Test and measurement or T&M

  • Fluke offers Multifunction Installation Testers alongside free extras

    Until 30th June 2019 and while stocks last, Fluke UK is offering two 1660 Series Multifunction Installation Testers with a free innovative T6 Electrical Tester plus data management software.

  • MCS Rentals Acquires Electro Rent UK

    MCS Rentals Ltd, the rental division of UK test and measurement company MCS Test Equipment Ltd, announces the acquisition of Electro Rent UK.

  • Exclusive infrared camera offer from Fluke

    Until 30th June 2019 and while stocks last, Fluke is offering TiS45 Infrared Cameras at a special recommended offer price, plus a free Fluke 971 Temperature Humidity Meter – the TiS45/971 Kit. 

  • Primary protection

    When you need to test a relay protection system, particularly during commissioning, you need to decide early on whether to use primary or secondary injection. Both approaches have their merits, yet according to Marius Averitai of Megger, there are many cases where primary injection has the edge.

  • Why all the fuss, asks Metrel?

    The current spat between tester manufacturers over the suggested need for voltage drop measurements and power quality in the 18th Edition amendment 1 is unfounded according to Metrel.

  • Leakage clamp measures up to the 18th Edition

    Ever had a problem with nuisance RCD tripping? Now the 18th Edition Wiring Regulations and the latest leakage current clamp meter from Martindale Electric, the CM69, come to the rescue.

  • Extended scope

    BSRIA has extended the scope of its UKAS (United Kingdom Accreditation Service) accredited testing services, which now includes the latest version of the BESA UK HIU Test Regime: October 2018.

    The accreditation follows the announcement BSRIA became a BESA approved laboratory in accordance with the above mentioned standard in October, as well as BSRIA attaining UKAS accreditation to the December 2016 version of the test regime in January of this year. 

  • Rhys Boni wins SkillELECTRIC gold

    23-year old Rhys Boni has been crowned 2018 SkillELECTRIC champion after excelling in a two-day practical competition at WorldSkills UK LIVE – the nation’s largest skills and careers event with over 80,000 visitors. 

  • Portable and scalable

    With its new CPCSync feature, Omicron’s modular and mobile CPC 100 multi-functional testing system can be used as a mobile and scalable HV source for on-site induced voltage tests on dry-type transformers (DTT) and gas-insulated switchgears.

  • Global Skills Exchange launches certification exam program with Pearson VUE

    Global Skills X-change (GSX)has entered into an agreement with Pearson VUE, a company specialising in computer-based testing, to deliver the Certified Mission Critical Operator (CMCO) exam.

  • Protection Testing Conference & Workshop

    From October 16–18, 2018 OMICRON will host the third UK Omicron Protection Testing Conference & Workshop at magnificent Crewe Hall.

    Protection experts from around the world will gather to exchange experiences, engage in an interesting conference and participate in practical workshop sessions.

  • Extended spectrum of IEC 61850 testing solutions

    The IEC 61850 international standard for power utility communications defines two types of communication to be used for substation protection, control and automation: real time communication with Goose and sampled values on the one hand and client/server (C/S) on the other. For all these cases, Omicron as a market leader for IEC 61850 testing solutions, offers internationally well accepted testing tools, such as IEDScout or Test Universe with its configuration tools for Goose and sampled values.

  • New London facility offers customers real choice

    Electro Rent, one of the world’s largest providers of electronic test equipment for rent, lease or used sales, is opening a new office in London.

  • Sophisticated test and measurement

     

    TJ|H2b is proud to announce its recent partnership with DV Power, bringing you the newest and best products DV Power has to offer.

    Founded in 2002, DV Power is internationally known to develop, manufacture and supply the most sophisticated test and measurement equipment for the power industry.

  • Test & measurement - Substation surveillance Part 1

    In service high-voltage (HV) substation equipment is exposed to many stresses, from the electrical, mechanical and thermal to the environmental. These stresses can act to accelerate the deterioration of the insulation and the electrical integrity of the HV equipment eventually leading to failure. Partial discharge (PD) is both a symptom and a cause of insulation deterioration, so the detection and measurement of PD phenomena can provide early warning signs of insulation failure.

    Critical to this detection is the availability of accurate and cost effective surveillance tools, which, if non-invasive, can provide early recognition and location of possible sites of electrical degradation while components are still in service. Gathering and trending PD activity over time is essential to monitor the rate and severity of degradation. Maintenance can then be planned in an effort to avoid unplanned outages, interruptions and inevitable loss of revenue.

