Relays are used to allow a low-power switch from a high-power electrical device. A relay can be used to switch electrical motors or condenser units in a refrigerator. Motors, condenser units and mechanical relays contain inductive electrical components that store electrical energy. Under some conditions, inductive electrical loads can cause a relay to fail.
An inductor is an electrical component that stores electrical energy by converting this energy into electromagnetic energy. An inductor requires time to charge up once voltage is applied across the inductor's leads. Similarly, when voltage is withdrawn from the inductor, the inductor will maintain a charge for a short period before the electromagnetic field collapses completely. A relay uses an inductor as an electromagnet, which closes the relay switching mechanism. Electric motors and condenser units contain several inductors that, when used with permanent magnets, convert opposing magnetic forces into mechanical energy.
Flyback Diode Failure
Many inductor-based circuits contain a fail-safe measure called a "flyback diode." A diode is placed in parallel with the inductor to allow the inductor to discharge at a slow rate if a sudden voltage change occurs. However, many commercially available diodes are designed to handle only small electrical currents. If the flyback diode has been subjected to a large current (likely caused by a power surge), the diode will fail and the circuit will lack power protection. If the circuit goes through several on/off cycles without a flyback diode in place, one of the inductors in a relay circuit may become damaged when the power comes back after a blackout.
Premature Failure Due to Overuse
Mechanical relays can be actuated (turned on or off) thousands of times before the relay fails. If a relay is required to switch on or off several times a day, the relay will last longer than if the relay is required to switch on or off several times an hour. Constant switching -- coupled with frequent power outages -- can cause the relay to fail prematurely.