Surge protection – Maximum protection from induced surges


The visible damage caused by lightning can be spectacular but the damage caused to sensitive electronic systems can have a far more profound effect on operations and profits says Andy Malinski of Omega Red Group

Most modern companies place heavy reliance on the uninterrupted functioning of electrical systems used to power everything from sophisticated IT networks and telecoms to lighting and heating systems. Yet many buildings, including those only a couple of decades old, were simply not designed with surge protection in mind even though a strong surge can completely disable the electrical system in place. Protecting electronic equipment from the consequences of surge or lightning activity is essential and not just because of the immediate disruption it can cause.
Big increases in insurance claims for surge related damage have led some insurance companies to increase premiums for companies heavily reliant on electronic technology and to impose exclusions of cover pending this particular problem being addressed. Correctly following BS6651:1999 Annex C general advice on protection against lightning of electronic equipment within or on structures should ensure that any site has adequate surge protection and will help to secure the relevant insurance for the site.

The problem
During lightning activity, and switching operations, transient surge voltages are generated. Surges are short duration voltage spikes appearing on a mains power or low voltage signal line such as a computer or telephone line.
The amount of energy contained within the surge is dependant on the magnitude and duration of the event but values up to several thousands of volts lasting for microseconds can be generated. Two main causes of transient over-voltages are:
• Switching. This results from an electrical load being switched on or off with typical loads including motors, transformers, welders, photo copiers etc. These types of surges happen many times every day, are of a short duration and value and do not usually cause major problems.
• Lightning and atmospheric disturbances: Surges generated by lightning activity tend to be of a much higher level and are consequently much more dangerous. They can be generated either by direct or nearby (up to 1.5km away) lightning strikes though most damage tends to be from nearby strikes as the magnitude of discharge current is more concentrated.

How does it get in?
Copper conductors used for electrical, mains, data, computer and telephone wiring are prime routes for transient surges to enter buildings and there are three easy main ways in which these transient voltages affect wiring systems.
• Resistive coupling – a ‘cloud to ground’ lightning strike injects a massive current into the ground. This raises the ground potential in the area of impact to a high level and for the current to dissipate it will seek the path of least resistance to earth. Cables running between buildings are usually connected to different earthing systems at each end and a cable connected to an earth of a lower value forms an ideal route for the induced current to follow.
• Inductive coupling – a lightning discharge causes a huge current to flow. This in turn sets up a massive magnetic field. Any conductor passing through this magnetic field will have a surge voltage induced on the cable, this is the same principle as used by a transformer, and it can happen either above or below ground.
• Capacitive coupling – atmospheric disturbance causes high voltages to be generated. A low voltage conductor in the area of influence of these voltages can be charged to that same voltage, this is the same effect as charging a capacitor.
Dealing with a surge
The British Standard addresses lightning protection for both external structural strikes and internal surges.
Appendix C of the standard looks at internal protection of electrical and electronic systems. More recently the new European standard EN 62305/4 Electrical and Electronic Systems covering surge protection has been released to work in tandem with the British standard but within a few years the national standard will be withdrawn in favour of the new European standard.
For both external and internal protection the first step is to undertake a risk assessment. This is a comprehensive, complex assessment as many factors need to be considered. Internal surge protection takes into account all factors affecting the electronic equipment and systems within the building.
A risk assessment looks at many factors including the number, length and types of cables entering and leaving the building, equipment types, exposure and risk levels, recommended levels of protection, and cable routing, amongst others.
Results taken from the assessment will determine if protection is required and the correct protection methods to use. This can take the form of surge voltage protection devices, the repositioning of cabling, a more effective earthing system or by other means.
Zones of protection are defined, with the corresponding levels of protection required, and co-ordinated protection devices are fitted at zone interfaces. By using a co-ordinated system the high surge current present at the outer zone will be dissipated and the attenuated surge then handled by devices fitted at subsequent zone interfaces – limiting possible damage.

Surge protection in practice
All electronic equipment has a transient safety level, a maximum surge voltage value that can be applied to the equipment without causing damage.
The protection device must reduce the surge voltage to below this value with any excess voltage shunted to earth in the quickest time possible. The let-through voltage of the protection device needs to be as near as possible to the nominal voltage of the line being protected.

Mains power protection
When applying surge protection to a site the first system to protect is the mains power as the large diameter of the cable will allow surges to pass into the system with minimum attenuation. Of greatest concern, the mains is common to all other systems and a surge entering via this route will quickly spread into all other associated systems.
Protection at the main Low Voltage (LV) incoming supply is necessary to control large transients before they enter the distribution system.
Additionally, protection should also be installed locally to important equipment or sub-distribution boards feeding outside equipment.
This is to guard against both internally and externally generated transients, which may be injected back into the distribution system.
Low voltage telephone signal protection devices fit in series with the cables being protected. On older termination systems the Surge Protection Device (SPD) is wired directly into the circuit.
Plug in protection devices are available for LSA Plus (registered trade mark of Krone) termination strips, these come in either single or ten way configurations.

Building Management System
Typically Bulding Management Systems (BMS) consist of a network of separate stand-alone slave controller panels.
Across the site a RS485 backbone cable will interconnect all panels. A drop cable making the network connection to each panel.
On most systems the drop cable is a single twin twisted pair utilising RS485 protocol. It is recommended to fit a surge protection device in series with each drop cable, as near to the interface card as possible.
The input/output (I/O) from each slave controller is usually internal to the building being monitored and controlled. Any I/O external to the building requires an SPD to be fitted.

Fire and Intruder Alarm Systems
These systems tend to be wired using the same type of backbone network as for the BMS.
The main difference being that they will have more I/O cabling connected to outside sensors and alarms. SPD’s are required to be fitted to any cable connected to an outside device.
The positioning and routing of the cables either side of the SPD is of the utmost importance. Incoming and outgoing cables either touching, or closely running in parallel with one another, can cause surge voltages to be induced ‘across’ cables.

Close Circuit Television (CCTV)
The two most popular types of camera are:
• Pan, Tilt and Zoom (PTZ) cameras
• Fixed cameras
Pan, Tilt and Zoom (PTZ) cameras are controlled from a central location with the vertical and horizontal positioning of the camera being via signals sent over the RS485 data loop. As the central processor would consist of expensive monitoring and control equipment, SPD’s would usually be fitted to this end of the system. It is possible to offer protection to the camera by fitting surge protection devices within the camera.
The level of protection required depends on the vulnerability of the camera. Is it located within the zone of protection of the building? Does the building have a lightning protection system fitted? Is the mounting pole correctly earthed?
Usually these would be monitored from a central point, the main difference being that motion control of the camera is not possible. Protection requirements and the methods of achieving them are the same as for the PTZ cameras.
The initial surge assessment will identify which ends of each system require protection.
As a surge can travel both ways in a cable it is important to protect both ends of the system if necessary.

Professional Contractors
Of course, none of the above holds true unless a competent contractor is used to survey, design, specify, install and maintain the surge protection system.
If a surge protection system is to work effectively to prevent equipment failures then a number of other factors need to be taken into account in addition to the technical expertise that is required.
Look for evidence of a proven track record, particularly for major or technically challenging projects.
There are a lot of suppliers in the market but the best will be able to provide customer references attesting to the professionalism of their work and thoroughness of approach.
Make sure the organisation has the resources needed to do your job –?can they handle large scale, multi-site operations across the country using their own staff or will the job be largely subcontracted to organisations that may not have the same quality standards or professionally trained staff?
By combining technical and service factors, the maximum level of protection from induced surges and over-voltages is delivered to the electronic systems within the building, this protects productivity and, ultimately, profits.