    The use of radio frequency interference (RFI) measurement is an efficient, non-invasive surveillance technique to detect and locate partial discharges in individual HV apparatus. This article will look at the benefits of combining the assessment of RFI emissions with the targeted deployment of complementary, non-invasive electromagnetic interference (EMI) detection techniques. Specifically, frequency sweep data and time-resolved traces can be compared with follow up assessments using complementary EMI couplers such as high frequency CTs (HFCT) and transient earth voltage (TEV) couplers. This combination of tests provides an increased level of confidence in the location, identification and assessment of the severity of degradation and is beneficial when dealing with complex HV apparatus.

    The detection and measurement of RFI emissions from PD phenomena involves the measurement of complex waveforms varying considerably and often erratically in amplitude and time. RFI signals from such phenomena are considered to be broadband and impulsive in nature with low repetition rates.


    Measurements carried out on PD activity within oil-insulated HV equipment demonstrate that the discharges produce current pulses with rise times less than a nanosecond and therefore capable of exciting broadband signals in the VHF (30 to 300MHz) and UHF (300MHz to 3GHz) bands. Other investigations in open-air insulation substations show that signals from PD and flashover occupy a frequency range up to 300MHz.

    When PD occurs inside a metal enclosure, such as in a transformer tank, the signal propagates within the structure, suffering frequency attenuation, reflection, etc. Detection of RFI emission relies on the placement of apertures in the tank walls and penetrating conductors to allow the RFI emissions to propagate and radiate externally.

    In the following examples, the instrument used (Doble PDS100) has two different detection modes: spectrum analyser and time-resolved mode. Within spectrum analyser there are three separate detection techniques: peak detection, average detection and separated peak and average detection.

    Case Study 1
    Dissolved Gas Analysis (DGA) carried out on a South African, 275/88/11KV, 250MVA transformer showed signs of a discharge type fault. RFI measurements (using the Doble PDS100) and conducted EMI measurements (using a HFCT) were performed to establish correlation between the measurements. RFI measurements were taken around the periphery of the transformer. The frequency traces (FIGURE 2) exhibit a discrete appearance as pulses are accumulated. Short bursts of pulse accumulation were interspersed with long intervals of no or low energy activity. Triangulation based on signal intensity of the higher frequencies locates the source of propagation in the vicinity of the HV B-phase. Observing the RFI at 900MHz for a period of time in spot frequency mode sees the measured peak amplitude reaching -45dBm at that location. This mode also confirms the burst nature of the pulse sequence.

    The conducted EMI was measured using a 300MHz split-core HFCT at the HV neutral connection to earth. The measured conducted EMI is subjected to significant attenuation through the HV neutral connection and requires an extended observation time. In time-resolved mode both the RFI and conducted EMI measurement confirm the measured pulse behaviour. However, the pulsed activity is more easily captured and more of the lower energy pulses are detected.


    Partial discharge activity is indicated by both the RFI and EMI techniques. In each, the dynamic behaviour of the activity is characterised by very short burst activity interspersed by intervals of no or low energy activity. The sequence exhibits the characteristics of a floating type discharge. A secondary source of discharge is evident in the time-resolved traces. The results confirm the conclusions drawn from the DGA analysis. This study proves the use of RFI as an assessment tool while the use of an HFCT coupler provides increased sensitivity to internal PD activity, offering an increased level of confidence in the identification and assessment of PD activity.



    Case Study 2
    At a distribution substation, RFI measurements were undertaken to survey the condition of each of the oil-filled circuit breakers making up a typical 11kV distribution switchboard configuration commonly found in the UK. A high percentage of 11/33kV switchboards have an installed age of over 25 years. They are subjected to various types of duty plus a varied level of maintenance. The trend is to extend the maintenance period for medium-voltage (MV) switchgear, which in turn creates the need for interim non-intrusive condition monitoring techniques to offer confidence in the equipment’s safety and reliability.

    A baseline RFI scan was captured in an adjacent room away from the surveyed switchboard. Measurements at the rear of each circuit breaker were captured and compared with the baseline. The observed uplift of frequencies indicated a nearby discharge source, which was eventually triangulated to one particular circuit breaker by comparing the uplift in higher frequencies while moving the receiving antenna along the rear of the switchboard. Further RFI measurements were captured at the front of the switchboard. A comparison of the front and rear RFI measurements shows that the uplift in the lower frequencies was strongest to the front of the unit. These tests were followed by complementary EMI measurements to gather more information. 

