Demystifying RCDs

by Pat Ward - Western Automation Research & Development

Starting With The Definition.
An RCD (residual current device) is a device designed to cause the opening of one or more contacts to disconnect a load when the residual current flowing in the circuit protected by the RCD reaches the rated residual operating current (IΔΝ) of the RCD.

That immediately raises the question as to what is a residual current. It is not clear why the term “residual current” was chosen, but the French translation for an RCD is a “differential current circuit breaker”, and this offers a clearer interpretation of the principle of operation of an RCD. Figure 1 shows a typical single phase circuit comprising of an AC supply fed to a load via Live and Neutral conductors. The Neutral is connected to Earth. The circuit is protected by an RCD.

Under normal conditions, a current IL flows from the supply in the Live conductor to the load and returns to the supply in the neutral conductor as IN. The Live and Neutral conductors pass through a current transformer, CT, whose output is connected to a residual current detector. The current flow in each conductor as seen by the CT is of equal magnitude but opposite direction, and as a result the vector sum as seen by the CT is zero, so the CT produces no output.

If a person touches a live part, as demonstrated in the Figure 1, a current IR will flow through the person’s body to earth and return to the supply via the earth return path. The current IL will now be greater than IN and the CT will produce a resultant output in response to this differential or residual current. This output will be sensed by the residual current detector, and if it is above a predetermined level it will cause the RCD contacts to open and disconnect the supply from the load and thereby provide protection.

A Residual Current therefore refers to any current over and above the load current. The key factor in the ability of the RCD to detect a residual current and provide protection is the connection of the supply neutral conductor to earth at the origin of the installation. It follows that all residual currents flow to earth, and residual currents are therefore often referred to as earth fault currents. These two terms are used interchangeably throughout this document.

As stated earlier, (IΔΝ) is the rated residual operating current of the RCD. Typical values used around the world are 6mA, 10mA, 30mA, 100mA and 300mA. Values up to 30mA are used for direct contact (personal) protection, and values above 30mA are generally used for indirect contact (earth fault) protection.