Safety & Security – Surge Protector

Surge protector

A surge protector (or surge suppressor) is an appliance designed to protect electrical devices from voltage spikes. A surge protector attempts to regulate the voltage supplied to an electric device by either blocking or by shorting to ground voltages above a safe threshold. The following text discusses specifications and components relevant only to the type of protector that diverts (shorts) a voltage spike to ground. Many power strips have surge protection built-in; these are typically clearly labeled as such. However, sometimes power strips that do not provide surge protection are erroneously referred to as surge protectors.

Important specifications

Some specifications which define a surge protector for AC mains and some communication protection.

  • Clamping voltage – better known as the let-through voltage. This specifies what voltage will cause the metal oxide varistors (MOVs) inside a protector to conduct electricity to the ground line. A lower clamping voltage indicates better protection, but a shorter life expectancy. The lowest three levels of protection defined in the UL rating are 330 V, 400 V and 500 V. The standard let-through voltage for 120 V AC devices is 330 volts.
  • Joules – This number defines how much energy the surge protector can absorb without failure. A higher number indicates longer life expectancy because the device will divert more energy elsewhere and will absorb less energy. More joules conducting the same surge current means a reduced clamping voltage. Its often said that lower joule ratings is undersized protection since harmful spikes are significantly larger than this. Better protectors exceed 1000 joules and 40,000 amperes. If properly installed, for every joule absorbed by a protector, another 4 or 30 joules may be dissipated harmlessly into ground.

The joule is a common misleading parameter for gauging surge protectors. Any ampere and voltage combination can occur in time, but surges commonly occur for microseconds to nanoseconds, and experimentally modeled surge energy has been far under 100 Joules. Well designed surge protectors should not rely on MOVs to absorb surge energy but more to survive the process of redirecting it. A MOV should blow gracefully, like a fuse, while diverting most of the surge energy to ground thus sacrificing itself, if needed, to protect equipment plugged into the surge protector. As energy in a MOV is stored as potential energy and if released as kinetic energy, a lower joule rating reduces fire and explosion hazards.

Manufacturers commonly design higher joule rated surge protectors by cascading MOVs in parallel. Since MOV have non-linear responses, when exposed to the same over voltage individual MOV can be more sensitive than others, causing one MOV in a group to conduct more, leading to overuse and eventually premature failure. If an inline fuse is placed as a power-off safety feature, it will trip, and fail the surge protector even if MOVs are intact.

  • Response time – Surge protectors don’t kick in immediately; a slight delay exists. The longer the response time the longer the connected equipment will be exposed to the surge. However, surges don’t happen immediately either. Surges usually take around a few microseconds to reach their peak voltage and a surge protector with a nanosecond response time would kick in fast enough to suppress the most damaging portion of the spike.

Response time is not a useful measure of a surge protector’s ability in MOV devices. MOV have response times measured in nanoseconds. Test waveforms used to design and calibrate surge protectors, are all based on modeled waveforms of surges measured in microseconds.

  • Standards – The surge protector may meet IEC 61643-1, EN 61643-11 and 21 , Telcordia Technologies Technical Reference TR-NWT-001011, ANSI / IEEE C62.xx, or UL1449. Each standard defines different protector characteristics, test vectors, or operational purpose. For example, to pass UL1449 a protector has to remain functional after a series of 22 test surges. ( NOTE: the latest revision of UL1449 is 3rd Edition, the previous sentence is no longer valid. Please see UL1449 3rd Edition requirements ) The protector can safely fail later tests, including sustained overvoltage. EN 62305 and ANSI / IEEE C62.xx define what spikes a protector might be expected to divert. EN 61643-11 and 21 specify both the products performance and safety requirements. IEC only writes standards and does not certify any product to meet those standards. IEC Standards are used by members of the CB Scheme to test and certify products for compliance. None of those standards say a protector will provide proper protection. Each standard defines what a protector should do or might accomplish.

Primary components

The principal components used to reduce or limit high voltages can include one or more of the following electronic components:

