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I recently ran a cable along a massive steel beam in our garage to power an electric water heater. I haven't secured the cable properly yet. It just lies on the beam for most of its path. Whenever I switch on the associated MCB, the RCD (safety switch) that is seemingly unrelated to this circuit trips. I have investigated all possible causes. The only thing left is that maybe running the cable along the beam has anything to do with it. By the way, the ground and neutral are mixed up in the switch board so that may be another issue but it hasn't caused any such nuisance before.

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    Unless the cable insulation is damaged(maybe only tiny bit), the beam should not be the cause. Mixed up ground and neutral is better choice depending on where. Inside main panel is usually okay, outside of main panel can cause problems. Use dry wood to raise cable from beam to check.
    – crip659
    Jul 7, 2021 at 10:34
  • If ground and neutral are mixed up and a metallic type cable metal clad, armor clad , I would expect the rcd to trip, non metallic I would be looking at the water heater as the source of the excess leakage.
    – Ed Beal
    Jul 7, 2021 at 13:41
  • Did you split off and securely cap off each of the wires in the cable? Or are they just however the cable shear left them, hot possibly mashed into ground, or hot or neutral touching the beam? Also, re: ground and neutral mix-up, are they the same physical bar? Generally RCDs are very finicky about grounds and neutrals being in good order. Is there a good reason not to correct that ASAP? Jul 7, 2021 at 15:29

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Some things to check:

  • What does the RCD protect? Are you sure its RCD - Residual Current Device? (Country is important - it is common in Europe is to install one servicing whole panel or set of rooms).

  • Is the heater ok? Can you connect it temporary with extension cord to a socket on same circuit to see if that causes issue? If It has a ground=neutral short, it will trip RCD on its circuit.

Having neutrals and ground mixed before RCD is messy but ok.
However, the neutral coming out from RCD must be kept separate from ground and other neutrals, forming separate circuit, and the hot wires from RCD must obviously only connect to loads using said neutral. If there is any unexpected connection between two, the RCD will trip.
This is because RCD watches for any current that went out and didn't return, and will cut the power as soon as it detects any leaks. (ex. someone touching wire when standing in puddle of water, or short between hot and ground)
It is also possible to accidentally connect neutrals from two circuits, or neutral to ground, without anything tripping, because leak is not enough. Then it trips when load is connected somewhere else on that circuit.

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  • I would saying neutrals and grounds mixed is unsafe. Having them combined at the same potential at a main panel is one thing and that is different than mixed.
    – Ed Beal
    Jul 7, 2021 at 13:38
  • The problem is solved. I had mistakenly connected the water-heater's neutral to the neutral coming out of the RCD. I connected it to another neutral bar as the water-heater's circuit is not protected by the RCD. I appreciate the advice.
    – Reza
    Jul 8, 2021 at 2:12
  • Happy to hear it was just mixed neutrals after RCD :-)
    – Thomas
    Jul 8, 2021 at 7:25

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