I have been working on my shower today and when I was cutting the cable the main rcd went. The circuit is turned off at the consumer unit and there is no live power to the cable. I have even double checked this with a multimeter.
What could have caused this? I think it is shorting the earth and neutral together whilst cutting that caused the trip. There is no potential between neutral and earth either.
I have not had any other problems with tripping, but I am interested in why this would happen.
A small current can flow between neutral and earth which is enough to create the imbalance required to trip the RCD.
If it's irritating, you can disconnect and tape the neutral for the circuit at the CU while working on it, remembering to reconnect afterwards of course....
There is normally a small voltage between neutral and earth.
RCD's trip when they sense a difference in the current flowing out through the live and back through the neutral. This difference in current means it's flowing somewhere it shouldn't - usually to earth (such as you shorting neutral to earth) and possibly through a person.
So you should only work on your wiring with the main switch OFF. Turning off the MCB at the consumer unit is not sufficient.
Ah, you are not looking hard enough - it will be there - just small (but enough to trip a RCD)
If you think about how power is provided to your house and your neighbours, it will become a little clearer.
Live O=============================================== A _| _| _| _| |H | |H | |H | |H | |__| |__| |__| |__| Neutral | | | | O=============================================== B \\ \\ Earth O============================================= C
Live, Neutral and Earth are shown at the substation. Neutral and earth are joined and hence will be at the same potential, and Live will be at (say) 240V
Simple application of ohms law will tell you that if each of the houses are drawing current, and the wires connecting them up have some resistance, then there must be a voltage drop along them. That means that the voltage at point "A" will tend to fall below 240V, and the Neutral measured at point "B" will tend to rise above zero. The earth however should in normal circumstances have little or no current flowing in it, and hence should remain at or near the same potential along its whole length.
So when you join neutral to earth, you are in effect connecting a low voltage source to earth, and current flows. Since the current flowing in the neutral did not come from the live the RCD sees the imbalance between and opens. (This is why technically speaking Neutral is *is* a live conductor (as opposed to a actively driven Phase conductor)
(the same effect can be observed in a single house - where other circuits will tend to pull the neutral away from earth potential. Hence you can get trips even on houses that have TN-C-S supplies with the earth and neutral joined together at the house.)
This is a warning that you shouldn't be working on a non-isolated circuit. Tripping the MCB does not isolate the circuit. If there is no isolator specific to the circuit you are working on, you should trip either the (double pole) RCD (if it covers the circuit) or the main switch.
But the risk here is not to life and limb, but that the RCD will trip, in which case he will be no worse off than if he had completely isolated the circuit in the first place. And if it doesn't trip, there may be less disruption to other activities in the house. That's a risk assessment he is perfectly entitled to make.
Some fault conditions will result in shock. If you don't isolate the circuit, you should consider it live working and take all precautions that this entails. Personally, I wouldn't cut through a "live" neutral, particularly with no attempt to segregate it from earth, phase and myself during the cut.
OK, the risk of shock is much less when the circuit is supposedly deenergised, but the risk is still there.
I'd check any wires before cutting them as a matter of course, whether I'd switched everything in sight off or not, but I don't see how that is the same as live working. The risk in this case is effectively zero.
Following BS 7671, it depends on whether it's a TN or TT earthed installation (see page 34 in the OSG). For TN-x BS 7671 only requires isolators to interrupt all phase conductors (other than at the main switch of a domestic installation). OTOH isolators in a TT installation must interrupt all live conductors - i.e. phase and neutral. The inference is clearly that the neutral of a TN supply is considered reliably earthed and will not be a dangerous voltage relative to the local equipotential zone, whereas in a TT system neutral could be quite hot wrt the local earthing.
Nuisance tripping of RCDs caused by N-E shorts during work in progress is a separate issue, really. A simple practical way to avoid the problem is to disconnect the neutral(s) of the circuit you're working on from the neutral bar as well as pulling the fuse or opening the MCB.
On Oct 23, 11:51 am, "Pyriform" wrote: . And if it doesn't trip, there may be less
Once I had established there was no potential between live and neutral I carried on. I had to, as my wife was watching Formula 1 and I had already upset her when it tripped the first time..I think she missed an interesting pit change, or a car following another one around the track!
Strictly speaking, you had in fact established the exact opposite. You only needed enough potential to push 30 mA through a rather hefty piece of cable! I think you meant "no *dangerous* potential"...
Miniscule risk of electrocution versus major risk of wife-rage. No contest...
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