We have just bought a house and when I look at the electrical switchboard, I see a 4-pole residual current detector (RCD) while the mains supply to the house is single phase? Any idea why this has been installed instead of a 2-pole one?

  • 1
    What makes you think the RCD is 4-pole? I think it is only 1-pole. I don't believe a trip interrupts the neutral (earthed conductor). You have only one hot phase and this RCD will interrupt it in the event of a trip. Dec 26, 2023 at 11:48
  • From the diagram on the RCD it seems the neutral might just be passed right through or the breaker might also interrupt the neutral and so in that sense be a 2-pole RCD. There are three (hot) phase connections shown but they are all the one phase. Dec 26, 2023 at 11:56
  • In the US the breakers only interrupt the phases and not the neutral so my first thought would be that the neutral (grounded conductor) bypasses the breakers but I don't know this for sure in this case. Dec 26, 2023 at 12:03
  • Where is this, what city and country/ Dec 26, 2023 at 16:33
  • 1
    GFCI/RCD often interrupt the neutral. Thus in a 3-phase distribution it has 4 throws. Note that overcurrent devices typically do not have an overcurrent sensor on neutral even if they interrupt it. Dec 26, 2023 at 23:17

2 Answers 2


Most likely that is what the electrician had on hand at the time.

I can't see any other reason to install a breaker intended for 3 phase in a box that would probably need replacing if the service was upgraded to 3 phase.

  • So is this RCD in fact capable of 3-phase circuit interruption? Dec 26, 2023 at 16:32
  • Abosolutely. the horizontal lines are just the wires that connect N and L1 to the test button. (I found a clearer image on e-bay)
    – Jasen
    Dec 27, 2023 at 3:15
  • So does this breaker also interrupt the neutral current path? Dec 27, 2023 at 3:18
  • it certainly looks that way. the diagram shows sets of 5 contacts, one is on 'N' and another connects the test button to N, the remaining three ad for L1,2,3
    – Jasen
    Dec 27, 2023 at 3:25

A 4-pole residual current device (RCD) can be used to protect two single-phase circuits, provided that it is appropriately rated and designed for such use. In this configuration, each pair of poles on the 4-pole RCD is used to protect one single-phase circuit.

Here's how it typically works:

  • The first pair of poles (pole 1 and pole 2) are used to protect one single-phase circuit. The live conductor (phase) of that circuit is connected to pole 1, and the neutral conductor is connected to pole 2.

  • The second pair of poles (pole 3 and pole 4) are used to protect the other single-phase circuit. The live conductor (phase) of the second circuit is connected to pole 3, and its neutral conductor is connected to pole 4.

This setup allows both single-phase circuits to be monitored for any imbalance in current between the live and neutral conductors. If a ground fault or leakage current is detected in either circuit, the 4-pole RCD will disconnect both circuits simultaneously.

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