Overload protection of A-C instruments

The behavior of indicating instruments subjected to overloads is described. The principles underlying design requirements of protective devices are presented and certain features of instrument design which enhance the effectiveness of protective devices are mentioned. Test results on current circuit and voltage circuit protective devices which are mounted within the case of a single-phase wattmeter are given. The current circuit tests cover overloads ranging up to over 200 times rated current. The latter represent direct connection of the current circuit across a 240-volt power circuit while the voltage circuit of the wattmeter was energized.