In one form, tester for an electrical system, such as for a vehicle, develops repetitively applied, short duration, control current pulses for reducing load current pulses through the electrical system. An operator connects a pair of input contacts across a fuse holder. A circuit between the input contacts is closed momentarily and repetitively to produce repetitive current pulses from the electricity source through the system. A perceptible alert is produced when the magnitude of the current pulse exceeds that of a prescribed reference current value. In another form, a tester for an electrical system is designed to plug into the fuse terminals in a fuse box. The tester can include a plurality of interchangeable circuit breaker modules to allow the tester to be used on circuits having differing current ratings.