.

What sort of service life can I expect from my VRLA battery?
For standby applications, battery manufacturers produce a range of batteries that are designed to give a pre-determined service life, usually 5, 10, and 15 years if used as per the manufacturer’s specification.
Towards the end of service life, battery capacity will reduce resulting in reduced run times. Operating the battery at temperatures above ambient 20 to 25°C for prolonged periods will shorten the service life of the battery.
For cyclic applications the service life of batteries depends on (a) quantity of cycles of charge and discharge and (b) the depth of discharge as expressed as a percentage of the capacity required per duty cycle. Most VRLA products can be cycled.

What can I do if I need support on a VRLA product?
If unsure of any safety or operational aspects associated with the use of VRLA batteries, always check with the supplier or original manufacturer.

Over the last decade or so, field wiring for automation installations has been completely transformed by the widespread adoption of fieldbus systems. Surprisingly, however, this revolution has had little effect on the wiring inside control panels. That’s about to change, says Paul Bennett of Moeller Electric

Few would deny the fieldbus systems used in automation applications bring major benefits. Not only do they dramatically reduce the amount of field wiring required, they also virtually eliminate the risk of wiring errors and, possibly best of all, they make it easy to modify the installation if the configuration of the plant changes.

But open the control panel door, and inside you’ll almost invariably find conventional control wiring, like that previosly used in the field. And this old-fashioned panel wiring shares all the disadvantages of old-fashioned field wiring. It’s complicated, open to wiring errors and the only way to make changes is to re-wire the circuits involved.

Surely there has to be a better way – why can’t the fieldbus benefits be enjoyed within the control panel? There’s no fundamental problem with the technology. Fieldbus technology would work within the panel, but present-day fieldbus systems are not a good match for in-panel applications.
The first reason is cost. With fieldbus systems, the cost of the hardware is invariably more than balanced by the savings made on cabling. Inside the panel, however, the situation is different – connections are short, so the reduction in cable cost is minimal. The hardware for in-panel bus systems must, therefore, be inexpensive if it is not to increase the panel price unacceptably.

In-panel systems also need to be simple to use as panel builders have neither the inclination nor the time to work with complicated configuration systems just to link, for example, a few motor starters. Finally, the system must be compact, given today’s pressures for ever-smaller panels.
For these reasons, automation product suppliers have seen little attraction in developing in-panel bus systems. However, progress in technology means that this situation has now changed – the time is right for bus technology to move into the panel, so let’s take a look at what an ideal in-panel bus system might look like.

To minimise space requirements, add-on auxiliaries for contactors are designed so that they add little to the overall dimensions of the contactor, and it makes good sense to adopt the same idea for in-panel bus adapters.

But what of the connections themselves? Screw-clamp terminals, for instance, are a poor partner for modern bus technology. Plug-in wiring is a much better choice; it cuts wiring time dramatically and also virtually eliminates the possibility of wiring mistakes.
To be easy to use, the system should offer automatic addressing, and should also be capable of automatically reconfiguring itself when items are added to or removed from the bus.

Naturally, the in-panel bus must offer direct connection to PLCs, since this will enable significant cost savings to be achieved by eliminating the need for conventional PLC I/O modules. And, users shouldn’t be tied to PLCs from one specific manufacturer.

Open connectivity could be achieved by providing support for one or more of the popular automation communication protocols such as CANopen or Profibus. This approach has the additional benefit of allowing the in-panel bus system to connect easily with conventional fieldbus installations.
We’ve mentioned some of the benefits that an in-panel bus-based connection system can bring but there are many others. The time required to build the control panel is reduced, which makes it easier for panel builders to meet tight customer deadlines. Panel builders can also start building panels sooner, even if requirements have not been finalised, safe in the knowledge that last minute alterations can easily be accommodated.

The time needed to test panels is also reduced, since the potential for wiring errors is minimal. Finally, maintenance throughout the life of the installation is made easier – if a starter is damaged, for example, it’s only necessary to unplug the connections to it, clip a replacement in place, and replace the plug-in connections. No conventional wiring is needed.
Before we close, however, it’s necessary to sound one note of caution. To maximise the benefits of an in-panel bus system, it’s important to choose one that has been specifically designed with this application in mind. While it may be tempting to try to shoehorn a conventional fieldbus system into the panel, this is not ideal. The modules are, for example, likely to be too big, too expensive and probably too complicated.
When the appropriate technology is used, in-panel bus systems, as we’ve seen, have a lot to offer. In fact, it’s very likely that, in a few years’ time, conventionally wired control panels will be as rare as conventionally wired field installations are today. But there’s really no need to wait – the technology and the benefits are available right now!

Faster to wire, easier to test, neater in appearance and simpler to maintain – that was the considered opinion of Best Conveyors after the company had finished building its first control panel to use Moeller Electric's new SmartWire wiring system.

"Although SmartWire is very new, it didn't take long for Moeller Electric and local dealer, Automation Technology of Daventry, to convince us of its benefits," said Gavin Hogan, Managing Director of Best Conveyors. "We could see immediately that it offered opportunities for simplifying panels and for cutting build time."

"These factors are very important to us, as it's becoming increasingly difficult to find skilled panel wirers," he continued. "Another attraction was the use of standard starters, so we could work with easy-to-obtain components, and the changes needed to our designs were minimal."

The project selected by Best Conveyors to evaluate SmartWire was a telescopic conveyor system for a major distribution warehouse. Developed in conjunction with Automation Technology and Moeller Electric, the control system uses a programmable relay, in conjunction with a remote I/O unit at the far end of the telescopic conveyor, which greatly simplifies the field wiring.

