Surge protection of electronic equipment

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Introduction

In the previous sections, we learnt about the necessity of grounding electrical equipment and non-current-carrying parts of electrical equipment for human safety under insulation failure situations. We also covered the basic facts about lightning, the need for lightning conductors and their grounding to protect a building and its inhabitants from lightning strokes and the accompanying surges during the discharge of lightning into ground. In this section, we will discuss the subject of surge protection in detail.

What is a surge?

A surge is a temporary but steep rise of voltage in a power system, usually as a result of lightning activity (but also sometimes due to internal causes). We had seen an example of such causes in an earlier section where the opening of current through an inductor was seen to give rise to a switching surge. The rise may last for a fraction of the power cycle wave. It may consist of a single spike or multiple diminishing spikes as we saw in Section 4 on lightning protection. A surge, unless properly protected against, causes failure of insulation in electrical wiring or devices due to excessive voltage. The energy contained in a surge may destroy the parts of a power system through which it passes (a result of the high magnitude of the current wave). Circuits with electronic components are especially highly vulnerable since the devices used in these circuits don’t have much ability to withstand high voltages or currents.

Ways of protecting against surges are:

• Lightning protection systems as per relevant codes

• Quality grounding/earthing grid/bonding

• Surge arrestors on power circuits

• Multi-level protection on signal paths based on protection zone concept

• Continuous maintenance of systems.

Bonding of different ground systems as a means of surge proofing

In Section 3, we covered the basic principles of grounding the enclosures of electrical equipment to ensure safety of personnel in the event of ground faults. In Section 4, we covered in detail the physics related to lightning phenomenon and how the surges due to lightning strokes are safely conducted to ground using a lightning protection system consisting of air terminations, down conductors and grounding electrodes. Both these grounding systems are inherently noise prone, since conduction of surges and fault currents into ground is accompanied by a rise of voltage of the conducting parts connected to these systems with reference to the local earth mass. When sensitive electronic equipment first started appearing in the work place, it was usual for manufacturers of these equipment to demand (and get) a separate isolated ground reference electrode since it was claimed that connecting these systems with the building ground would affect their operation due to the ground noise. Thus, the concept of 'clean' ground was born as opposed to the other 'dirty' ground.

While this did give a solution of sorts to the problem of noise, it violated the fundamental requirement of personnel safety.

In FIG. 1, we see three different types of ground each isolated from the others: the power system ground, the lightning protection ground and the 'clean' electronic ground.

While this is perfectly trouble-free most of the time (when no lightning discharge or power system faults occur), the situation becomes positively dangerous when there is a surge due to lightning or faults. As we saw earlier, when lightning strikes a building, it produces a momentary high voltage in the grounding conductors due to the inherently fast rise time of the discharge and the inductance of the grounding leads (which is solely a function of their length). Similarly, when there is an insulation failure, the flow of substantial earth fault current causes a perceptible rise of voltage in the metal parts exposed to these faults and the associated grounding conductors (limited to safe touch potential values, but a rise all the same).

FIG. 1 Isolated grounding systems

So while the clean ground which does not develop these high potentials remains at true ground potential, other metal parts or building structures or flooring in its vicinity can all assume a high potential, albeit briefly, during surges and faults. It means that a high potential can and does develop between the electronic ground and the equipment connected to it and the building ground or the lightning protection ground, which gives rise to inherently unsafe situations both for personnel and for the equipment connected to the 'clean ground'. Another problem with an isolated ground is that the ground resistance of a system, which uses one or two electrodes, is much higher than the common ground. The touch potential of the electronic equipment enclosures in the event of an earth fault within the equipment may therefore exceed safe limits. The answer to these problems therefore lies in connecting all these different grounding systems together (refer FIG. 2).

FIG. 2 Grounding systems brought to a common electrode

FIG. 3 An integrated grounding system

FIG. 2 shows all three grounding systems tied at a single point to the ground.

Theoretically, this arrangement will prevent differential potentials between different grounds. But in practice, such a common ground electrode will have a high value of impedance, which cannot properly disperse lightning surges and will cause an undue potential rise in the grounding system with respect to the earth mass. The arrangement is therefore not of much practical value.

FIG. 3 shows a system with multiple grounding points with different types of electrodes bonded together to form a low-impedance ground path which ties together all forms of grounding within the building. It prevents the grounding system from attaining dangerous potential rise with reference to the general earth mass and also avoids differential voltages between the building's exposed metallic surfaces and equipment enclosures.

