Transient voltage suppressors exist across a range of voltage levels. Low voltage suppressors are designed to protect residential electronics in the 120/240 volt range, and high voltage suppressors in the 2.4 kilovolt to 15 kilovolt range, where power is drawn directly from the electrical grid for industrial use and channeled through step-down transformers. Low level transient voltage suppressors are often built upon suppression diodes, such as the zener diode, which, unlike typical diodes, permit current flow in both directions. Mid-level transient voltage suppressors are based upon metal-oxide varistors (MOV), but they are degraded over time by smaller electrical spikes on the line. Gas tube suppressors are a third category of transient voltage suppressors, and the only variety that can handle repeated high voltage spikes, such as from lightning, without degrading.
Since most electrical spikes occur in very fast, short durations of less than one nanosecond or one billionth of a second, transient voltage suppressors must be capable of nearly instantaneous reaction. They, therefore, are designed to let through a certain amount of voltage as they engage, which is officially rated as the Let-Through Voltage, and this is the most important rating for any voltage suppression system. Secondly, since electrical spikes are short in individual duration, they have no heating effect on wiring. Therefore, transient voltage suppressors do not need to be sized to meet the surge ampere ratings of the circuit they are plugged into, giving them more universal application across a range of electrical devices.
Industrial level transient voltage suppressors are built upon the assumption that the high voltage step-down transformer that is feeding power to a facility from the electrical grid does not offer adequate protection from spikes on the line. High power switching events or lightning strikes cannot be controlled by a utility. These transient voltages suppressors also account for inductive kick, which are voltage spikes generated inside a plant when breakers are opened or closed. Internal inductive kicks alone can be 10-20 times the rated voltage for electrical systems in a industrial facility, with the possibility of a 4.15 kilovolt circuit experiencing voltage spikes as high as 83.2 kilovolts. These systems are often built using MOV components designed to shunt high voltage spikes to a low impedance path.
Residential and standard commercial transient voltage suppressors work at restricting interior spikes first, as most transient voltages are generated from within a building instead of from outside electrical feeds. In homes, the common source for electrical spikes is usually the cycling of large appliances, such as refrigerators and freezers, washers and dryers, furnace motors, and air conditioning units. Most of these transient voltage suppressors are designed with a Let-Through Voltage of no more than twice the normal system peak for voltages on the circuit. For a 120-volt circuit, this is considered to be 339 volts, and for a 240-volt circuit, 679 volts. All transient voltage suppressors are also rated in terms of Peak Pulse Power dissipation as well, which is the amount of excess wattage allowed to flow, and, at lower levels, is restricted to around 150 watts.
Each design for transient voltage suppressors has its unique advantages and disadvantages. Zener diodes shunt voltage spikes away from low voltage circuits by being capable of transmitting electricity in two directions instead of one as typical diodes do, and they are economical and easy to incorporate into suppressors. Metal-oxide varistors are designed to shunt higher level voltage spikes away from a circuit, yet cannot shunt sustained lower voltages that exceed the peak levels of the circuit. Gas discharge tubes are expensive by comparison, but can handle repeated high voltage spikes on a line without degrading significantly as MOVs quickly do.
