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Lightning Surge Protection - Current trends and some contentious issues

Due to the random and statistical manner in which lightning related events occur and cause equipment damage, lightning surge protection equipment is often only tested in the field for a brief moment, sometimes years after its purchase and installation. If the surge protection was correctly rated for the application, manufactured correctly and installed correctly there is no reason why it should not operate and adequately protect the equipment installed ‘downstream’ of the surge. If the rating, manufacture or installation of the product is not adequate, however, the surge protector may remain installed without incident until required to handle a large surge. When the product then fails, people search for reasons and due to the uncertainty associated with the events surrounding a given a lightning strike, reasons are sometimes difficult to substantiate and mistrust of the product and the industry which supplied it are often the end result.

Over the last few years, there have been a proliferation of articles claiming to ‘explain away the myths’, surrounding the field of lightning surge protection. Many of these articles have been written either in order to bring the layman into the picture or to promote a particular product. This article will highlight some of the contentious issues and trends which have arisen in the field of lightning surge protection recently and give an impartial analysis of what the implications are.

Surge component criticism

Very often, specific surge protection components are criticised for their general performance in favour of another product or component. What is seldom highlighted in the argument is the fact that there are many different situations in which the three main groups of surge protective components, Metal Oxide Varistors (MOVs), Gas Arresters and Silicon Clamping Diodes (SCD’s) may be applied. Each has characteristics making it the best choice for a given application. The sensitivity and cost of the equipment to be protected coupled with the risk of sustaining surge related damage determines the component or combination of components that is most suitable. The following discussion reviews the three main component types and what application they are suited for, their advantages and disadvantages:

MOVs: Metal Oxide Varistors are commonly used for mains protection applications and are available in a wide range of clamping voltage and peak impulse current variations. MOVs are extremely cost effective components and have been proven by time to work efficiently in many applications. There are three main drawbacks in the use of MOVs namely their tendency to degrade with use, a relatively high terminal voltage when clamping high current impulses and a response time which could be considered slow when compared with silicon clamping technology. Despite these weaknesses, the MOV is perfectly suited to a number of applications.

• The ‘aging’ characteristic describes the degradation of the micro-granular structure of the material comprising the MOV. The result is an increase in leakage current drawn by the MOV at normal system voltages. If this current increase is suitably high, it is liable to drive the MOV into thermal runaway – a condition where the device is absorbing energy more quickly than it can be dissipated and it heats up to dangerous temperature levels. This hazard is effectively countered with the use of an effective thermal disconnect device which disconnects the MOV from the supply if thermal runaway occurs and should visually indicate the failure on the device on the outside of the casing. MOVs should, therefore, be chosen so that they will be exposed to surges which are well within the limits of their specified energy handling capability and should, like any other piece of industrial equipment, be checked and replaced if found to be faulty. I this is done correctly, the service lifetime of MOVs is increased dramatically and they provide a very cost effective surge protective solution.
• There is often confusion surrounding the quoted clamping voltage of a MOV and the fact that during a surge, the voltage across its terminals may reach three or four times this value. The clamping voltage is the voltage at which the MOV will begin to lower its impedance and begin drawing surge current. As this current flowing through the MOV increases, it raises the voltage across its terminals (the MOV is after all just a non-linear resistor with a very small but finite resistance and as the current through it increases, so must the voltage across it!). The fact is that in most mains applications, the equipment input is quite robust and will more than adequately be protected by a correctly chosen MOV product even if the terminal voltage does exceed the clamping voltage by 3 or 4 times for a short duration.
• The manufacturer quoted response time of the MOV is usually of the order of tens of nanoseconds which is once again more than adequate for most mains applications. Response time becomes critical when the surge is applied across sensitive silicon-based inputs which react quickly to a change in terminal voltage. The speed of response of the MOV material is almost instantaneous, however, and it is largely the inductance in the leads connecting the surge protector to the supply which causes a delay in response.

Gas arresters operate on the principle of electrical breakdown across a gap between two conductive plates. They are capable of repeatedly diverting huge surge currents without the degree of degradation experienced by MOVs (although they may be damaged by repeated operation due to large surges) and are used extensively as the front end to hybrid communications and data protection circuits. Gas arresters have two main drawbacks: follow on currents at large system voltages and they often experience a lengthy statistical breakdown delay before operation leading to a large transmitted voltage spike before clamping.

