Three wire connection of RCDs

Our Technical Team have received a number of enquiries regarding three wire connection of RCDs.

A RCD will still function without a neutral connection, however the test button may not operate.

Some RCDs have the trip test circuit connected between phases while others are between phase and neutral. There will be a circuit diagram on the RCD which will show the trip circuit configuration.

In the case of a RCD with a phase to phase test circuit, there are no special requirements and the test function will operate with a three wire connection.

RCDs with a phase to neutral test circuit will require the connection of a resistor between the load side neutral terminal and a load side phase that is not connected to the test circuit - see wiring diagram below.

There will be a wiring diagram in the installation instructions supplied with the device and these should be referenced to ensure correct connection for the specific RCD. Your supplier will be able to supply the correct resistor for the RCD to be installed.