Facilities Support Services

What is an RCD?

RCD stands for Residual Current Device. In short, an RCD is an electrical protection device that disconnects a circuit in the event of a fault.

The Residual Current Device operates on the principle of continuously monitoring the balance of currents between the Line conductor and the Neutral conductor. If this balance is lost that means that the current from the Line conductor is going somewhere else other than the Neutral conductor – that is to Earth, trough a fault somewhere within the installation.

A regular domestic RCD is set to a 30mA limit. The reason for this is because the human body can withstand a current of that level. Anything above this would be dangerous for life, so the RCD would cut the supply off.

How to perform an RCD test?

This is a live test so extra care is advised!

Step 1. Use a dedicated RCD test meter, or the RCD function of a multifunctional tester such as the Megger 1553. Make sure that the installation is fully energised.

Step 2.  Connect the test leads on Line, Neutral and Earth terminals anywhere on the installation but after the RCD (i.e. socket).

Step 3. Testing should be done with 50% In, 100% In and 500% In on both 0° and 180°. Depending on your test meter, this could be done automatically or manually. Please refer to the user instructions of your RCD tester.