• Follow on currents make conventional gas arresters impractical for use on mains systems. Once the device operates in response to a surge, the impedance across the arrester is extremely low and normal mains voltage can sustain the arc leading to the rapid heating and destruction of the device. Modern developments in the field have yielded special gaps which use circuit breaker principles where the heat of the arc is used to stretch it between two diverging electrodes thus raising its impedance and eventually quenching it. These gaps may be used on mains systems and are normally used in conjunction with MOVs and a series impedance providing good protection against large surge currents.
• The statistical delay causing a voltage spike in the clamping characteristic of the gas arrester is normally removed by placing a series impedance behind the gap (to prevent the spike from driving a large current into the device being protected) followed by a MOV or silicon clamping device which operates quickly enough to clamp the spike until the voltage across the gas tube drops. Modern hybrid spark gaps include the tiny hybrid components between the terminals and legs of the gas arrester component and function without the series impedance.

Silicon Clamping Diodes (SCDs) are well know for their rapid response times, excellent clamping characteristic and extended life-time. Unfortunately, their inability to deal with high surge currents and the relatively high cost are the drawbacks associated with silicon surge technology. SCDs are used extensively as the secondary stage of the hybrid protection described previously. The rapid response times (a few nanoseconds once again dependant on component lead length!) make them ideal for circuits used to protect sensitive silicon inputs such as those found in computer networking and most digital telecommunications systems. There are a few silicon based products which have been designed to cope with larger currents and are sometimes used for specialised mains applications. These products are usually enormously expensive and cannot handle surge currents which MOV or the new gas arrestor technology can. As mentioned previously, the majority of mains protection is placed in front of reasonably robust inputs which can handle high voltages for a short duration and there is subsequently no need for nanosecond response times and rigid clamping in this application.

From the above synopsis, it should be clear that no one surge protective component provides the ‘ultimate solution’. The answer lies in choosing or combining components to achieve the desired performance for a particular application at an acceptable cost.

The ‘numbers game’

A common claim in the lightning protection industry is that the device able to withstand the highest current magnitude is the superior product and is worth the extra expense. This has led to a race in which competitors are producing devices capable of withstanding extremely large lightning strike currents when this is almost never required in any practical situation. It is well known that, during a lightning strike to a structure, a conservative estimate of 50% of the strike current is dissipated by the structure itself and the rest divides itself up more or less equally in the cables entering the building. Assuming a lightning stroke with a magnitude of 100kA which, according to the IEC-SABS 1024-1-1, is large, has a 5% chance of occurring and is thus quite rare, we can make some simple deductions about the common mode currents entering a building via power and signal cables. If we assume that we have a conservative number of five cables entering the structure (3 phases of the supply and telephone/fax for example), a 10kA impulse could be expected on each incoming line. The decision to place a 100kA surge protection device on each phase of the incoming power lines would then, for example, be unnecessary and a poor economic decision.

Lightning Protection Zones

The concepts of defined lightning zones and boundaries are sometimes misunderstood and poorly defined. A lightning protection zone is an area in which a similar electromagnetic environment can be described. A significant change in this environment occurs at the boundary of a lightning protection zone. A zone boundary may be established by a structure exhibiting a certain degree of electromagnetic shielding or the placement of a surge protective device. Zones may be classified as follows:

LPZ0A : Area exposed to direct lightning strike and unattenuated electromagnetic fields.

LPZ0B: Area shielded from direct lightning strikes but still exposed to unattenuated electromagnetic fields.

LPZ1: Area shielded from direct lightning. Fields and currents on conductors are significantly attenuated compared with LPZ0A and LPZ0B.

LPZ2: Area shielded from direct lightning. Fields and currents on conductors are significantly attenuated compared with LPZ1.

etc.

How this concept affects the ratings and placement of surge arresters is probably best described by the following diagram:

The diagram shows that by slightly increasing the clamping voltage of the surge protective devices at consecutive zone boundaries, the surge current may be diverted in a controlled manner with most of it being dealt with at the Zone 0B/1 boundary. As the surge currents then entering the building are smaller, lower rated and cheaper surge arresters may be used inside the building. This approach is often called cascading. Note that one of the main reasons why cascading surge arresters is a valid practice is not that the initial device at the Zone 0B/1 boundary is inferior but that in addition to any transmitted surges, the cabling lying inside Zone 1 & 2 etc. may be exposed to further induced surges, making it important to place further surge protective devices closer to the equipment to be protected. If these surge protectors can be smaller and cheaper yet provide completely adequate protection, it is economically beneficial. Cascading surge arresters makes this possible.

Conclusions

An attempt has been made to highlight some of the issues which pertain to the field of lightning surge protection at the present moment. The components commonly used in the manufacture of surge protective devices have been reviewed and critically analysed. It is clear that each has its benefits and short comings and that effective surge protection often requires the use of more than one component to achieve the required performance. The need for surge arresters which protect against huge impulse currents was critically analysed for realistic scenario and the concepts of zones, boundaries and the cascading of surge arresters were explained.

Information Supplied By:

Contact:	Company:
Grant Walliser, MSc (Eng)	Kuell Lighting Protection
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