CS12

Residual Current Detectors (RCD’s)

A residual current detector is a safety device installed in homes and businesses globally in an effort to save lives. They do this by preventing electrocution through direct and indirect contact and are responsible for saving countless lives each year ……. And most of the people don’t even know that they were at risk.
RCD’s work on the principle of measuring the current balance between to conductors using a “differential current transformer” [1]. The device will open its contacts if it detects any difference between the live and neutral conductors, it does this by using the idea of the product of the two currents should equal zero. When this criterion is not met the devive knows that the current must be leaking out somewhere else and it cuts the power.
To understand the function of and RCD Faradays Law is very important. This is so because of the design of the RCD. The live and neutral current create a pair of equal and opposite emf’s in the differential current transformer so if the live of the neutral wires were to suddenly gain or lose current the net induced emf in the differential transformer would no longer equal zero so the RCD puts its contacts out to cut the supply.
Taken from www.energomera.com


RCDS should be tested twice a week at least and every time you think of doing it. This is done by pressing the test button on the RCD (shown above) and the appliance that the RCD is controlling should turn off. The reason behind the testing is that is vital that a faulty RCD be replaced.
RCBO's combine the operation of the MCB and the RCD into one unit. The RCD part operates in exactly the same way as the standard RCD, however for overload protection the RCBO contains magnetic or thermal trips. The MCB ( miniature circuit breaker) is also a current leakage device but it concentrates on the thermal trips instead of electronic current deficit detection
RCD
RCBO
All images taken from Google images
[1] Taken from Wiki