A means of removing the high-voltage noise that can occur when switching on mains-level voltages; for example, thermostats in domestic central heating and refrigerators, fluorescent light starters, etc., if unsuppressed, can generate bursts of high-voltage peaks of up to a kilovolt for a time duration of a few microseconds. Transient suppressors remove the high-voltage content, and often also slow down the rise time of the voltage peaks so that they are no longer a problem.

The following are examples of transient suppressors. Voltage-dependent resistors (VDRs) have a resistance that varies (reduces) with applied voltage, so the device provides a low-resistance path for the high-voltage peak thereby protecting the downstream equipment. Varistors are solid-state devices that effectively ‘clip’ the high-voltage peak to a predetermined maximum level. Varistors are faster acting than VDRs. They are often manufactured under commercial model names. Gas discharge devices are gas-filled glass envelopes containing a pair of electrodes, attached to the mains. When the high-voltage peaks are present, the gas becomes ionized and forms a low-resistance path for the high voltage. These are faster acting than VDRs.