Within the panel are five reversing and two unidirectional motor starters which feature motor circuit breakers and convenient plug-in connections between the contactor and the protection device. In this application, the power to the starters is provided by a prefabricated busbar system.
For use with the SmartWire system, each starter is fitted with an interface module that clips into place like an auxiliary contact block. Since the SmartWire modules fit on top of the contactors, no extra space is needed in the control panel.

With the modules in place, all Best Conveyors had to do to complete the control wiring was to link the modules together with pre-assembled daisy-chain plug-in cables, and then make one final plug-in connection to the gateway on the easy 800 relay.

"It sounds simple," said Gavin Hogan, "and it genuinely was. It's virtually impossible to make a wiring mistake with SmartWire. Even the module addressing is taken care of automatically. It really would be hard to go wrong.”

Best Conveyors found that a further benefit of using Moeller Electric components was the availability of the easy MFD multi-function display module. With its integral function keys and LCD screen, this provides the features of an entry-level HMI module at low cost. Linked to the easy800 relay, the MFD module allowed Best Conveyors to develop an intuitive user interface for the conveyor project.

"Adding the SmartWire modules did slightly increase the cost of components for the panel," said Gavin Hogan, "but the extra cost was outweighed by the savings on wiring, on wiring time and on testing time."
"Using the easy800 and the MFD unit instead of the ordinary PLC and HMI panel that we used on previous projects provided further cost savings," he said, "without us having to make any compromises in the performance of the system. In fact, the easy800/MFD combination is so cost effective it's like getting the HMI functionality free."

Following its positive experience with SmartWire, Best Conveyors is now planning to use it extensively in future projects.

The recent widespread flooding in the UK has undoubtedly damaged electrical equipment worth millions of pounds. In some cases, however, it may be possible to restore this equipment to safe and reliable operation, thereby avoiding the need for costly replacements. Jeff Jowett of Megger looks at what can be done

The key to salvaging flood damaged electrical equipment is to find ways of drying it out effectively, without risking further damage. A number of options are available for this. Probably the most satisfactory is to use a temperature controlled oven with efficient air circulation but, in many cases, this is not possible either because the equipment is too large to be moved to an oven, or because no oven is available.

In these cases, infrared lamps can be used, or a housing can be built around the equipment, with steam coils or electric elements used as the heat source. It is important to make provision for free circulation of air, so that moisture is allowed to escape, and the use of blowers can be helpful.
Another method of heating sometimes used with items like motors and transformers is to pass a current at low voltage through the windings. To avoid the risk of further damage, however, this should not be done until the insulation resistance has been raised to at least 100,000 ohms by other methods. Insulation testers that have kilohm ranges are invaluable in this type of work.

On occasion, welding sets are used as a current source for drying out windings. It’s important to note that these are not intended to supply high currents continuously and they must, therefore, be used at only a fraction of their rated current.

Whichever method of heating is used to dry out the equipment, it’s vital to monitor the insulation resistance for a long enough period to ensure that it has reached a stable value. It is very common, during the drying process, for the insulation resistance to rise to a comparatively high value then dip again. In fact, this rise and fall is often repeated several times as moisture works its way out of the equipment.

While the comments above give general information on salvaging flood-damaged equipment, it is worth looking in more detail at what can be done with various specific types of equipment.

Switchboards and Electrical Controls
- Thoroughly clean and dry out all equipment, dismantling where necessary. After drying, re-varnish all coils. Check contacts for corrosion and oxidation, and make sure that all moving parts operate freely.
- Drain all oil-filled devices, clean them and re-fill with fresh oil of the correct dielectric strength. The oil can be tested for conformance with British Standards using a test set. Dry all insulating barriers, or replace them if they have warped.
- Meters and protection relays will usually have to be reconditioned by the manufacturer. To ensure fast return to service, it may be preferable to fit replacements.
- Clean and dry thoroughly all busbar insulators and control wiring. A minimum of two megohm insulation resistance must be achieved before the equipment is energised, and can readily be confirmed by any good quality insulation tester.
- Check standby batteries for functionality using a battery impedance tester, and check battery straps for corrosion or excessive resistance using a purpose-designed low-resistance ohmmeter.

Electrical Tool and Portable Appliances
- Many of the techniques outlined in the introductory section of this article are suitable for salvaging wet tools and appliances. Before these items are returned to service, however, it is essential that final proof testing is carried out with a portable appliance tester (PAT) in line with IEE code of practice for in-service inspection and testing. As a further precaution, it may also be desirable to flash test Class 2 assets

Rotating Electrical Machines
- Completely dismantle all parts and, except for ball and roller bearings, and either wash them with clean water or steam clean them. Follow this with a thorough cleaning using a grease solvent.
- Thoroughly clean all bearings and housings paying particular attention to oil grooves and reservoirs. Disconnect and swab oil lines, or steam clean them.
- Dismantle the brush rigging and clean the insulators. Some types retain water and must be dried very thoroughly.
- Monitor the insulation resistance of the machine with a modern tester that uses a low applied voltage for the kilohm ranges. Once a value of at least 100,000 ohm is reached, the megohm ranges of the instrument can be used for further monitoring.
- Commutators can be hard to dry out, and it may be necessary to loosen or even remove the clamps to let water out of the inside of the commutator. On large commutators, it may be necessary to use drying temperatures as high as 130°C to achieve effective results.
- Check the bands on armatures or rotors for tightness, as the drying out of the underlying insulation may loosen them. If this happens, they will need to be replaced.
- Some slot wedge materials may be affected by moisture. If this has happened, new wedges must be installed.
- Field coils from DC motors, generators and synchronous machines can present particular problems, and it may be necessary to remove them from the machine for drying in an oven and re-varnishing. After this, the coils should be checked for shorted turns with a digital low-resistance ohmmeter.
- After cleaning and drying, most windings will need re-varnishing. Dip-and-bake varnish is recommended but, it the original varnish is in good condition, an air-drying varnish may be used.
- Before starting the machine, check the entire installation, paying particular attention to lubrication and electrical connections. For three-phase machines, check the phase rotation.

