How to wire an ABB RCD with no neutral line?
One of the most recent changes to the Wiring Rules AS/NZS 3000 included significant changes to when and where RCDs (Residual Current Devices) are to be installed. This has resulted in a large number of new RCD installations, some of which are more difficult to implement.
IPD and ABB offer superior protection for power distribution with the range of Type A, Type F, and Type B residual current devices (RCDs) and the range is compliant with amendment 2 of AS/NZS 3000:2018. RCDs provide protection against leakage current by having the ability to detect an imbalance of electric current. Once the detection is made, the electrical system is tripped to reduce the risk of serious injury or death.
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:
- If there is a neutral in the distribution board, use that and you are good to go.
- 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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