It’s this type of system that is installed in any modern facility to ensure that no unsafe conditions develop during lightning strokes or ground faults. The grounding system safely conducts away the surge currents led through by different surge protection devices into the ground path when surges occur in a system.

Bonding of the different earthing systems is thus the first step toward protection of sensitive equipment against surges. Practical examples of problems that can arise if this is not done are illustrated in Section 10.

Surges and surge protection

Extensive studies have been done in USA on the subject of surge protection requirements for housing sensitive electronic and automatic data processing (ADP) systems and the results were originally published in the form of a publication called Federal Information Processing Standards (FIPS). The FIPS publication 94 gave the guidelines on electrical power for ADP installations. Subsequently, these have been incorporated in the standard IEEE 1100 and FIPS publication 94 was withdrawn. The IEEE 1100 gives definitions of surge protective and related devices. A surge-protective device is defined as: A device that is intended to limit transient over-voltages and divert surge currents. It contains at least one non-linear component.

A surge suppressor is defined as: A device operated in conformance with the rate of change of current, voltage, power, etc., to prevent rise of such quantity above a pre-determined value.

A transient voltage surge suppressor (TVSS) is defined as: A device that functions as surge-protective device (SPD) or surge suppressor.

Any power system operates normally between certain voltage and frequency limits.

Usual limits are ±10% of voltage and ±3% for frequency. These limits are valid only under 'normal' conditions. Abnormalities in the power system such as loss of major generation capacity, outage of a transmission line or a power transformer failure can cause these limits to exceed. A brownout is one such condition when the voltage becomes low on a sustained basis (called as 'sag'). A voltage 'swell' is the opposite of this condition with a voltage rise for prolonged duration.

In the event of system faults such as a short circuit or a ground fault, the system voltage can become much lower but for brief duration within which time, the protection systems come into operation and safely isolate the faulty circuit or equipment. In lower voltage circuits, this is done by a fuse or miniature circuit breaker whereas larger power systems are provided with relays, which are extremely complex in nature. Once the isolation is complete, the system bounces back to normalcy very quickly. Between the appearance of a fault and its isolation, the voltage can dip as low as 10% of its normal value for a fraction of a second for close faults. For faults which are far away, the dips of 50% or so of normal values are usual and last up to a couple of seconds at the most.

These disturbances (sags or swells) can be safely handled by voltage-stabilization devices, constant voltage transformers or where required, by UPS systems with battery backup. Also, most electronic power supplies are designed to ride-through short disturbances.

Surges, however, pose a more serious hazard. One of the main causes of surges is lightning. While shielding prevents direct strikes on electrical lines, induced surges cannot be altogether avoided. A lightning surge when superimposed on an AC power wave is shown in FIG. 4. Note the sharpness of rise and the very small duration of the disturbance.

FIG. 4 Lightning surge superimposed on AC supply waveform

The other main reason for surges is the opening of inductive loads. Magnetic energy is stored in an energized coil, which tends to continue the current flow when the circuit is broken. This gives rise to a high voltage pulse, which creates an arc across the switch contacts when they are in the process of opening. As the switch gap keeps on increasing, the arc is quenched and once again gives rise to a high voltage pulse, which can cause a re-strike. This happens a few times before the current is finally interrupted completely when the switch gap becomes too high to permit a restrike. The resulting waveform is a series of diminishing spikes superimposed over the AC sine wave (refer FIG. 5). Such switching surges can happen not only in large power transformers of high voltage but also in a building distribution system employing choke coils, small power supplies with transformers or relay coils used in different devices.

All these are clubbed under the name of transients or transient surge voltage.

FIG. 6 and 7 show some common causes of transients.

FIG. 5 Surge superimposed on the AC supply waveform

FIG. 6 Atmospheric transients

FIG. 7 Earth current transients

5 Principle of surge protection

FIG. 8 explains the basic principle involved in surge protection. A surge involves a high magnitude voltage spike superimposed on a sine wave. The transient voltage surge suppressor (TVSS) or surge protection device (SPD) is a component, which is of high impedance at normal system voltages but is conductive at higher voltages (but still below the basic insulation level of the system). When a surge with a steep wavefront comes into the system, the portion of the wave above the breakdown voltage of the suppressor is conducted to the ground, away from the downstream equipment. The top portion of the wave is chopped of, resulting in the clipped waveform shown in FIG. 8.