- Remove inspection cover plates and check the condition of the windings, looking particularly for signs of failure. Check all connections for looseness and signs of heating. With oil insulated transformers, draw oil samples from top and bottom, and check them with an oil test set. Breakdown should be at least 22kV, or 25kV if an askarel is used. If it is lower, the oil/askarel will need to be replaced.
- Check the insulation resistance. This should be at least one megohm for each 1,000 volts rating, with a minimum of two megohms. Ideally, the resistance should be comparable with the pre-flood values, which may be available from maintenance records. This is best confirmed by an insulation tester with an extended range, such as Megger’s MIT400 and MIT510 products.
- Note the condition of the bushings, external connections, operating switches and protective devices, and take remedial action where needed. If necessary, clean the transformer externally and paint the tank.
- If water has entered the tank, flush the windings with clean insulating oil. If the transformer is small, remove the coil and core, and dry in an oven at up to 90°C. If necessary, dip and bake the windings. Windings for larger transformers can be dried in the tank by forcing hot, dry air (not above 90°C) around the windings after the tank has been drained; by short-circuiting one winding and energising the other with a low voltage; or by using a combination of these methods.
- During the drying process, plot a curve of insulation resistance against time, initially measuring with a low-voltage tester and subsequently, if the process proceeds successfully, changing to a high-voltage insulation tester. If the process is not successful, and the curve shows no sustained increase in insulation resistance, the transformer will need to be re-wound.
- When the insulation resistance has reached an acceptable value, a final test should be made with a transformer turns ratio tester to confirm the transformer has been returned to full performance.

Cables and Wiring
- All open wiring, including non-metallic sheathed cable, can usually be retained after thoroughly cleaning and drying the cable and the junction boxes, and remaking connections.
- Armoured cable will usually have to be replaced, as will lead cable if the ends have been under water.
- Rubber-covered cable in rigid conduit can sometimes be reused, but it must be pulled out of the conduit so that the conduit can be cleaned. The conduit must be thoroughly cleaned to remove all silt and moisture before being used again.
- Check and clean potheads and other insulators, and inspect them for cracks or other damage
- Perform a comprehensive insulation resistance test before returning the installation to service.
Hopefully, this article will have given a useful indication of the measures that can be taken to salvage electrical equipment after it has been subjected to flooding. It is essential, however, to remember, in every case, safety is of paramount importance. This can only be assured by careful testing of the salvaged equipment, during and after the repair process, using appropriate test equipment.

Mike Frain looks at the dangers of live testing and fault finding in industrial and commercial low voltage electrical installations

In advising industrial and commercial companies on electrical safety rules and procedures, I make enquiries to every delegate on training courses about their own experiences. A good percentage have said that they have experienced electrical flashover (arc flash) and I have yet to meet anybody who has never received an electric shock. Most of us know it takes a very small amount of current flow through the body to cause death. Where death has not occurred this is due simply to the fortuitous nature of the current path in missing vital organs. What is required therefore to survive an electric shock from mains voltage is luck!
When someone is killed or injured by electricity what first comes to mind is electrocution but electric shock is not the only hazard. There are several hundred serious burn victims each year as a result of electrical flashover and is a source of long-term injuries and sometimes a slow and painful death. Many electrical staff routinely work on live high power equipment, carrying out tasks such as fault finding and diagnostic testing without fully understanding the consequences of what will happen to them should an electrical flashover occur. This article is written to highlight the dangers from electrical flashover and recent research to better understand its effects.

Live working is defined as “work on or near conductors that are accessible and live or charged”. This is anywhere a worker is exposed to energised conductors, terminals, busbars or contacts and that often includes the removal of fuses and links. In industrial and commercial environments this usually means live diagnostic testing, faultfinding, non-invasive inspections such as thermal imaging and some tests in accordance with BS7671 Wiring Regulations. Even if it is intended to carry out work on dead conductors, an assumption should be made that they are still live until proven dead.

Whilst live connection work is routinely undertaken by utility distribution network operators (DNOs), any work that requires the connection and disconnection of live conductors and components in most industrial and commercial premises would be extremely difficult to justify.

