Surge Protector - Wikipedia

In an AC circuit, a voltage spike is a transient event, typically lasting 1 to 30 microseconds, that may reach over 1,000 volts. Lightning that hits a power line can cause a spike of thousands of volts. A motor, when switched off, can generate a spike of hundreds of volts.[3] Spikes can degrade wiring insulation and destroy electronic devices like light bulbs, battery chargers, modems, televisions (TVs), and other consumer electronics.

Spikes can also occur on telephone and data lines when AC main lines accidentally connect to them, or lightning hits them, or if the telephone and data lines travel near lines with a spike and the voltage is induced.

A long-term overvoltage surge, lasting seconds, minutes, or hours, caused by power transformer failures such as a lost neutral or other power company error, is not protected by transient protectors. Long-term surges can destroy the protectors in an entire building or area. Even tens of milliseconds can be longer than a protector can handle. Long-term surges may or may not be handled by fuses and overvoltage relays.

Surge currents

A building's wiring adds electrical impedance that limits the surge current that reaches the loads when a voltage transient arrives at the service entrance (the point where the supply company's wiring enters a property). There is less surge current at longer wire distances and where more impedance is present between the service entrance and the load.[4]

Category A loads are more than 60 feet of wire length from the service entrance to the load. Category A loads can be exposed to 6 kV and 0.5 kA surge currents. Category B loads are between 30 and 60 feet of wire length from the service entrance to the load. Category B loads can be exposed to 6 kV and 3 kA. Category C loads are less than 30 feet from the service entrance to the load. Category C loads can be exposed to 20 kV and 10 kA.[5]

A coiled extension cord can be used to increase the wire length to more than 60 feet and increase the impedance between the service entrance and the load.[6]