FIG. 8 Clipping of transients

There are many devices with different voltage and power levels to suit the system which is being protected, ranging from the spark gap arrestors in power systems of high voltage to gas arrestors commonly used in communication systems. We will learn more about these devices later in this section.

Surge protection of electronic equipment

Generally, power circuits have components that have large thermal capacities, which make it impossible for them to attain very high temperatures quickly except during very large or long disturbances. This requires correspondingly large surge energies. Also, the materials that constitute the insulation of these components can operate at temperatures as high as 200 ºC at least for short periods.

Electronic circuits, on the other hand, use components that operate at very small voltage and power levels. Even small magnitude surge currents or transient voltages are enough to cause high temperatures and voltage breakdowns. This is so because of the very small electrical clearances that are involved in PCBs and ICs (often in microns) and the very poor temperature withstanding ability of many semiconducting materials, which form the core of these components.

As such, a higher degree of surge protection is called for if these devices have to operate safely in the normal electrical system environment. Thus comes the concept of surge protection zones (SPZs). According to this concept, an entire facility can be divided into zones, each with a higher level of protection and nested within one another. As we move up the SPZ scale, the surges become smaller in magnitude, and protection better.

• Zone 0: This is the uncontrolled zone of the external world with surge protection adequate for high-voltage power transmission and main distribution equipment.

• Zone 1: Controlled environment that adequately protects the electrical equipment found in a normal building distribution system.

• Zone 2: This zone has protection catering to electronic equipment of the more rugged variety (power electronic equipment or control devices of discrete type).

• Zone 3: This zone houses the most sensitive electronic equipment, and protection of highest possible order is provided (includes computer CPUs, distributed control systems, devices with ICs, etc.).

The SPZ principle is illustrated in FIG. 9.

FIG. 9 Zoned protection approach

We call this the zoned protection approach and we see these various zones with the appropriate reduction in the order of magnitude of the surge current, as we go down further and further into the zones, into the facility itself. Notice that in the uncontrolled environment outside of our building, we would consider the amplitude of say, 1000 A. As we move into the first level of controlled environment, called zone 1, we would get a reduction by a factor of 10 to possibly 100 A of surge capability. As we move into a more specific location, zone 2, perhaps a computer room or a room where various sensitive hardware exist, we find another reduction by a factor of 10. Finally, within the equipment itself, we may find another reduction by a factor of 10, the effect of this surge being basically one ampere at the device itself. The IEEE C62.41 indicates a similar but slightly differing approach to protection zones. The same is shown in Section C to this guide.

The idea of the zone protection approach is to utilize the inductive capacity of the facility, namely the wiring, to help attenuate the surge current magnitude, as we go further and further away from the service entrance to the facility.

The transition between zones 0 and 1 is further elaborated in FIG. Here we have a detailed picture of the entrance into the building where the telecommunications, data communications and the power supply wires all enter from the outside to the first protected zone. Notice that the SPD is basically stripping any transient phenomena on any of these metallic wires, referencing all of this to the common service entrance earth even as it’s attached to the metallic water piping system.

FIG. 10 The transition from zone 0 to zone 1

Similarly, the protection for zone 2 at the transition point from zone 1 is shown in FIG. 11.

Here as we address the discrete level between the first level of controlled zone 1 and perhaps the plug-in device taking it into the zone 2 location, we can see surge protection devices are available that handle the telecommunications, data and different types of physical plug connections for each, including both the RJ type of telephone plug as well as coaxial wiring.

FIG. 11 The transition from zone 1 to zone 2

This is a common design error where there are two points of entry and therefore two earthing points are established for the AC power and telecommunication circuits. The use of the TVSS devices at each point is highly beneficial in controlling the line-to-line and line-to-earth surge conditions at each point of entry, but the arrangement cannot perform this task between points of entry. This is of paramount importance since the victim equipment is connected between the two points. Hence, a common-mode surge current will be driven through the victim equipment between the two circuits despite the presence of the much-needed TVSS. The minimal result of the above is corruption of the data and maximally, there may be fire and shock hazard involved at the equipment.

No matter what kind of TVSS is used in the above arrangement nor how many and what kind of additional individual, dedicated earthing wires, etc. are used, the stated problem will remain much as discussed above. Wires all possess self-inductance and because of -e = L dI/dT conditions cannot equalize potential across themselves under normal impulse/surge conditions. Such wires may self-resonate in quarter-waves and odd-multiples thereof, and this is also harmful. This also applies to metal pipes, steel beams, etc. Earthing to these nearby items may be needed to avoid lightning side-flash, however.

