VOLTAGE SPIKES
In an AC circuit a voltage spike is a transient event, typically lasting 1 to 30 microseconds, that may reach over 1,000 volts. Lightning that hits a power line can give many thousands, sometimes 100,000 or more volts. A motor when switched off can generate a spike of 1,000 or more volts. Spikes can degrade wiring insulation and destroy electronic devices like light bulbs, battery chargers, modems and TVs etc.
Spikes can also occur on telephone and data lines when AC main lines accidentally connect to them or lightning hits them or the telephone and data lines travel near lines with a spike and the voltage is induced.
A long term surge, lasting seconds, minutes, or hours, caused by power transformer failures such as a lost neutral or other power company error, are not protected by transient protectors. Long term surges can destroy the protectors in an entire building or area. Even tens of milliseconds can be longer than a protector can handle. Long term surges may or may not be handled by fuses and overvoltage relays.
Protectors
A transient surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting current to reduce the voltage below a safe threshold. Blocking is done by using inductors which inhibit a sudden change in current. Shorting is done by spark gaps, discharge tubes, zener-type semiconductors, and metal-oxide varistors (MOVs), all of which begin to conduct current once a certain voltage threshold is reached, or by capacitors which inhibit a sudden change in voltage. Some surge protectors use multiple elements.
The most common and effective way is the shorting method in which the electrical lines are temporarily shorted together (as by a spark gap) or clamped to a target voltage (as by a MOV) resulting in a large current flow. The voltage is reduced as the shorting current flows through the resistance in the power lines. The spike’s energy is dissipated in the power lines (and/or the ground), or in the body of the MOV, converted to heat. Since a spike lasts only 10s of microseconds, the temperature rise is minimal. However, if the spike is large enough or long enough, like a nearby hit by lightning, there might not be enough power line or ground resistance and the MOV (or other protection element) can be destroyed and power lines melted.
What is a surge?
A surge is a temporary, steep rise of voltage in an electrical system ranging from a few microseconds to a few milliseconds and less than a quarter cycle of power frequency wave, usually as a result of lightning activity but also sometimes due to internal causes such as the opening of current through an inductor (called a switching surge). It may consist of a single spike or multiple diminishing spikes as we saw in the chapter on lightning physics.
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 ‘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 a 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 back up. 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.
Surge protectors for homes can be in power strips used inside, or a device outside at the power panel. Sockets in a modern house uses three wires: line, neutral and ground. Many protectors will connect to all three in pairs (line–neutral, line–ground and neutral–ground), because there are conditions, such as lightning, where both line and neutral have high voltage spikes that need to be shorted to ground.
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 and the resulting high potential differences). Circuits with electronic components are especially vulnerable since the devices used in these circuits do not have much ability to withstand high voltages or currents.
A surge can find its way into an electrical circuit in various ways. Power circuits, which are directly exposed to lightning strikes, propagate surges galvanically. Surges can also be induced in adjacent conductors by electromagnetic coupling. Surges can also pass through circuits by capacitive coupling, since capacitances (such as those between the windings of transformers) freely conduct surges having steep wave fronts. Galvanic linking is possible indirectly due to the phenomenon of ground potential rise when a lightning surge current passes through soil.
Some ways of minimizing the effect of surges and protecting vulnerable devices against surges are as follows:
• Lightning protection systems as per relevant codes
• Quality grounding/Grounding grid/Bonding
• Surge arrestors on Power circuits
• Multi-Level protection on signal paths based on Protection Zone concept
A surge protective device is defined as follows:
• A device that is intended to limit transient over voltages and divert surge currents. It contains at least one nonlinear component.
A surge suppressor is defined as:
• A device operated in conformance with the rate of change of current, voltage, power, etc., to prevent the 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.
Figure 7.5 explains the basic principle involved in surge protection. A surge in an AC power line 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 acts as high impedance at normal system voltages but is conductive at higher voltages. This causes the voltage of the system to be limited to the breakdown voltage level of the SPD which is higher than the normal system voltage, but is lower than the basic impulse voltage withstand level (BIL) of the insulating materials.
SURGE PROTECTION DEVICES:
Surge protection has the objective of preventing damage to expensive equipment and components, which may be subjected to surges far beyond their voltage and current ratings.
The common protective devices that perform this function are:
• Fuses
• Surge Relays
• Circuit Breakers
• Spark gaps
Special devices that divert the surges away from sensitive equipment of these, fuses and circuit breakers are normally not considered fast enough to prevent the surges from passing through, especially very steep-fronted impulses such as a lightning discharge. Surge relays, although faster than circuit breakers and fuses are still not considered fast enough to stop lightning-induced surges. Also, once these devices operate, the loads get interrupted till the fuse is replaced or the circuit-breaker is closed again.
Spark gaps are quite fast in their action and can definitely prevent the surges from reaching sensitive equipment by flashing over and establishing a path away from the protected circuits. Both air-gap and carbon spark-gap arrestors have been used in earlier times for surge protection of power circuits. However, the voltage of operation of these devices is seldom below 600 V. In addition, the voltage at which the device operates is uncertain and may depend on environmental factors such as humidity. Their insulation resistance may fall after a few operations and the device itself may have to be replaced. Also, arrestors of this type have to be used in conjunction with other components such as a non-linear resistor (usually silicon-carbide) so that they do not continue to conduct after the surge has been discharged, since the ionized air between the gaps may continue to breakdown at power frequency voltages. Such arrestors were however popular in medium and high voltage substations till they were replaced by metal-oxide devices (discussed later in this chapter).
Protection of sensitive components in communication and instrumentation circuits thus calls for devices which are faster, can breakdown at much lower voltages and exhibit good, repeatable voltage clamping characteristics. Such devices are called Transient Voltage Surge Suppressors (TVSS) or simply as Surge protection Devices (SPD).
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 metal oxide varistors. 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 do not 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 to the chapter on lightning protection). It is necessary to keep the grounding lead 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 devices 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.
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
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, represented by the BIL value. Once the voltage of the system returns to normal value after the surge passes, the device should stop conducting. In other words, there should be no follow through current at normal system voltage/frequency.
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 in earlier in this chapter, the location can fall under different zones classified by the IEC 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, an example being the Gas Discharge tube type of SPD.