    RFI Peak Measurements: Front and Rear of Circuit Breaker
    Legend: Front, Rear


    The HFCT uses inductive coupling to detect PD pulses flowing to earth and is capable of picking up both local PD in the cable end box and also the lower frequency PD pulses coming from down the cable. The results of this method confirmed the observations from the RFI survey, with uplifts of up to approximately 50dB at 75MHz and 40dB at 200MHz. Time-resolved measurements also showed pulse behaviour is similar to those obtained from RFI measurements (FIGURE 6).

    Lower Frequency ( circa 50MHz)
    Legend: RFI, HFCT, TEV

    Mid Frequency (circa 150MHz)
    Legend: RFI, HFCT, TEV

    Higher Frequency (circa 200MHz)
    Legend: RFI, HFCT, TEV


    The placement of HFCTs provides a means to trace the likely source of the signals by comparing the uplift in frequencies. The uplift reduces significantly as the location of the HFCT is moved away from the suspect circuit breaker. Repeated measurements on earth straps placed on adjacent circuit breakers indicate the circuit breaker identified is the source of the measured discharge activity.

    The most advantageous setup for metal-clad switchgear is to use an HFCT sensor in conjunction with a TEV sensor. Transient Earth Voltage (TEV) measurements work on the principle that if a PD occurs within metal clad switchgear, electromagnetic waves escape through openings in the metal casing. The electromagnetic wave propagates along the outside of the casing generating a transient earth voltage on the metal surface. TEV sensors are “capacitive couplers”, which when placed on the surface of metal cladding can detect TEV pulses as a result of PD internal to the switchgear.

    Observed peak TEV measurements on the main circuit breaker tank reach 0dBm at a frequency of 100MHz. Comparative measurements taken with the TEV sensor located on the cable end box show a reduction in uplift of approximately 20dB. The main circuit breaker tank is identified as the likely source of the discharge. Time-resolved measurements show pulse behaviour confirming the results obtained from both RFI and HFCT measurements.
    The utility opened up the circuit breaker and found signs of carbon at the cable end in the main tank of the switchgear. Results of this study confirm that deploying frequency spectra measurements and time-resolved patterns from RFI, HFCT and TEV probes can be used to pinpoint PD issues within switchgear. Using TEV sensors in conjunction with RFI surveillance on metal clad switch gear offers an additional capability in confirming and localising the partial discharge source.

    Conclusion
    RFI monitoring offers, and has proven to be, a routine non-invasive and cost-effective surveillance technique that complements and provides added value to other well established HV asset monitoring techniques such as thermal imaging and DGA analysis. As the practical examples illustrate, measurements logged with an RFI instrument platform specifically designed for substation surveillance can assist in effective discrimination and recognition of the RFI emissions radiated from potential sites of insulation deterioration.

    There are great benefits of combining the assessment of RFI emissions with targeted deployment of other complementary non-invasive electromagnetic interference (EMI) detection techniques using the same RFI instrument platform. The deployment of both ‘far field’ and ‘near field’ probes provide a diversity of sensors, which is of particular importance with complex HV apparatus such as transformers where the propagation paths for RFI are less well defined.

    This article is based on the paper Substation Surveillance Using RFI and Complementary EMI Detection Techniques, which was recently presented at the 78th International Conference of Doble Clients in Boston, Massachusetts USA. The paper was written by Alan Nesbitt, Brian Stewart and Scott McMeekin of Glasgow Caledonian University and Kjetil Liebech-Lien and Hans Ove Kristiansen of Doble Engineering Company.

    Part 2 of this article will appear in the June 2011 issue of Electrical Review.

  • Test & measurement - Substation surveillance Part 2

    At a distribution substation, RFI measurements were undertaken to survey the condition of each of the oil-filled circuit breakers making up a typical 11kV distribution switchboard configuration commonly found in the UK. A high percentage of 11/33kV switchboards have an installed age of over 25 years. They are subjected to various types of duty plus a varied level of maintenance. The trend is to extend the maintenance period for medium-voltage (MV) switchgear, which in turn creates the need for interim non-intrusive condition monitoring techniques to offer confidence in the equipment’s safety and reliability.

  • LEM publishes 2005 Test and Measurement Instruments catalogue

    LEM publishes 2005 Test and Measurement Instruments catalogue

    LEM’s 2005 Test and Measurement Instruments catalogue includes individual product descriptions and the most important technical data on the company’s ranges of installation testers, machine and device testers, network analysers, isolation and earth testers, probes, multimeters etc. LEM’s wide ranges of supporting accessories and software are also described.

