Abstract: An interruptible power supply can supplant a primary power source with a secondary power source such as a battery when the primary power source fails. The supply has a switching device, an inverter and a pair of output terminals. The inverter is coupled to the output terminals. The inverter can couple electrical energy from the battery to the pair of output terminals. The switching device is responsive to the presence of electrical potential at the primary power source for coupling and decoupling the primary power source or the secondary power source to the output terminals. The switching device can also provide a current path from the primary source to the output terminals, which bypasses the inverter. A force commutation device responds to the voltage at the primary power source for driving the switching device, causing it to decouple the primary power source from the output terminals.