Live working should never be accepted as the norm and Regulation 14 of the Electricity at Work Regulations 1989 makes clear three conditions must be met for live working to be permitted. These conditions are:
1. It is unreasonable in all the circumstances for the conductor to be dead
2. It is reasonable in all the circumstances for that person to be at work on or near that conductor while it is live
3. Suitable precautions (including where necessary, the provision of personal protective equipment) have been taken to prevent injury
If live working can be justified through the rigorous tests of reasonableness in conditions one and two, judgements must be made about suitable precautions against electric shock and the effects of electrical flashover to satisfy the requirements of condition three

Electrical flashover is usually caused by inadvertent contact between an energised conductor such as a busbar with another conductor or an earthed surface. This is often the result of incorrect use of test probes, faulty or poorly specified instruments or dropped tools and can be made more likely when the equipment is subject to condensation, dust or corrosion. The magnetic field from the resultant fault current will cause the conductors to separate or the tool to be blown back producing an arc, which ionises the air, making a conducting plasma fireball. Electric arcs produce some of the highest temperatures known to occur on earth and can be up to 35,000 degrees Fahrenheit, which is four times the surface temperature of the sun. This is enough to immediately vaporise all known materials and this sudden release of thermal energy at the point of the fault can cause severe burns to the skin, internal burns to throat and lungs due to inhaling vaporised metal or heated air, ignition of clothing, blindness from the resulting ultraviolet light and even death. The effects of the flashover can radiate several metres away from the point of the arc, injuring other people that might be nearby.

When an electrical flashover occurs, conductors can vaporise expanding to thousands of times their original volume and the high release of thermal energy superheats and rapidly expands the surrounding air. The result can create a pressure wave called arc blast, which is literally an explosion. During this violent event, molten metal particles, destroyed equipment and related components will be ejected as shrapnel at speeds of up to 700 miles per hour.

European statistics demonstrate that serious electrical accidents resulting from fault arcs occur mainly in low voltage power installations. Fault levels in many industrial and commercial networks can be surprisingly high and can lead to larger and more catastrophic flashover events. This does not necessarily mean you are safe working on systems that have lower prospective short circuit currents as at lower levels, overcurrent devices can operate more slowly allowing the arc to last longer producing a high overall amount of energy at the point of flashover.

The electrical utility industry adopts a rigorous task based approach to risk assessment of live working on low voltage systems and yet the fault energy in many cases is actually lower than can be found in some medium sized manufacturing plants. There are many instances of electrical installations that incorporate low voltage distribution transformer sizes of between 2MVA and 3 MVA as well as having generators and large motors that will contribute to total fault energy. At least one UK utility company (DNO) uses protective measures to cover 7,000A to 10,000A of arc current. In industrial processes and some commercial premises the amount energy that can be generated in an arc can be much higher than this, low voltage fault levels of 40,000A are quite common which will raise the level of arc energy.

Experience shows many employers have no idea of the destructive nature of such fault energy should an incident be initiated by an employee. The difficulty has been that until recently there has been limited knowledge available to predict the amount of harm to a worker whose screwdriver or test probes slip whilst working in an energised control panel. Recent studies particularly in the United States have started to address this and there are now methods of calculating the amount of harm to workers and bystanders. This has enabled the North Americans to specify relevant work place precautions from flashover or arc flash hazard as it is commonly referred to. There are two US standards that apply to arc flash; NFPA70E-Standard for Electrical Safety in the Workplace and IEEE 1584- Guide for Performing Arc Flash Calculations.

As a very brief summary, the basis of the calculations relies on input data such as; fault level, voltage, gap between conductors, enclosure type, working distance, method of earthing and protective device characteristics. From this data, the amount of thermal energy from an arc can be determined that can reach a surface such as a person’s skin. A figure of 1.2cals/cm2 is considered to be the energy required to produce the onset of a secondary degree burn and this is used as a benchmark for the need and type of PPE such as flame retardant clothing and the Flash Protection Boundary. The Flash Protection Boundary is defined as “An approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electrical arc flash were to occur.” The IEEE 1584 calculation methods have been derived from actual laboratory experiments involving the simulation of electrical flashover events. There is ongoing research in the US into the arc flash hazard and a further $6million has been allocated this year. However, the effect of arc blast, which accounts for the pressure wave and shrapnel from a flashover, is still relatively unknown.

You should make sure that you have written – and up to date – safe working practices to for people who carry out any work on or near low voltage electrical systems. The HSE guidance note HSG85 Electricity at Work – Safe Working Practices is an excellently written and valuable resource in this respect. It clearly sets out decision-making flowcharts on whether to work live or dead and also gives some guidance on live working procedures.

The ability to calculate flash protection boundaries will enable managers to arrive at better-informed judgements before allowing live proximity work to proceed in the first place and provide an ability to apply quantitative techniques to risk control. There is software available but only trained individuals should undertake arc flash calculations, as any study should take account of variables due to system configuration for example.
Finally, if you have influence over the design of electrical systems then a goal will be to eliminate the need to work live at all in maintaining those systems. Some of the measures that can be employed are the segregation of power and control circuits, safe control voltages and currents, finger safe shrouding of terminals and built in test facilities.

Mike Frain FIET MCMI is MD of Electrical Safety (UK), advising industrial and commercial organisations on electrical safety procedures. He has held senior management positions in contracting, utilities and facilities maintenance companies having direct responsibility for putting people to work on a full range of complex and large power electrical systems.

