There are two other shunt-based options: gas discharge tubes and silicon avalanche diodes. Gas discharge tubes use specially designed electrodes housed in a tube with one or more inert gases under pressure. The gas acts as the conductor between two lines. At normal voltages, it has low conductivity. But when the voltage is high enough, the gas is ionized and becomes highly conductive, passing the excess voltage to the ground wire. Gas discharge tubes have low breakdown ratings, and they can reduce voltages in a matter of nanoseconds. But though they're adept at handling very high peak voltages, lower voltages that still have enough power to cause damage can sometimes get through.
Silicon avalanche diodes (SADs) are also semiconductors that can respond very quickly to transient voltages, and they have a wide clamping voltage range (the amount of energy they let through before activating). As long as their rated capacity isn't exceeded, they won't degrade. But a suppressor usually needs multiple SADs to dissipate extra voltage without sacrificing the device, which makes this kind of suppressor more expensive.
Because each approach has its limitations, many of the better surge suppressors use a combination of these devices, most typically MOVs and SADs. And because of the sacrificial nature of MOVs, many suppressors have backup fuses in case the MOVs fail. Many also have a light that shows when the unit's not functioning properly.
Because of the limitations of MOV-based suppressors - and the argument that shunting excess voltage to ground can contaminate the system - companies such as ZeroSurge and Brick Wall prefer so-called "series-mode" suppressors. These suppressors use an inductor (also called a choke or a toroidal choke coil) that inhibits surge frequences while allowing power frequencies to pass undeterred. Essentially a copper wire wound around a core, an inductor acts as a low-pass filter that inhibits surge energy, some of which is stored in capacitors and then slowly released to the neutral wire.
Proponents of series-mode protectors claim several advantages. Because there's nothing to "turn on," reaction is instantaneous. They have very low dynamic clamping levels and, perhaps more important, even lower let-through voltage. Also, because excess energy is contained in the capacitors rather than diverted to elsewhere in the power system, that energy can't degrade the system or find its way into other cables and interconnects. Critics of the approach say that unlike shunt-mode suppressors, series-mode devices don't protect against common-mode surges (on the hot and neutral conductors simultaneously) or surges on other lines, such as coaxial cables from antennas or cable-TV hookups.
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