Surge protector

Single-outlet surge protector, with visible connection and protection lights

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. File:Surge Arrestor 716 Connectors.jpg

Important specifications

Many surge and noise protectors have multiple outlets

UK style surge protector

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 greater protection and 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. Generally, 200 joules is undersized protection since harmful spikes are significantly larger than 200 joules. 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.

• 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.

• 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. 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.

Typical low power lightning protection circuit. Click for more details.

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 400MHz; 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.

A telephone network connection point with spark-gap overvoltage suppressors. The two brass hex-head objects on the left are the suppressors. They act to short overvoltage on the tip or ring lines to ground.

• 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 completely differently than the above suppressors, and are vastly superior, offering no materials that wear out over time, even after thousands of surges and spikes. Their starting prices are thus higher, starting at around 130US 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, and the fact that MOV types wear out over time, or even with one large surge or spike.

External links

• Template:ODP
• Transient Protection Notes from the Clemson EMC web site
• TR-NWT-001011, Generic Requirements for Surge Protective Devices (SPDs) on AC Power Circuits