There are estimated to be about 10 serious flashover (arc flash) explosions and 30 flashover incidents involving injury per day in the US. The seriousness and scale of the problem has been recognised by the regulatory bodies there and resulted in the introduction of code 70E by the National Fire Protection Association (NFPA). This only addresses risk mitigation during the incident. It does not address how to reduce the risk of the incident occurring, argues Ross Kennedy, managing director of QHi Group

In the USA ‘live’ thermal imaging inspections on medium voltage switchgear, where they open the panels, is quite common. There are around 15 deaths a year resulting from contact with live switchgear in the US, according to statistics from the US Bureau of Labor and from industry.
NFPA code 70E attempts to mitigate the risk of death and injury by zoning the area and prescribing increasing levels of personal protection equipment (PPE). There is a common misconception that thermal windows offer protection against the effects of flashover (arc flash). This is because the manufacturers indicate ‘compliance’ with NFPA code 70, but this is only with the covers on, in other words, when not being used. With covers removed they do not offer protection and the operative needs full PPE protection. Bad connections are the single most common cause of flashover. These ‘bad’ connections cannot be detected other than thermally, hence the widespread use of thermal imaging. However, while being a significant technology step at the time of its introduction, thermal imaging does not resolve two key issues:
1) It is only periodic - usually one or two days a year.
2) it is measuring the temperature of equipment which is located inside the enclosure, from the outside, requiring correlation and operator experience. Locating infrared sensors inside enclosures enables 24/7 continuous thermal monitoring with on-going trend analysis and alarm thresholds, which can be easily integrated into any BMS system.
QHi Group’s Exertherm can be fitted by switchgear manufacturers and panel builders as OEM equipment into new power systems, or retro-fitted and subsequently expanded. Self-powered intrinsically safe infrared (IR) sensors measure the rise above ambient, with only the assumption being that the sensor body is at the same temperature as the local ambient temperature. Local signal conditioning occurs outside of electrical panels using wired or wireless data transmission to a monitoring computer with appropriate software.

This predictive technology is already successfully installed in numerous multinational blue chip organisations and data centres, and is becoming increasingly adopted as ‘best practice’ in providing a significant improvement over periodic thermal imaging for mission critical equipment operating in high downtime cost locations. Furthermore, in an arc flash situation, you cannot apply standard mean time to repair (MTTR) calculations because it may take one or two days to assess the damage, and on average, a minimum of say five weeks (= 840 hours) of downtime.
In conclusion, continuous thermal monitoring is the only safe and reliable method of avoiding not only flashover, but also the cost of litigation and settlement, which in the case of the death of an employee can be as high as $15.75m in the US.

Electronic loads draw power from the supply with high levels of harmonics in the current. For IT equipment this has started to reduce in severity and the situation has improved from typically 50% down to 30% THCD (total harmonic current distortion). However, even at the lower figure these harmonic currents can cause high levels of voltage distortion in transformers, emergency generator alternators and all forms of UPS based on rotating electrical machines. Rob Tanzer, Technical Support Manager at Chloride Power Protection explains

Impedance is directly proportional to frequency so it is the spectrum of distorted load current harmonics that dictate the level of voltage distortion. For example, high-order harmonic currents will produce higher levels of THVD than the same THCD produced from low-order harmonic currents. So it is both the quantity (%age load) and quality (harmonic spectrum) that drives the resultant THVD.

In the case study that follows we have used a 2/3 pitch-wound synchronous alternator. Unlike a transformer, whose impedance at each harmonic frequency is a multiple of that frequency, this type of machine (used in gensets and rotary UPS) has lower impedance to the Triplen harmonics. Hence the same current will produce higher THVD in a distribution transformer.

Gensets tend to be partially loaded with harmonic generating loads and often oversized so this study concentrates on rotary UPS. The machine chosen is rated at 800kVA and has very low impedance (sub-transient reactance or forward transfer impedance, if you prefer) of 6.1% at the fundamental frequency, 50Hz. This is ‘low’ in comparison to a typical distribution transformer (8-10%) or, more significantly, a standard diesel-genset (10-15%).

We shall model the changing level of voltage distortion in a Tier IV type computer environment – with dual-corded loads being fed by a dual-bus UPS system, each bus comprising an N+1 rotary based UPS system. We shall take 2400kVA as the system load being fed by 2x 4x800kVA UPS systems, so that each system is capable of delivering 2400kVA with N+1 (33%) redundancy.

The point of interest here is in the wide variation of the four load scenarios as the system changes state due to maintenance or failure: • Normal operation with 2x(N+1) systems running at a maximum of 50% of the system load and the loading on the UPS is 38% • Maintenance in one system by removing one module from service, resulting in that system running without the +1 redundancy but still with <50% of the system load and the loading on the UPS is 50% • Maintenance in one system by a total shut-down (e.g. annual inspection) such that the other system carries the full load, with N+1, and the loading on the UPS is 75% • Emergency mode – one system out-of-service and the other system without N+1 redundancy, with the loading on the UPS at 100%.

To analyse the THVD we need to predict a load current profile. Fig. 1 (all figures below) shows the 1920kW full load with 32% THCD and CF=1.83. The Displacement (Power) Factor of such loads today is close to unity. On the right of the diagram is shown the harmonic spectrum. The distortion factor calculation (and for completeness the Neutral Current profile) is shown in Fig. 2. It is interesting to note that the Neutral Current is only 76% of the phase current with this modest harmonic load.

In Fig. 3 the system is modeled in ‘normal’ mode, all systems running with full redundancy. With 3200kVA capacity of UPS feeding 1200kVA of load the overall effective impedance is very low at 2.29% and hence the resultant THVD is 5.34% - although specific IT hardware OEM’s would regard that as excessive.

In Fig. 4 one module has been removed (for maintenance for example) and, as the overall impedance rises, the THVD rises to 6.62%, well above the normally acceptable 3% in most National power systems. The obvious distortion will not find favour with IT OEM’s looking for a high-fidelity voltage waveform and will cause heating in cables, transformers, motorwindings etc.

Next we come to the less frequent ‘emergency’ conditions – although it has to be said that this is what the client is actually paying for via the high levels of infrastructure redundancy.

