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I am installing a small electrical board for a home renovation. There is a single RCD 30mA, EU standards (I am in France). There are ~20 circuits, each with their own differential, most are OFF, I am only using the lights and some power points.

One circuit is posing a problem since yesterday when I was pulling another cable through the box, it is causing the RCD to trip ever since.

The problem is strange, only the +ve cable of this circuit trips the RCD (neutral does not even need to be connected). If I connect the live cable to live terminal of another differential, or directly to RCD I have the same problem (CONCLUSION 1 : the circuit's differential is not faulty)

The RCD does not trip the other circuits (CONCLUSION 2 : the RCD is not faulty)

If I take (only) the live cable from the circuit and connect it to the neutral terminal of a differential, or RCD the RCD does not trip. (CONCLUSION 3: something strange is happening, why does earth fault only trip RCD live ?)

This circuit has several power points connected in parallel with some solder points along the way. The solder points are wrapped in tape and embedded in a stone wall. While the concrete was drying the circuit used to trip, but I let it dry for a few days and no problems since, until yesterday. I cut the cable close to the entry of the wall to check whether the fault was in the wall, or between the wall and circuit board. There is definitely bad insulation inside the wall connection. But it is hard to access and I would at least like a temporary fix that does not require breaking into the wall.

Is there any.

  1. Why does the RCD trip when the earth fault is connected to RCD live, and not RCD neutral?
  2. Why was this not a problem until yesterday?
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  • For (2) perhaps pulling the other cable may have damaged the suspect cable - perhaps rubbing it against a nail or metal edge?
    – Brian Drummond
    Jan 2, 2016 at 12:02

2 Answers 2

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AC power does not have positive and negative, it has active (or live) and neutral

The voltage between active and ground is much greater than the voltage between neutral and ground, so connecting a weak fault between active and ground will trip the breaker while connecting it between neutral and ground does not.

The only temporary fix is to stop using the faulty wire. You could run a temporary wire on the surface of the wall.

Next time use electrical junction boxes and conduit inside the wall so that the cables do not bind to the concrete and can be serviced or replaced should they develop a fault.

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schematic

  1. Somewhere in your electricity supply the neutral wire is connected to ground. This is normal. The voltage on N is close to zero in your house.
  2. The RCD (residual current device) monitors the current in and out of each circuit. If all is OK then I L = I N and nothing happens.
  3. If there is a fault and more the 30 mA is returning via the ground then the coil will sense this and trip (switch off) SW1 and SW2 to make the circuit safe.

Q: What live wire leakage resistance, R2, will cause a fault?

A: We can calculate this from V = I R.

R = V / I = 220 / 0.03 = 7400 Ω = 7.4 kΩ.

So if we have a fault with < 7.4 kΩ on the live wire the RCD will trip.

Q: What about a fault on the neutral wire?

A: The neutral wire is nominally 0 V but when there is current flowing in it we can expect it to be a few volts above ground in the house. Let's make the maths easy and say that it is 3 V above ground. Now what value of R1 will cause 30 mA to go to ground?

R = V / I = 3 / 0.03 = 100 Ω

So we can see that the leakage resistance would have to be much lower to cause a fault on the neutral wire.

What to do?

You need to fix this problem. I am sorry to tell you that you have done a bad job and it will most likely fail an inspection.

  • Soldered connections are not used on domestic wiring.
  • Each wire should go from one screw terminal to the next.
  • Ideally there should be a maximum of two wires at each connection.
  • If junction points are needed then these should be in plastic boxes with screw terminals. Metal boxes can be used if earthed.
  • If there is danger of water then waterproof boxes with waterproof cable entries must be used.
  • Insulating with tape and burying in a wall is very bad practice.

You need to open the wall and fix the problem.

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  • Use of solder joints is acceptable in the UK wiring rules, but I don't know about France. But wrapping tape around a join is poor - it should really be something more permanent, such as heat-shrink. The chances are that pulling through another cable has ripped the tape off a join, and it's now leaking current through the wall.
    – Simon B
    Jan 2, 2016 at 15:31

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