There is provided an electrical device (10) comprising electrical elements communicating with an integral power supply (12), wherein a power connection means (22) associated with the electrical elements (13, 13′ and 14) is adapted to receive a temporary external source of electrical power such as a battery (24) to allow removal and replacement of the integral power supply without interrupting power to the electrical elements. The integral power supply (12) is adapted to receive an enduring external power source, with a second switch (32) provided to disable connection of the integral power supply (12) to the enduring power source while the battery (24) is connected.