DD Usually, with exception of some earlier types of aircraft,the master switch grounds one side of a relay (normally adjacent to the battery), thus energising it and applying power to the aircraft. The starter relay works in a similar way and cannot be activated until the master switch is turned on. On your typical C172/PA28 there is also a third relay which operates when ground power is applied, this is designed to isolate the battery on start up. You will also find a diode across the coil of the relay to ground any high voltage caused by the collapse of the coil when master turned off.