Fig. 5 shows the system’s response to the full load being transferred to one of the two buses. This could occur under system failure or, more likely, should occur during the annual maintenance shut-down. The resultant THVD has now risen to an unacceptable 9.18%, even though the machines are still only loaded to 75%. Lastly, in Fig. 6 the ultimate emergency mode prior to load loss through UPS shut-down (and then the load might be transferred to mains or generator where the THVD will be markedly worse) – where the load is carried by one system which has lost its redundant module. The THVD is an un-operational 11.74%.
Conclusion It should have become clear that rotary systems’ critical output THVD is 100% load dependent (both harmonic spectrum and quantity). Unless the client can specify the loads harmonic profile the THVD will be unknown until after startup.

If the spectrum contains larger proportions of higher-order harmonics than those modeled above then the THVD could be far higher.

If the load conditions are known then a rotary system can be selected that will produce acceptable levels of THVD (generally by over-rating and achieving a lower impedance) under all operating conditions. In general the system designer should be aware of the dangers of loading rotary UPS to high levels with high-order harmonic loads – say in Tier II systems with no redundancy. Also the smaller the rotary machine generally the higher the fundamental impedance so, for example, putting 500kVA of the above load onto a 500kVA solo-machine produces nearly 20% THVD whilst 250kVA of the load still produces 10% THVD.

On the other hand, due to the PWM algorithms of the IGBT inverters all forms of static UPS, both series-on-line (double conversion) and line-interactive, will control the output voltage sine-wave to around 3% and always be less than 5% regardless of harmonic spectrum or percentage load.

Geoff Brown, Drive Applications Consultant, ABB Limited

Servo drives are generally used in applications requiring high precision, for example machine tools, and other high-precision machinery in industries such as material handling, packaging, paper, textile and woodworking.

The majority of servo drive solutions utilise a motor with low inertia rotor to provide rapid torque or speed response, and these motors may use permanent magnet or brushless d.c. technology, together with, a feedback device in the great majority of cases to give high accuracy. In addition to the motor the controller or amplifier traditionally has little programmability, and relies on an external controller.

But like many other areas of drive technology, the servo drive is being challenged in its market niche by standard AC drives that are becoming increasingly accurate and provide high performance at a lower cost. There is a new class of machinery drives around that is able to run a standard AC motors as well as specialist servo motors, with or without feedback devices, using a single drive platform.

Historically servo motors have used “square frame” construction, reminiscent of d.c. motors, and a range of “square frame” asynchronous motors matching d.c. motors has also become available. These can readily incorporate fixed speed fans, encoders and brakes.

This gives the designer an opportunity to select the most cost-effective solution amongst a range of options – servo, square frame or standard squirrel-cage motor, with feedback or without, depending on the requirements. The same type of drive can be used throughout the application, regardless of performance requirements.

The biggest leap forward for conventional drives in recent years has been improved torque control as a result of more powerful processors; this has enabled them to be used in the high performance arena. More computer power means quicker execution. This has opened up a.c. drives and motors to applications with a wide speed range, for instance dropping to a dead speed and then rapidly accelerating to high speed.

This all means that the required performance can often be achieved with standard AC motors without feedback devices, and drives with on board functionality, reducing the cost and complexity of the installation. If higher performance is needed, then a feedback device can be added to the standard or square frame a.c. motor. For still higher performance, add a servo motor, with low inertia rotor. For the highest performance achievable, a servo drive will still be needed. But for many applications, the performance needed can be achieved more simply and at a lower cost.

This is a good time to shop around for a high performance drive at a reasonable cost. Don’t just instinctively reach for the servo drive option, ask your drive supplier for advice.

Recently the trade and industry secretary, Alistair Darling, released the energy white paper, which sets out the details of the government’s energy strategy for the years - and decades - to come. Among the issues discussed are security of energy supply and the environmental impact of energy policy, but not apparently whether future government ministers will be given a fleet of Jags to take their wives 100 yards down the road. Open Circuit looks at some of the less well-known elements of the strategy…

Badly lit tunnels and incomplete diagrams were among the obstacles encountered by Geoffrey Lilleker an engineer working on a hydropower plant in the Himalayas’ foothills as he told Electrical Review

Located on the river Satluj in the north east of India this is a run of the river scheme to provide up to 1500MW of power from 6 machines at the village of Jhakri. The water is directed from the dam site at Nathpa via a 26km tunnel including 3 silt flushing chambers all dug into the mountains that are a part of the foot hills of the Himalayas in that part of Himachel Pradesh.

His brief initially, was to provide the software package for a number of Allen Bradley PLC systems that were to control the silt flushing gates and the guard valves in a tunnel with a length of 26km leading from the Naptha dam to the village of Jhakri where six machines were due to deliver up to 1500MW of electric power. His brief also comprised the electrical commissioning of the ports at the dam including the radial ports as well as various gate hoists that cut off individual areas off the plant for maintenance work and in case of an emergency.

“Unfortunately the drawings I was provided with locally were causing me many problems! They were out of date which meant there were many instances where they did not match the equipment supplied. Because of a shortage of relay contacts extra relays had been added which were not even shown on the drawings. Many device terminal numbers were missing making it difficult to follow them. Perhaps worst of all, the drawings had been done on a package that allowed more and more devices to be added to the same sheet by scaling the existing drawing down, which meant when they were printed out they could only be read with a magnifying glass. When a lot of the job involved working in poorly lit tunnels underground this was obviously an impossible situation. By the very nature of commissioning many circuit modifications had to be made, which also needed incorporating accurately into the drawings for final issue to the client and included into the of maintenance manuals,” Lilleker said.