Achieving graded surge protection

From the above, it will be clear that the type of surge protection depends on the type of zone and the equipment to be protected. We will further illustrate this by a few examples, as we proceed from the uncontrolled area of zone 0. Let us begin by talking about what happens when a lightning strike hits an overhead distribution line.

Here in FIG. 12, we see the picture of the thunderstorm cloud discharging onto the distribution line and the points of application of a lightning arrestor by the power company at points #1 and #2. We notice that the operating voltage here is 11 000 volts on the primary line and the transformer has a secondary voltage of 380/400 V typically serving the consumer. We need to understand what is known as traveling wave phenomena. When the lightning strike hits the power line, the power line's inherent construction makes it capable to withstand as much as 95 000 V for its insulation system.

FIG. 12 Protections in zone 0

We call this the basic impulse level (BIL). Most of the 11 000-V construction equipment would have a BIL rating of 95 kV. This says to us that the wire insulation, the cross-arms and all of the other parts, which are nearby to the current-carrying conductors, are able to withstand this high voltage.

Traveling waves and sparks over the lightning arrestor applied on a 11 000-V line might have a spark-over characteristic of approximately 22 000 V. This high level of spark-over protection is to enable the lightning arrestor to wait until the peak of the 11 000-V operating wave shape is exceeded before discharging the energy into the earth. The peak of the 11 000-V RMS wave would be somewhere in the neighborhood of 15 000 V. As the voltage comes to the 22 000-V level and then stays there as the lightning arrestor performs its discharge, that voltage waveform travels on the power line moving very fast to all points of the line. At places where there is discontinuity to the electric line, such as points #3 or #4 in our chart, the traveling wave will go in at 22 000 V and then will double and start back down the line at 44 000 V. This type of phenomenon is known as reflection of the traveling wave and it occurs at open parts of the circuit or even the primary of transformers. When the primary of our distribution transformer serving the building achieves 44 000 V, the secondary supplying the building is going to have an over-voltage condition on it. Thus, points #5 and #6 on our chart require us to think in terms of some type of lightning protective devices at the secondary of the transformer, the service entrance to the building and then further on into the building such as point #6 for the sensitive equipment to be fully protected in this facility.

Positioning and selection of lightning/surge arrestor

FIG. 13 poses some serious questions. Where do we put a lightning arrestor? Do we need one at all? The final question is in answer to #1 and #2 above. What is it you wish to blow up? The computer? The UPS? And which are the most appropriate devices for protecting your equipment? It may be a humorous type of description, but the practical, real world finds many people placing lightning arrestors and discharge devices inside of occupied spaces where the equipment and the personnel represent a heavy financial burden as well as a safety factor. Most of the safety standards will advise that discharge devices don’t need to be located where there are either personnel or equipment to be protected. The location of a discharge device, such as a lightning arrestor, is to be at the large service entrance earth, where the electric utility makes its service connection to the premises. Here, at this point, this discharge device, which has large levels of current, then has a sufficiently large earth plane into which to discharge that current without a damaging effect on sensitive equipment.

Typical lightning arrestor ratings call for 65 000 A of discharge capacity for distribution class arrestors, 100 000 A of discharge capacity for station class arrestors, and even at the 600-V and below level, 40 000 A of capability at the minimum. So we notice that these questions are not silly, but they do point out that we need to locate our lightning arrestor product as close to the service entrance as possible. The appropriate device for this location will be a metal oxide varistor (MOV) type of lightning arrestor (or the older spark-gap type of arrestors) with the required surge current and voltage rating. The voltage rating of the device is selected in such a way that the device does not break down even at the peak value of the highest system voltage that is encountered under normal conditions.

FIG. 13 Arrestor positioning

FIG. 14 shows the overall electric utility supply and internal wiring; we see the recommendation that is consistent with what we have been talking about before. The black boxes marked on our drawing as SPDs first appear connected at the service entrance equipment inside the building where it receives power from the service transformer. An appropriate device for this duty is the MOV type of surge arrestor. Next we see an SPD at a panel board or sub-panel assembly. Here one would preferably select a silicon avalanche device whose surge current rating may be lower but the speed for operation and low clamping voltages make it more suitable than an MOV. Finally, we may find a lower voltage style device as a discrete device either plugged in at an outlet or perhaps approaching the mounting of this device within a particular piece of sensitive equipment itself. The SPDs of silicon avalanche type are once again most appropriate in this location.

