Surge suppressors are also commonly referred to as surge protection devices (SPD) or transient voltage surge suppressors (TVSS). They are devices used to prevent high voltage transients (spikes) on mains power supplies damaging electronic circuits. It is common for power lines to have spikes of several hundred volts caused by customers with highly reactive load switching or fault conditions. Lightening (even when not a direct hit) can also cause extremely high voltage glitches that need to be arrested. Spikes are typically very short in duration (<1mS), and as a result lower power, enabling a safe dissipation by rapidly detecting and shunting the unwanted energy.

Energy shunting is achieved by a variety of non-linear devices that tend to be high impedance until a clamping (or ‘let-through’) voltage is exceeded. When this condition happens, the SPD will rapidly switch to a low impedance state and divert the energy from the protected line. These characteristics are regulated by mandatory safety standards. The amount of energy a surge protector can absorb in a single event without failure is referred to as its joule rating. The joule rating does not necessarily reflect the life expectancy of an SPD. Devices with lower shunt impedance may encourage dissipation in other source impedances in the current path. Good design practice is to encourage this to occur rather than dissipating the surge in the device itself.

There are several different types of non-linear devices used for surge suppression. A metal oxide varistor (MOV) is a type of voltage dependent resistor (VDR). MOVs act as low impedance when presented with a voltage larger than the clamping voltage specified for the device. These are combined with fuses to protect from dangerous failure modes due to excessively long shunt currents. In this event, the fuse should be chosen to ensure it would blow.

Transient voltage suppressor (TVS) diodes, also commonly referred to as Transil^TM crowbar protection diodes, are a type of zener diode. These diodes have the fastest responding reaction times, typically measured in picoseconds. They are not the most robust option for large amounts of energy dissipation. They are typically utilized in low voltage circuits to protect against transient conditions caused by electrostatic discharge (ESD) or hot insertion. Important characteristics of clamping diodes include the breakdown voltage (the voltage at which a current avalanche occurs) and clamping voltage (the voltage that the diode prevents from being exceeded).

Transient protection diodes are required in many interfaces to protect against ESD and enable hot plugging. An example of this is a USB interface. Such devices are required to not mismatch the data lines as this would impact the data rates and distances that can be met.