Voltage sensitive relays, just as the name implies, use voltage as the variable that triggers the energizing or de-energizing of a circuit. An example of these relays and their functions can be founding maritime applications. A voltage sensitive relay is frequently seen in maritime battery charging systems.
Most ships use two batteries. One powers the engines during startup and the other is used to power the ship’s electrical equipment. A ship’s engine starting equipment needs a strong spike of electricity while the electronic equipment needs to be insulated from these spikes. The voltage sensitive relay detects the voltage being put out by each battery. It will allow both of them to be charged at once under certain conditions but, when the voltage changes, it can disengage and separate the batteries. This prevents the electronic equipment from being spiked by the starter battery and prevents the electronic battery from being drained by the starter.