Frequency sensitivity is another way that relay switches can be used to energize or de-energize a circuit. These devices are sometimes variable, sometimes not. They can be set up so that they will energize a circuit when it goes below 50Hz or above 70Hz, for example. Any frequency can be selected on some models. This provides another way that these devices can help to protect electronic equipment. Equipment that is sensitive to frequency changes will simply be shut down if the circuit goes into an over- or under-frequency state.