“The nights were long in that remote part of the world,” he continued, “Using Elcad I could create all electric drawings from scratch in a short time. Thus, to the immense relief of the customer, the huge project was saved in time, and with new complete and safe documentation.”

Jim Wallace, research and technology manager at Seaward Electronic explains the importance of ensuring the safety of electrical appliances and equipment used in the workplace

The safety testing of electrical appliances and electronic equipment on the production line has become an integral part of the manufacturing process - but what happens when those items leave the factory for ‘in-service’ use in a factory, office, construction site or other type of workplace?
Although there are legal duties on manufacturers and suppliers covering the performance and integrity of new electrical or electronic equipment, responsibility for the safe operation of equipment in the workplace rests firmly with the employer.
The HSE claims nearly a quarter of all reportable electrical accidents involve portable and transportable electrical equipment and reports around 1000 workplace electrical accidents each year.
Poor electrical installations and faulty appliances are also a major cause of workplace fires that are responsible for extensive property damage, as well as posing a further risk to staff. For example, faulty equipment and leads have been known to cause over 6000 separate fires a year

Records indicate that a large number of workplace deaths and injuries are due to electrical shock from misused or faulty electrical equipment and most could have been avoided if proper electrical checking procedures had been applied.
For example, how can gradual deterioration in the electrical integrity of a power tool be identified or a potentially dangerous fault in a vending machine, kitchen appliance, or other electrical item be diagnosed?
The following examples on the effects of current on the human body are worthy of consideration:
0.9 – 1.2mA ~ Current just perceptible
15.0 – 20.0mA ~ Release impossible: cannot be tolerated over 15 minutes
50.0 – 100.0mA ~ Ventricular fibrillation, leading directly to death
100.0 – 200.0mA ~ Serious burns and muscular contraction of such a degree that the thoracic muscles constrict the heart.
When these values are compared to the fact that 250mA of current is required power a 25-watt lamp, the lethal potential of a faulty electrical appliance is easily understood.
The Health & Safety At Work Act 1974 puts a duty of care upon both employer and employee to ensure the safety of all persons using the work premises. However, the particular legal requirements relating to the use and maintenance of electrical equipment are contained in the Electricity at Work Regulations 1989 (EAWR).
Regulation 4(2) of the EAWR requires all electrical systems be maintained, so far as reasonably practical, to prevent danger. This requirement covers all items of electrical equipment including fixed, portable and transportable equipment – essentially anything connected to a building’s electrical system with a plug.

Although people carrying out the testing of portable electrical equipment should be appropriately trained for this work, since the introduction of the EAWR many electrical contractors have set up specialist portable appliance testing operations – as have other FM companies and electrical service firms.
Other organisations have responded with the introduction of in-house testing protocols managed by electrical engineers, maintenance managers, safety engineers and/or site electricians or facilities management personnel.
Whichever way it is implemented, planned and proactive safety policies must be capable of detecting potential problems with electrical appliances before they occur and this is the role of preventative maintenance programmes.
The majority of equipment defects can be found during visual inspection – the HSE says that just looking can identify 95% of faults or damage. For example, a detailed examination by a competent person is likely to eliminate hazards caused by cable or plug damage, faulty wiring or other obvious signs the equipment’s condition could create faults or a danger to users.
However, to identify all potentially dangerous faults, visual inspection needs to be linked with a programme of periodic inspection and testing that is capable of revealing any ‘invisible’ electrical faults such as earth continuity, insulation integrity, correct polarity, unacceptable earth leakage and other potential problems.
Clearly such combined inspection and testing measures should be appropriate to the particular risk posed by the equipment and its environment. This means maintenance procedures in some commercial environments might be required less frequently than in other high risk environments such as factories, engineering or industrial premises and construction sites – but will still be needed to verify safe working conditions.
For example, smaller offices or similar workplaces with only a few electrical appliances might be regarded as relatively low risk environments. Here, a responsible attitude might be regarded as a regular process of formal user checks and visual inspection, combined with some limited periodic testing.
A different view, however, might need to be taken in a large commercial operation or by an engineering or manufacturing organisation, with different departments and having many different types of electrical equipment used by staff. In this case, ensuring the safety of appliances may not only be a matter of ensuring the correct test equipment is available, but also having the ability to show that the right tests have been performed at the right time in the correct sequence - with records of test levels and results.
Also, overall frequency of inspection and testing of equipment will depend on whether the electrical items are rated as Class I or Class II and in what environment they are used. For example inspection and testing of some types of industrial and construction equipment might be advisable every 1-3 months and the interval can range to up to 12 months for other industrial locations, commercial kitchens and other workplaces, to 24 months and above for hotels, some offices and shops.

