Changing some light fittings today, MCB for circuit turned off. RCD tripped when I cut a (not live) cable (all three cores).
Similar thing happened in a different property about a year ago. Both reset OK.
AIUI an RCD compares current @ L & N and trips if there is any difference.
Why would it trip when the circuit being worked on is disconnected?
That's a normal occurance. The MCB only isolates the live wire. If there are any devices switched on on any other circuit, there will be a current flowing through N causing a voltage drop due to the (small) resistance of the N wiring. When you cut the cable, you connected N and earth, creating another path. Current will now flow from L through the appliance to N, then through your cutters into the earth wire, then to a point where earth and N are bonded (present in any earthing system) bypassing the RCD. The RCD registers less current in N than in L and trips. The only remedy would be to disconnect N as well as L on the circuit being worked on.
Wolf
Dave - I'm surprised at you working on a circuit, only isolated by the MCB - that is NOT ok, and you should know better.
Neutral and earth often have a slight difference in voltage. You shorted neutral to earth, some current flowed - so the RCD tripped.
Neutral leakage from another circuit, through you/your pliers to earth. There'll be 4 or 5 other circuits (probably) on the same RCD. All you have done when turning off the MCB is break the live on the circuit you are working on. The Neutral will still be 'live', by being connected to the other 4 circuits neutral bar. The RCD is 'measuring' in/out the current going through, you make a short between earth and neutral, and it is an easier path for all the neutrals to take, rather than going back to the sub-station 100's of metres away. Hence there will be less current coming back than going in, so it trips. Also, if you dont know the house you are working on, then there is always the chance of a shared neutral on the lighting circuits, so even if you have turned off the MCB, it could still be live (though unlikely on a RCD protected CU).
After doing a lot of training in the last year, I now make sure the circuit is completely dead before doing any work on the circuit - you cannot always rely on a RCD to trip to save you. In future, I'd tell the customer that you have to turn off the power when doing the work - far safer.
Alan.
See back posts on residual current.
Try and find out the voltage involved. It may be quite high but have little ampage.
Because the circuit is not disconnected - only the live. Any voltage difference between N & E will result in a current flow from one to the other when you short N & E as you cut the cable. The RCD will see this flow in N and not L and hence trip.
Because a domestic circuit breaker only cuts the Live.
The Neutral remains connected, to the circuit in not fully isolated.
You created a E-N fault, and that tripped the breaker.
To avoid this, disconnect the N from the bus-bas also. ( or nibble the snip carefully and don't cause the E-N fault! )
Remember that neutral is also "live wire" - you can get reasonably significant current flow between N & E. The voltage ought not be more a than a few volts away from earth on a TN-S system, and much closer on a TN-C-S obviously.
Other way round!
A few volts maximum, even in large commercial buildings.
But in those settings a lot of current may flow if neutral and earth are shorted.
I always check with a volt stick before & after switching off the MCB or removing the fuse.
Whats the alternative to switching off the MCB?
Main switch>
That'll show you if there is a direct live on the circuit at that time. It wont show you if anything else is on that side of the RCD. When something else is turned on, you have the potential to get a shock from the neutral, as the neutral is connected to all the other circuits.
Yes. or the RCD on that circuit.Much preferable to turn off the main switch, no chance of any cross-joined circuits then[1], so you know everything is safe (of course, you check it is safe before starting). Also, if there is someone else in the house, put a lock on the main switch so it cannot be turned on while you are working. They are only £3 or so.
[1] it is possible that someone has taken a feed off one circuit, then joined it to another unknowingly in the past, so you turn off one of the 2 RCDs thinking it is now safe, when there is a cross-connection somewhere, meaning the circuit is still live. On one of the elec forums last week someone came across a live circuit when the CU at been turned off at the main switch - it turned out the tenants of a flat had run a cable from an adjacent flat to use their electric to reduce their own bill.
Or RCD if its a circuit on a bit of the split load protected by a double pole RCD (note modern MCB width RCBOs are often only single pole).
If working on my own place where I know there are no borrowed neutrals etc, then I will sometimes work with just the MCB off, and take care not to short N & E. However, on a place I am not familiar with, then its safest to pop the front off the CU, and disconnect the neutral for the circuit from the bus bar (for quick jobs, its obviously quicker to turn off at the main switch).
See - you knew the answer all along.
In some non-domestic settings, a lot of current can flow by shorting neutral and earth.
That is a bit overkill.
It should be safe to work on a circuit that has it's MCB turned off. It won't stop a shared RCD from tripping, but you should be at no risk of a shock.
The ring needs to be disconnected and grounded at both ends, and the cable spiked with a big nail before it's considered safe :-)
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