RCD tripping, N-E earth fault.

[Kev2632]

I have an RCD side of the board, with two ringmains on it and both MCB's was off, but the RCD was on.
Should an RCD trip N-E when there was no current flowing in any of the circuits? because it tripped for me everytime, why would this happen?

thanks.

 

[telectrix]

RCDs will trip with a N-E fault when the MCB is off. this is because the fault will cause a current to flow N-E when the current in L is 0. therefore there is an inbalance. happens often when working on a circuit, isolated by MCB/Fuse if you short the N to E , then the RCD trips.

 

[ackbarthestar]

I have an RCD side of the board, with two ringmains on it and both MCB's was off, but the RCD was on.
Should an RCD trip N-E when there was no current flowing in any of the circuits? because it tripped for me everytime, why would this happen?

thanks.

The RCD was on but it kept on tripping ? So it couldn't be reset ?
Therefore it couldn't be On ?

Need more input

But in the meantime ........

 

[Kev2632]

RCDs will trip with a N-E fault when the MCB is off. this is because the fault will cause a current to flow N-E when the current in L is 0. therefore there is an inbalance. happens often when working on a circuit, isolated by MCB/Fuse if you short the N to E , then the RCD trips.

yea, but there was not current flowing in either of the two ringmains,the MCB's was off? so how can the RCD still detect an imbalance when there was no current flowing?. all I was doing was working on a socket and touched neutral to earth and it tripped the RCD.

 

[ackbarthestar]

yea, but there was not current flowing in either of the two ringmains,the MCB's was off? so how can the RCD still detect an imbalance when there was no current flowing?. all I was doing was working on a socket and touched neutral to earth and it tripped the RCD.

The electrical installation is live even though you may not be drawing current via any specific load but because it is live there may well be circulating earth currents from else where and when you touch earth and neutral together you create a new earth path via the neutral connection in the RCD. That would cause a sufficient imbalance to trip the RCD.

Isolate the installation and then touch the downstream neutral to the cpc and see what happens

 

[Kev2632]

The electrical installation is live even though you may not be drawing current via any specific load but because it is live there may well be circulating earth currents from else where and when you touch earth and neutral together you create a new earth path via the neutral connection in the RCD. That would cause a sufficient imbalance to trip the RCD.

Isolate the installation and then touch the downstream neutral to the cpc and see what happens

Yea I understand what your saying, so even though there was no Circuits live on that side of the RCD, It can still be affected from circulating earth currents of the non rcd side is that correct?, so if i try and switch off all the other circuits on the non-rcd, it should then solve that wee problem yea?

 

[telectrix]

you only need a current of up to 30mA flowing N-E to trip the RCD. for arguments sake, assume a 10 ohm resistance applied between N and E. using ohms law a voltage of 300mV will give rise to a 30mA current. when you accidentally short N-E the resistance applied is the resistance Rn + R2 of the cable from the CU to the socket, hopefully well under 10 ohms.

 

[Octopus]

Yea I understand what your saying, so even though there was no Circuits live on that side of the RCD, It can still be affected from circulating earth currents of the non rcd side is that correct?, so if i try and switch off all the other circuits on the non-rcd, it should then solve that wee problem yea?

Switching off all the MCB's won't solve the problem but might point you in the direction of where next to look for a fault.

How are the IR readings?

 

[Deleted member 26818]

This is something that occurs predominately with TN-C-S systems.
As the earth and neutral are combined, any current flowing in the neutral will also flow in the earth.
By touching the neutral to earth, you are creating a path for current to flow.
This will flow through the RCD, and as there is no current flowing through the line, the RCD registers this as an imbalance.
This wouldn't happen, if the MCBs were double pole, or if you turned off the RCD, or the main switch.

 

[telectrix]

but surewly, spin, for a current to flow between N and E, there must be a p.d. between them

 

[needasparks]

the OP said he touched N to E so this would imply the circuit was disconected so may not have had an earth potential.I'm sure running next to other cables may have given it enough eddys to make it trip unless other circuits on the rcd side were energised with a common neutral or did i miss-read.

 

[ackbarthestar]

the OP said he touched N to E so this would imply the circuit was disconected so may not have had an earth potential.I'm sure running next to other cables may have given it enough eddys to make it trip unless other circuits on the rcd side were energised with a common neutral or did i miss-read.

All your wiring downstream of the RCD can be 'perfectly installed' without any back emfs, even on a TT system and then touching neutral cpc together will still trip the RCD. Remember it is most unlikely that there will be 0V on both the Neutral and cpc, so even a small voltage differential of 0.2V. The RCD is an active little rascal and only needs a slight voltage variation causing a current of between >15mA (22 - 28mA) to activate.

 

[needasparks]

Thats where i was going.
''active little rascal'' think i would have used a stronger term but i love that term, I'm gonna write that on a cert one day
such a friendly was of saying it.

 

[Deleted member 26818]

but surewly, spin, for a current to flow between N and E, there must be a p.d. between them

No not really, it just becomes a parallel conductor.