FIG. 14 Protection locations

FIG. 15 shows in further detail the location of SPDs in zones with sensitive equipment. Note the combined placement of lightning arrestor products and surge protection devices called transient surge protectors in this FIG. (reproduced from the now superceded FIPS document). Notice that the location of the arrestor product is as close to the power source as possible. In addition, also note the use of the older style arresting products, which required capacitors to affect wavefront modification. Let us explain that. Wavefront modification means that the voltage rise is so fast that if something does not mitigate that rise, the wiring may be bridged by the extremely high voltage in the surge.

Downstream from the arrestor location, over a certain amount of distance, preferably greater than 10-15 m (30-50 ft), if possible, should be the second level of protection shown in this drawing as a transient surge protector. This device indicated as combination suppressor and filter package made up of a variety of different types of components, which will now protect against the residual energy that is flowing in the circuit. The structure that we see here is one in which the various components installed in the system, starting at the service entrance, then to a sub-panel and then, finally, to discrete individual protectors, will now attenuate more and more of the surge energy until it’s completely dissipated.

FIG. 15 Supplementary protection

A practical view of surge protection for sensitive equipment

Any building usually has separate entry points for power and communication cables.

FIG. 16 illustrates this situation. The electrical service lateral and communication central office feeder (COF) are located at different places. Both have independent protection for surges and are separately grounded although both ground connections are inter-connected through the building's cold water piping. The sensitive electronic equipment (tagged as 'victim' equipment) has connections to the power line through a branch circuit feeder and the communication system.

FIG. 16 A typical site showing power and telecommunication earthing

There are two problems in this installation. The first is that the victim equipment itself has no separate surge protection and is served only by the zone 1 protection of the branch power circuit. The second problem is that surge currents flow through the building piping between the power and communication grounds, which can give rise to high-voltage differential within parts of the victim equipment.

The situation will improve somewhat by adding an SPD at the power outlet of the victim equipment (refer FIG. 17). The problem of surge flow through the building still remains to be solved. There are two possible approaches to resolving this problem depending on whether you are planning a new facility or dealing with an existing one.

While planning a new installation, it’s possible to integrate the entry points and earth connections of both power and communication services. The connection between power circuit ground and the cold-water piping is still maintained but does not cause any problem, as there is no differential voltage possible between power and communication grounding. FIG. 18 shows such an installation.

FIG. 17 Surge protection added for victim equipment

It’s however, not possible to implement this ideal solution in an existing facility. In this case, the problems can be mitigated by a different approach; that of creating a common ground plane between power and communication grounds. This is done by running a pair of metallic conduits between the power service lateral and the communication cable entry point. The communication cable is taken into the building through a new pull box with an SPD inside. The cable is then routed to the power service lateral through one of the conduits and back to the communication distribution box through the other conduit. At the power service lateral, a common ground plane is created for accommodating the communication cable loop. The victim equipment is provided with its own SPD to divert any surge currents reaching up to its power outlet.

Such a system is shown in FIG. 19.

What is best from all points of view to achieve excellent surge protection is the fully integrated facility with single ground reference plane to which all equipment enclosures and SPDs are connected and which in turn is grounded using several grounding electrodes. Such a system is shown in FIG. 20. As far as possible, every new facility with sensitive equipment should be planned along these lines.

Codes A number of codes, recommended practices, standards and guidelines have been developed by international and national standard making bodies on this subject and are listed in FIG. 21. These can be used to advantage by design engineers of electrical power and data systems as well as contractors who install them.

FIG. 18 Common point of entry for power and communication cables

FIG. 19 Retrofitting with a common ground plane for power and communication cabling

FIG. 20 Integrated facility with a single ground reference plane

Selection of suitable device for surge protection

Selection of surge-protective devices (SPD) should consider the following:

• Compatibility with the system being protected

• Voltage level maintained during operation of SPD

• Survival of the device.

We will discuss these aspects in detail.

Compatibility:

A surge-protective device should not interfere with the normal operation of the system.

Normally, most surge protective devices connected across supply leads do have a small leakage current, but the value of such leakage is usually very small in comparison with the rated operating current of the equipment. When using SPDs in data circuits, it should be ensured that the quality of the data signal is not affected by the SPD both under normal conditions and when conducting a surge.

Voltage level The SPD should not conduct at the normal voltage of the system (including voltage variations to which the system is normally subjected during operation). At the same time, the voltage under abnormal conditions should not be permitted to go beyond the level, which the protected system can safely withstand without any insulation breakdown.