    The technical tables at the beginning of each product family offer a direct comparison between the instruments. The explanation of important technical expressions at the end of each section helps in the selection process.

    The catalogue is available in eight different languages, from LEM NORMA at: This email address is being protected from spambots. You need JavaScript enabled to view it., phone +43 2236 691-0.

  • Test & measurement - Power quality measurement...What does Class A mean to me?

    Power quality measurement is still a relatively new and quickly evolving field. Whereas basic electrical measurements like RMS voltage and current were defined long ago, many power quality parameters have not been previously defined, forcing manufacturers to develop their own algorithms.

    There are now hundreds of manufacturers around the world with unique measurement methodologies. With so much variability between instruments, technicians must often spend time trying to understand the instrument’s capabilities and measurement algorithms instead of concentrating on the quality of the power itself.
    The IEC 61000-4-30 CLASS A standard defines the measurement methods for each power quality parameter to obtain reliable, repeatable and comparable results. It also defines the accuracy, bandwidth, and minimum set of parameters. Going forward, manufacturers can begin designing to Class A standards, giving technicians a level playing field to choose from and increasing their measurement accuracy, reliability, and efficiency on the job.
    IEC 6100-4-30 Class A standardises measurements of:
    • Power frequency
    • Supply voltage magnitude
    • Flicker, harmonics, and inter-harmonics (by reference)
    • Dips/sags and swells
    • Interruptions
    • Supply voltage unbalance
    • Mains signalling
    • Rapid voltage changes.
    Examples of Class A requirements:
    • Measurement uncertainty is set at 0.1% of declared input voltage. Low cost power quality measurement systems with uncertainties greater than 1% can erroneously detect dips at -9% when the threshold is set at -10%. With a Class A certified instrument, a technician can confidently classify events with internationally accepted uncertainty. This is important when verifying compliance to regulations or comparing results between instruments or parties.
    Dips, swells and interruptions must be measured on a full cycle and updated every half cycle, enabling the instrument to combine the high resolution of half-cycle sampled data points with the accuracy of full-cycle RMS calculations.
    • Aggregation windows – A power quality instrument compresses acquired data at specified periods which are called aggregation windows. A Class A instrument must provide data in the following aggregation windows:
    - 10/12 cycle (200ms) at 50/60Hz, the interval time varies with actual frequency
    - 150/180 cycles (3s) at 50/60Hz, the interval time varies with actual frequency
    Harmonics must be measured with 200ms intervals according to the new standard, IEC 61000-4-7 / 2002. The old standard allowed 320ms intervals which cannot be synchronised with the 200ms aggregation windows of other Class A measurements.
    Using 200ms intervals allows harmonic calculations to be synchronous to all the other values like RMS, THD, and unbalance.
    The Harmonics FFT algorithm is specified exactly such that all Class A instruments will arrive at the same harmonic magnitudes. The FFT methodology allows for infinite algorithms that can result in vastly different harmonic magnitudes. By standardising on 5Hz bins and summing the harmonics and inter-harmonics according to specific rules, Class A instruments will be consistent and comparable.
    • External time synchronisation is required to achieve accurate timestamps, enabling accurate correlation of data between different instruments. Accuracy is specified with ±20 ms for 50Hz and ± 16.7ms for 60Hz instruments.
    • 10 min interval sync to clock
    • 2 h interval sync to clock.
    Latest product developments
    There have been a number of significant introductions to the market in the past 12 months of power quality analysers offering compliance with IEC 61000-4-30 CLASS A. These new products include both handheld devices and those designed for leaving in a fixed location for a time period set by the user. They will log a large number of parameters at user chosen time intervals for later analysis by a PC. Thus there is a choice of products, offering different capabilities, from which a technician can choose the most appropriate tool for the job.
    These new tools are designed for ease-of-use to uncover intermittent and hard-to-find power quality issues. Suitable handheld analysers will provide on-screen display of trends and captured events even while background recording continues. Some can be used to analyse disturbances, to validate incoming power compliance, for capacity verification before adding loads, and for energy and power quality assessment before and after improvements. The best tools provide powerful reporting software to enable rapid assessment of the quality of power at the service entrance, a substation or at the load according to EN50160 standards. The software can quickly analyse trends, create statistical summaries and generate detailed graphs and tables.

  • Test & Measurement - Restoring safety after the Storm

    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.

    Transformers
    - 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.

  • Test & Measurement - Checking the Earth

    Test & Measurement - Checking the Earth

    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).

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