The RCD does not provide phase fault protection such as short circuits. The original fault was a line/neutral short, tripping the single pole MCB disconnecting the live only. Subsequently the neutral conductor may have touched an earthed part and would then trip the RCD. The neutral voltage to earth is generally sufficient to drive the RCD with more than 30 mA and hence tripping the device.

I would think the first ‘bang’ was the neon connection fusing – but not taking the 45A (?) breaker – and the second was line to earth connection, possibly due to a reconnection due to an already melted conductor making a further short circuit? Just guessing, of course.

Edited By not done it yet on 08/04/2023 08:39:50

I fully endorse checking properly for isolation but in an ideal world a multimeter cannot be recommended due to the increased chance of user error. Back in the real world it is unlikely that any of us will have a tester and proving unit so we are back to the multimeter that many of us are likely to have. Check that the multimeter is set correctly by testing a known source then check L-N, L-E, and N-E then prove the multimeter is still working on a known source, of course if you have turned all the power off rather than one circuit the source may now be off which is why a proper tester has an independently powered proving unit. Although the proper test gear which is expensive and unlikely to be in anyone’s toolkit who isn’t a working electrician the process of checking is valid with whatever test equipment you have. I find it difficult to recommend shorting cables to earth to ensure they are isolated as if you get it wrong and it is still alive the flash and bang and the possible expulsion of molten copper could injure you. This may be marginally be better than electrocution. If you totally lack any test equipment then working on mains voltage equipment should be avoided.

Mike

"Should not" agreed, but there often is. I have measured 19 volts on a system which had passed an earth loop resistance test.

George

I'd go along with that. Fuses (including the conductors of a neon lamp) tend to go, "bang", but RCDs, etc. go, "click".

All a matter of scale. The earth leakage trip may be 30mA if the wiring is 0.1 ohms it only takes 0.003V to generate 30mA of current. Triple that for N-E loop resistance of 0.3 ( earth conductor is smaller)ohms and you get 0.009V. A load of 0.1A will generate 0.01V in a 0.1 ohm neutral.

So if there is pretty much any load on the ring connecting neutral to earth will trip the leakage device.

Robert.

Well, a PFC of say 1000 A would make any fuse go, "bang".

Good point, but a few volts present on a neutral – even in a circuit isolated from the line supply – when shorted to earth can cause enough current to flow to make an RCD see a sufficient current imbalance to trip out.

The potential difference, (in volts), between a neutral and an earth is what makes the current flow !

Interesting to hypothesise about where volts between neutral and earth come from. I offer this example:

In the circuit, to keep the maths simple, a 250Vac source is feeding a 100 Amp load through a cable with a 0.1 ohm resistance. If a voltmeter is read near the Neutral/Earth connection at position V1, the voltage will be close to zero. However, if the voltmeter is moved to position V2 at the load, it detects the voltage drop caused by cable resistance. Though about 10V above ground, there's no current imbalance at the source, so an RCD is happy. So is an earth resistance check. This example is all good I think (other than the power wasted in the cables!)

Another possibility is that the multimeter is detecting RF on the line, perhaps from a Ethernet over Power Line system. These piggy-back on mains wiring to carry a modulated radio signal through buildings. Power equipment doesn't react to the signal, but it can be detected by suitable electronics. A multimeter might 'see' both signals. Low frequency mains (50Hz), pretty much obeys ohms law inside the meter, which measures it correctly. Radio signals don't obey ohms law in the same way: high frequency AC sees the meter circuit as a mix of resistance, inductance, and capacitance, which allow it to bypass the meter's dropper resistors. The movement of an analogue meter is protected by diodes, which rectify the signal and produce an uncalibrated meter reading. Diodes in a digital meter can also cause anomalous meter readings. This cause is also OK – only the meter is confused.

My circuit diagram deliberately shows 3 earth connections, which is not normal! I did it to stimulate debate about earth faults – what happens when the earth system malfunctions, either by developing resistances, leaks, or part of it goes open circuit. Various strange things can happen, and there's at least one supply-side earth fault where consumer Neutral becomes Live with spectacular results.

And then there are electrical storms and solar activity…

Dave

Edited By SillyOldDuffer on 09/04/2023 11:18:18

Looks like it might be a good job our house has separate RCBOs in the consumer unit for each circuit.

Edited By V8Eng on 09/04/2023 13:52:36

The Electrician who did the work said they are all double pole and a main switch in there.

Edited By V8Eng on 09/04/2023 15:18:02