Surge protector

A surge protector 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 thus. However, sometimes any power strip is (mis)called a surge protector.

Important specifications

Here are 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 because the device will divert more energy elsewhere and will absorb less energy resulting in a lower voltage spike. Generally, 200 joules is undersized protection since harmful voltage spikes are significantly larger than 200 joules. Better protectors start at 1000 joules and 50,000 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.

*Surge current in kiloamperes (see Joules above).

*Standards — The surge protector may meet IEC 61643-1, BS6651, Telcordia TR-NWT-001011, ANSI / IEEE C62.xx, or UL1449. Each standard defines different protector characteristics, test vectors, or operational purpose. For example, a protector may obtain UL1449 approval even though it fails during testing. That standard tests only for fire hazards and other safety threats. Irrelevant to that approval test is whether the protector actually provides protection throughout testing. [http://downloads.eatonelectrical.ca/downloads/Transient%20Voltage%20Surge%20Supp/Tech%20Data/TVSS%20UL%20spec%201449.pdf "Tech Note #13: New UL1449 Safety Standard for Transient Voltage Surge Suppressors" states on page 2, "The TVSS is connected to AC power and subjected to two surge impulses ... It is acceptable for the TVSS to fail the first, second, or while connected to power, but only in a manor which will not cause a fire or expose the end user to live parts." ] ] BS6651 and ANSI / IEEE C62.xx define what spikes a protector might be expected to divert. IEC only writes standards and does not certify any product to meet those standards. 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 usually includes 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. [http://www.extremetech.com/article2/0,1697,1155237,00.asp Rosch, Winn. "Surge Suppressors: Anatomy Lesson ". "ExtremeTech" ] ] 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. [http://www.littelfuse.com/data/en/Technical_Articles/Littelfuse_SizingMOVs_EC921.pdf Walaszczyk, et al 2001 "Does Size Really Matter? An Exploration of ... Paralleling Multiple Lower Energy Movs". See Figures 4 & 5 for Pulse Life Curves. ] ] MOVs may be connected in parallel to increase current capability and life expectancy. "Degrading" is the normal failure mode. MOVs that fail shorted were so small as to violate the MOV’s "Absolute Maximum Ratings". [http://www.littelfuse.com/data/en/Application_Notes/an9311.pdf Application Note 9311 "The ABCs of MOVs". See "Q. How does an MOV fail?" on page 10-48. ] ] 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. [http://www.littelfuse.com/data/en/Application_Notes/an9773.pdf Application Note 9773 "Varistor Testing" Jan 1998. See "Varistor Rating Assurance Tests" on page 10-145 for definition of "end-of-lifetime". ] ] 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 [http://www.semtech.com/pc/downloadDocument.do?id=532 SemTech "TVS Diode Application Note" Rev 9/2000. See chart entitled "TVS Capacitance vs Transmission Rate". ] ] 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.

* Crowbar circuits — A crowbar (circuit) using a zener diode driving the gate of a silicon-controlled rectifier (SCR) latches any high voltages until power is turned off. The zener diode sets the trip voltage, and when exceeded, the SCR will latch on (conduct current).
*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). [ [http://www.legerity.com/getfile.php?bpd_14_AN_Overvoltage-Protection_A1_ID080942.pdf Overvoltage Protection of Solid-State Subscriber Loop Circuits] ] Carbon block suppressors are similar to a gas arrestor but with the two electrodes exposed to the air.

Other important components

*Inductor — An inductor resists sudden changes in current flow. Surge protectors using inductors are sometimes called "series mode" surge protectors, because the inductor is connected in series (acts as a low pass filter) with the load, rather than diverting current to neutral or ground as with MOVs and diodes. Inductors are usually used in hybrid circuits, in conjunction with the shorting devices above.

*Capacitor — A capacitor when connected in parallel resists sudden changes in voltage by absorbing current, and acts as a low pass filter, reducing spikes and noise.

* Resistor — A resistor can even be useful in limiting energy, providing that the voltage and energy absorption are within the limits of the component.
*Overcurrent protection — A fuse burns out or circuit breaker or polyfuse trips when excessive current flows to a device within the designed time lag and current rating. These can protect from current surges due to the operation a protector circuit (see Crowbar Circuits), or due to the failure of some other component. Fire protection is the most common purpose of these devices.

* An iron-poor transformer can transmit AC power similar to a normal iron core transformer (although less efficiently), but will be unable to transmit sudden surges that saturate the small iron core.

*Protector devices having several outlets may have all or some outlets protected. The better ones have higher ratings and offer more modes of protection against surges between the phase and neutral and grounding conductors; maybe for telephone and coax cable connections. The typical power strip protector may also provide equivalent protection to other electrical devices connected to the shared duplex wall receptacle. Protector components connect in parallel meaning devices on both sides of the protector may be provided equivalent protection.

*A UPS of the flywheel type seldom pass surges. Some models that use electrochemical batteries as the energy storage device may absorb spikes much like a capacitor acts as a low pass filter. Almost all battery-backed models also have protectors described above. UPSs of the "On line" topology provide the best protection.

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