Cost effective maintenance of portable electrical equipment can therefore be achieved through a combination of user checks, formal visual inspection and electrical testing.
Combined inspection and testing programmes require greater level of competence than for inspection alone. However, a range of portable appliance testers (PATs) are available that make the in-service safety testing process safe, fast and easy to carry out.
Test instruments are available which range from the relatively simple to operate pass/fail checkers which will carry out some of the basic safety checks on equipment to provide an immediate ‘go/no go’ display.
However, for more comprehensive test requirements, microprocessor controlled testers are available that combine user-friendly operation with a whole range of other features for particular test demands or routines.
Lightweight testers are now available that incorporate all Class I and Class II required electrical safety tests in a compact hand held instrument. Long life battery power eliminates the reliance on mains outlets for testing, making the instrument totally portable and suitable for universal testing applications.
The incorporation of Bluetooth technology in some modern testers allows the wireless connection of bar code scanners, label printers and other accessories – allowing totally cable-free testing, without the cumbersome and constant plugging in and unplugging of leads and cords.
In addition, the latest generation PAT testers also have the facility to record the results of other safety management data including emergency safety lighting conditions, or condition of fire extinguishers, for example, as part of more comprehensive safety equipment audits.
Although there is no formal requirement in the EAWR for records, the HSE does recognise that some records of maintenance and test results is a useful management tool for reviewing schemes and demonstrating that safety policies have been enforced.
The use of computerised portable appliance testers also enables test data to be transferred directly from the instrument to a PC-stored database allowing an automatic update of test records, the generation of test reports and advance testing schedules.

Against this background, for the electrical contractor offering PAT services the pressure to complete tests quickly is immense. Those companies that have succeeded recognise the need to combine a fast effective service with the proper quality of testing.

To help them do so, a new safety-testing concept has been introduced to help those involved in PAT testing the ability not only to work smarter – but to improve their productivity and effectiveness in the process.
The PATSolutions approach seeks to streamline the selection of integrated portable appliance testing systems by bringing different test elements into distinct product packages.
Linking hand held test instrumentation with new technology and service support, different packages have been introduced to meet the different test system needs of facilities and safety managers or service engineers that carry out in-house electrical safety testing.

At each level a choice of test instruments are provided in keeping with the sort of portable appliance testing being undertaken. The testers are supplied with various accessory and software options, together with service and calibration support.

With growing awareness of the importance of the safety of electrical appliances, many private and public sector organisations have come to recognise the importance of regular inspection, test and maintenance of all electrical equipment used in the workplace.

Thermal images of electrical systems can rapidly indicate the operating condition of electrical equipment. In fact, since the beginning of thermography more than four decades ago, the principal commercial application for thermal imaging has been electrical system inspection says Ken West of Fluke (UK)

Thermal imaging has typically been the exclusive domain of specialists in the field, and has required the use of costly and difficult to operate equipment. With recent advances in sensor technology, this non-contact, versatile measurement technique has become available to a wider audience through dramatic reductions in price and in developments of user-friendliness.
Essentially, thermal imagers locate potential problems by detecting temperature differentials between one location and another. This can be achieved with instant displays of data using today’s handheld imagers which employ ‘point and shoot’ technology from an ‘electrically safe’ distance.
By presenting a rich visual image, using colours to represent a temperature, thermal imagers facilitate a quick visual check of surface temperature and easy identification of hot spots, which are often an early indication of impending failure. Maintenance programs can be developed and re-traced periodically using in-built routing instructions on the best products now on the market.

Warning signs
New electrical components begin to deteriorate as soon as they are installed. Whatever the loading on a circuit, vibration, fatigue and age cause the loosening of electrical connections, while environmental conditions can hasten their corrosion. All electrical connections will, over time, follow a path toward failure. If not found and repaired, these failing connections lead to circuit faults.
The reason thermography is so applicable to the monitoring of electrical systems is that a loose, over-tight or corroded connection increases resistance at the connection and since increased electrical resistance results in an increase in heat produced, a thermal image will detect the developing fault before it fails. An unbalanced or overloaded phase will also show up as hotter compared to the other two phases.

Preventative maintenance
Detecting and correcting failing connections by comparing the temperatures of connections within panels before a fault occurs averts impending failures. The best solution is to create a regular inspection route that includes all key electrical panels and any other high-load connections. The latest thermal imagers enable capture of data which can be uploaded and stored on a computer at the end of an inspection and then compared with other measurements over time. The ideal imager will allow the images from the previous inspection to be downloaded, so that previous and current images can be compared side-by-side at the point of capture. This will help to determine whether a hot spot is unusual or not, and also help to verify that repairs have been successful.
Such predictive actions are important because when a critical system does fail, it inevitably increases costs, threatens a client’s profitability and may impact on safety.

What to look for?
In general, connections that are hotter than others should be looked for. This signals high resistance possibly due to looseness, tightness or corrosion. Connection-related hot spots usually (but not always) appear warmest at the spot of high-resistance, cooling with distance from that spot. Overheating connections can, with additional loosening or corrosion, lead to a failure and should be corrected. Equipment conditions that pose a safety risk should obviously take the highest repair priority.

3-phase problems
Thermal images are also an easy way to identify apparent temperature differences in three-phase electrical circuits, compared to their normal operating conditions. By inspecting the thermal gradients of all three phases side-by-side, engineers can quickly spot performance anomalies on individual legs due to unbalance or overloading. Even a small voltage unbalance can cause connections to deteriorate, reducing the amount of voltage supplied. A severe unbalance can blow a fuse, reducing operations down to a single phase. Overloading or unbalance should then be investigated using other types of measuring instruments such as a clamp meter or power quality analyser.

Reporting procedures
Analysis and reporting software is provided with the best thermal imagers. Whenever a problem is discovered using a thermal imager, this software should be used to document the findings in a report, accompanied by each thermal image and a digital image of the equipment. This is the best way to communicate the problems to the client along with the suggested repairs.

Keeping ahead of the game
Thermal imagers at affordable prices offer one more tool in the arsenal of the maintenance engineer who wants to keep one step ahead of the game by offering his clients even better preventative maintenance.