How to wire an RCD with no neutral line

One of the more recent changes to the Wiring Rules AS/NZS 3000 included some major changes regarding when and where RCDs (Residual Current Devices) are to be installed. This has led to a lot of new RCD installations, some of which present more of a challenge. 

Normally, you would wire the 3 active phases and neutral of the supply and load sides as shown below:


But what do you do when the device you are connecting the RCD to does not have a neutral line?

There are two options:
  1. If there is a neutral in the distribution board, use that and you are good to go.
  2. If there is no neutral at all, the RCD will be connected normally but a bridging resistor will need to be added.
The absence of a neutral line creates an issue where the test button would be supplied by the full concatenate voltage (typically ~400V), which could damage the circuit.


Adding a resistor across terminals 4/3 and 8/7/N, as shown above, causes a voltage drop which leaves the test circuit at a voltage of 254V-277V, a suitable range for normal operation.

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