ABSTRACT
A basic residual current device (RCD) is a device that is used to detect earth faults. The RCD monitors the amount of current flowing in the line conductor and the neutral. When an earth fault occurs and some of the current leaks to earth the result will be more current flowing in one live conductor than the other. Once the fault current reaches the trip rating of the RCD the device will automatically switch off. BS 4293 is an RCD that will detect an earth fault and switch off, but it will not switch off if there is an overcurrent. RCD protection can be incorporated into individual socket outlets and these would be referred to as SRCDs. The older types of RCDs can be tested using a tester with what is called a 'D' lock mechanism. This type of test instrument uses DC to desensitise the trip coil, which will prevent it from operating during the earth fault loop test.