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Organization Code, Article or Standard No.

Scope

ANSI/IEEE C62 Guides and standards on surge protection

C62.41 - 1980 Guide for surge voltages in low-voltage AC power circuits

C62.1 IEEE standard for surge arrestors for AC power circuits

C62.45 - 1987 Guide on surge testing for equipment-connected low-voltage AC power circuits

C62.41 - 1991 Recommended practice on surge voltages connected to low voltage AC power circuits (approved, not published)

IEEE C74.199.6 - 1974 Monitoring of computer installations for power disturbances. International Business Machines Corp. (IBM) UL UL 1449 Transient voltage surge suppressors (TVSS) NEC Article 250 Earthing

Article 280 Surge arrestors

Article 645 Electronic data processing equipment

Article 800 Communications circuits NFPA NFPA-75 - 1989 Protection of electronic data processing equipment

NFPA-78 - 1989 Lightning protection code

NFPA-20 - 1990 Centrifugal fire pumps MIL-STD MIL-STD-220A

MIL-STD 419A/B 50 ohm insertion loss test method. Earthing, bonding and shielding for electronic equipment and facilities FIPS FIPS PUB 94 Guideline for electrical power in ADP Installations ( Section 7)

=== FIG. 21 Codes, standards and guidelines

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Survival

A surge can contain a lot of energy, which the SPD should successfully divert away from the equipment being protected. The quantum of energy varies with the location. As seen earlier in this section, the location can fall under different zones classified according to the severity of probable surges, with zone 0 being the worst, diminishing progressively as we move up to zone 3. The power levels of surge in zone 1 are thus the highest and zone 3 the least. An SPD selected for each one of these locations must safely clamp the voltage of the protected circuits to specified values and absorb the energy contained in the surge without permanent damage to itself. It should be remembered that lightning surges can contain multiple wave components and the SPD should withstand all of them safely.

This will call for adequate energy absorption rating.

One of the important aspects of survival is that the SPD should stop conduction as soon as the surge incident is over. Some of the types of SPDs continue to conduct at relatively small voltages once the conduction mode is initiated and can therefore destroy themselves in the process unless used with other devices that can stop the conduction of current. Importance of shielding against direct lightning strokes on power equipment. It’s not always possible to design an SPD for the worst-case conditions such as a direct lightning stroke. A device that can withstand such a disturbance will be too large and prohibitively expensive to design and manufacture. The same principle will be applicable as we move up the zone of protection. The correct approach is therefore to mitigate the worst effects of surges by other means. For example, all exposed power line equipment should necessarily be shielded (refer Section 8). The SPD should therefore be required to protect against the residual surges only, which will make the design feasible and cost- effective.

Types of surge suppressors or protective devices

The IEEE 1100 contains the following description of SPDs. Various types of surge suppressors are available to limit circuit voltage. Devices vary by clamping, voltage and energy-handling ability. Typical devices are 'crow bar' types such as air gaps and gas discharge tubes, and non-linear resistive types such as thyrite valves, avalanche diodes and MOVs. Also available are active suppressors that are able to clamp or limit surges regardless of where on the power sine wave the surges occur. These devices don’t significantly affect energy consumption.

Thus, there are two basic types of devices:

• Devices that limit the voltage

• Devices that switch the voltage to lower values when they break down.

The former type acts by clamping the voltage to a safe value while conducting the rest of the surge voltage and energy to the ground through the grounding lead (refer also Section 4 on lightning protection). It’s necessary to keep the grounding lead itself as short as possible so that the voltage drop across the lead does not add to the voltage across the SPD. Metal oxide varistors (MOV as they are usually referred to) and zener (avalanche) diodes used in electronic circuits are examples of such devices. These work using the principle of non-linear resistors whose resistance falls sharply when the voltage exceeds a threshold value. But the resistance value is such that the voltage remains more or less constant even when large surge currents are conducted through the arrestor. The second type viz. voltage-switching type (or 'crow bar' type as IEEE:1100 calls it) are devices, which suddenly switch to a low-voltage state when the voltage exceeds a certain threshold value, thus lowering the voltage 'seen' by the protected circuit. Once this happens, the normal system voltage is enough to keep the device to remain in this state. The device can be turned off only when the voltage is switched off. Spark gaps and gas arrestors are examples of such devices.

The characteristic of these devices is shown in FIG. 22 and 7.23.