Testing and Test Instruments 81
For 30 mA ratings, it is required that the RCD be injected with a current five times its operating current and the tripping time should not exceed 40 ms. Where loop impedance values cannot be met, RCDs of an appropriate rating can be installed. Their rating can be determined from I n 50/ Zs where: IΔn is the rated operating current of the device 50 is the touch voltage Zs is the measured loop impedance All RCDs have a built-in test facility in the form of a test button. Operating this test facility creates an artificial out-of-balance condition that causes the device to trip. This only checks the mechanics of the tripping operation, it is not a substitute for the tests just discussed. All other items of equipment such as switchgear, controlgear interlocks, etc., must be checked to ensure that they are correctly mounted and adjusted and that they function correctly.
PROSPECTIVE FAULT CURRENT If we are able to remove all the protective devices in a system and connect a shorting link between line and neutral conductors, the resulting current that would flow at the point where the link was placed is known as the PFC or in this case PSCC. This is all very interesting, you may comment, but why would one need to know all this information? When a short circuit occurs, the device protecting the circuit in question has to open that circuit. If the device is incorrectly selected, a violent and possibly damaging explosion could result.