  • Metal oxide varistor – The metal oxide varistor (MOV) contains a material, typically granular zinc oxide, that conducts current (shorts) when presented with a voltage above its rated voltage. MOVs typically limit voltages to about 3 to 4 times the normal circuit voltage by diverting surge current elsewhere. MOVs have finite life expectancy and “degrade” when exposed to a few large transients, or many more smaller transients. MOVs may be connected in parallel to increase current capability and life expectancy, providing they are matched sets (MOVs have a tolerance of approximately 20% on voltage ratings). “Degrading” is the normal failure mode. MOVs that fail shorted were so small as to violate the MOV’s “Absolute Maximum Ratings”. MOVs usually are thermal fused or otherwise protected to avoid short circuits and other fire hazards. A circuit breaker is different from the internal thermal fuse. If a surge current was so excessively large as exceed the MOV parameters and blow the thermal fuse, then a light found on some protectors would indicate that unacceptable failure. Adequately sized MOV protectors will eventually degrade beyond acceptable limits without a failure light indication. MOVs are the most common protector component in AC power protectors.
  • Transient suppression diode – A type of zener diode called an avalanche diode or suppression diode will limit voltage spikes. These components provide the best limiting action of protective components, but have a lower current capability. Voltages can be limited to less than 2 times the normal operation voltage. If current impulses remain within the device ratings, life expectancy is exceptionally long. If component ratings are exceeded, the diode may fail as a short circuit. Protection may remain but normal circuit operation is terminated. Due to their relatively limited current capacity, transient suppression diodes are often restricted to circuits with smaller current spikes. Transient diodes are also used where spikes occur significantly more often than once a year. This component will not degrade with use. A unique type of transient diode (transzorb or transil) contains reversed paired avalanche diodes for bi-polar operation. Another type is paired in series with a diode to provide low capacitance as required in communication circuits.
  • Gas discharge tube (GDT) – These rely on a gas trapped between two electrodes that is ionized by the high voltage to conduct electrical current. GDTs can conduct more current for their size than other components. Like MOVs, GDTs have a finite life expectancy, and can take a few very large transients or a greater number of smaller transients. GDTs also take time to trigger permitting a higher voltage spike to exist before the GDT conducts significant current. It is not uncommon for a GDT to let through pulses of 500V or more of 100ns in duration. In some cases additional protection is necessary to prevent damage due to this effect. GDT create a short circuit when triggered, so that if any electric power (spike, signal, or power) is present, the GDT will short this, and will continue conducting until all electric current sufficiently diminishes. Unlike other protector devices, a GDT will conduct at a voltage less than the high voltage that ionized the gas. Gas arrestors are often used in telecommunication equipment. Due to an exceptionally low capacitance, GDTs are commonly used on high frequency lines.
  • A selenium voltage suppressor is a “clamping” semiconductor similar to a MOV, but it does not clamp as well. However, it usually it has a longer life than a MOV. It is used mostly in high-energy DC circuits, like the exciter field of an alternator. It can dissipate power continuously, and it retains its clamping characteristics throughout the surge event, if properly sized.
  • A quarter-wave coaxial surge arrestor is used in RF signal transmission routes. It features a tuned quarter-wavelength short-circuit stub that makes it pass a bandwidth of frequencies, but presents a short to any other signals, especially down towards DC. The bandwidths can be narrow (about ±5% to ±10% bandwidth) or wideband (above ±25% to ±50% bandwidth). Quarter-wave coax surge arrestors have coaxial terminals, compatible with common coax cable connectors (especially N or 7-16 types). They provide the most rugged available protection for RF signals above 400 MHz; much better than gas discharge cells typically used in the universal/broadband coax surge arrestors. Quarter-wave are useful for Telecom, Wi-Fi at 2.4 or 5 GHz but less useful for TV/CaTV. Since a quarter-wave shorts out the line, it is not compatible with systems sending power for a LNB up the coax downlink.
  • Carbon block spark gap overvoltage suppressor – an older technology still found in telephone circuits. A carbon rod is held with an insulator a specific distance from a second carbon rod. The gap dimension determines the voltage at which a spark will jump between the two parts and short to ground. The typical spacing for telephone applications in North America is 0.003 inch (0.076 mm). Carbon block suppressors are similar to a gas arrestor but with the two electrodes exposed to the air.
  • Series Mode (SM) surge suppressors are not rated by joules because they operate differently than the above suppressors, offering no materials that wear after repeated surges. Their initial costs are higher, typically US$130 and up. The main differences in this type of suppressor and the others is that this type absorbs the surge, whereas the others discharge it. Since SM devices absorb energy rather than redirect it, devices are bulkier, heavier and maintain a theoretical fire risk should the absorbed energy exceed design limits. SM designs do not include the ground path as part of a strategy for surge mitigation, a troubling philosophy given at least NASA and FAA mandate grounding as a critical source of protection against lightning and surges. Experimental results show most surge energies occur at under 100 J, so exceeding SM design parameters are unlikely, but it provides no contingency should rare events induce energies that exceed it. Surge energy is also limited via arc-over to ground during lightning strikes, allowing a surge remnant that often does not exceed a theoretical maximum, such as 6000 V at 3000 A with a modeled shape of 8 x 20 microsecond waveform specified by IEEE/ANSI C62.41. SM devices are essentially filters, allowing 60 Hz line voltages through, suppressing others and do their task between neutral and hot wires. While SM suppression focuses its protective philosophy on a power supply input, it offers nothing to protect against surges appearing between the input of an SM device and data lines, such as antennae, telephone or Internet connections or multiple such devices cascaded and linked to the primary device; in their philosophy, such events are already protected by the SM device before the power supply. The limitation of such filter approaches has been examined.