the protection of potentially incendive circuits is achieved by a power system which comprises a power supply connected by power distribution wiring to one or more modules , the or each module comprising an intrinsically safe circuit including a load , sensing means to detect the voltage supplied to the load , and a fast - acting switch arranged , in response to the detection of a decrease in the supplied voltage , to disconnect the load in such manner as to prevent any series break upstream from the switch from becoming incendive .