Earth-neutral contact trips RCD

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Whilst changing a broken socket outlet with the circuit breaker switched off the neutral cable made contact with the earth cable and the main RCD on the meter side of the consumer units tripped out leaving me in the dark both literally and wondering if there was some kind of fault and if so what it could be. Can any one shed any light on this please?
 
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it's supposed to do that...

an rcd "measures" the current running through the live to all the circuits, and then "compares" it to the current returning through the neutral from all the circuits..

when you touch the neutral to earth, some of the current returns back through the earth wire instead of the neutral..

so the rcd sees more current going out than is coming back and knows it's lost some somewhere and turns off..
 
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Yes garymo you're right about this but what can you say if the rcd will not trip when a neutral to earth fault?? ( Neutral to earth shorted trough a plug in socket outlet)

The RCD will trip when connected live to earth, but neutral to earth will not.

however there is neutral in socket outlet because the appliances working correctly.

Do you have any answer please? Thanks and regards Wayne
 
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As above, RCDs work on sensing a current imbalance, so if there's no current flowing there's no imbalance.

Short live to earth, and a large current flows through the live, but none through the neutral, so the RCD trips.

Short neutral to earth, and, as both are at the same potential, no current flows, so the RCD doesn't trip.

If there is a load on the circuit, there are equal currents in live and neutral conductors, so the RCD doesnt trip.

Short neutral to earth on a circuit carrying a load, and part of the neutral current flows through earth conductor instead of neutral. The RCD sees an imbalance and trips.

So an RCD trips on almost any live fault, but only trips on neutral faults if the circuit is carrying a load.
 
W

Woodendhead

N to E short not tripping an RCD can tell you a few things.

First it tells you to press the T button if that works then at least you have an idea it is still working, if that does not trip it get a new one put in.

It could just be that the potential difference between N and E is not enough to push the rating current though the RCD.

It could be the either E or N is disconnected so no complete circuit, hence the L to E on a dead circuit has not effect.

If L to E is tripping it (with the circuit dead), it could be wired the wrong way i.e. the wire you think is N is L and what you think is L is N. Why I test all wires before I start work.
 
H

holmslaw

W

Woodendhead

holmslaw

The MCB only disconnects the live wire but the RCD is connected to the neutral as well.

What it does to make sure the power going out on the live comes back via the neutral is within a defined rating i.e. 35 mA

Now if the potential between earth and neutral is big enough that is to say there is a voltage between them make a current flow it will flow though the RCD the RCD will detect this as a difference between the out on the live and back on the neutral and trip it. i.e. it is not completely dead.

Hence why some say you should disconnect all wire at the CU to make it completely dead.

A voltage is a potential difference between to point we have diffined earth as 0V.
If we diffined another point as 0V say the moon we could be all standing on 1 million volts, p.s. I have no idea what the potentail defference is, just an example.
Neutral is also considered 0V but in reality it is not the same.
Try it get your multi meter on a AC setting (make sure N and E are not live with a neon or whatever you use) and then test it now the impedance between them is about 200ohm so I am told (It might just be a big tank of water at the sub-station), so you can use ohms law and get the current.
Oh yes and that might just prove to you it is not dead even with the MCB off.

You might even find a voltage on the live as another circuit close by could be inducing a voltage in your dead circuit.
 
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Its a bit like saying 'how do you kill a dead person???'
What is?

Saying that an RCD can be tripped by a fault on a circuit whose MCB is off?

I don't think so.

And neither do you, but it will be instructive to see if you continue to pretend that you do.
 
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holmslaw

The MCB only disconnects the live wire but the RCD is connected to the neutral as well.

What it does to make sure the power going out on the live comes back via the neutral is within a defined rating i.e. 35 mA

Now if the potential between earth and neutral is big enough that is to say there is a voltage between them make a current flow it will flow though the RCD the RCD will detect this as a difference between the out on the live and back on the neutral and trip it. i.e. it is not completely dead.

Hence why some say you should disconnect all wire at the CU to make it completely dead.

A voltage is a potential difference between to point we have diffined earth as 0V.
If we diffined another point as 0V say the moon we could be all standing on 1 million volts, p.s. I have no idea what the potentail defference is, just an example
between the Earth and the Moon? I would say 0Vac
Neutral is also considered 0V but in reality it is not the same.
neutral is the very reason that earth is considered 0V..but I sort of take your point
Try it get your multi meter on a AC setting (make sure N and E are not live with a neon or whatever you use) and then test it now the impedance between them is about 200ohm so I am told
on An ac setting?
(It might just be a big tank of water at the sub-station),
..Eh?
Its a bit like saying 'how do you kill a dead person???'
What is?

Saying that an RCD can be tripped by a fault on a circuit whose MCB is off?

I don't think so.

And neither do you,

And neither do I
 
W

Woodendhead

between the Earth and the Moon? I would say 0Vac
You might be right then again you can't prove that (unless you have a very long wire on your meter)

neutral is the very reason that earth is considered 0V..but I sort of take your point
No the reason is we need a point of reference, the earth make a good point we can use all over the Earth.
on An ac setting?
Any induced voltages will be AC you could also have a DC standing voltage with a AC induced one on top.

(It might just be a big tank of water at the sub-station)
Okay just something I was told once a long time ago but it sort of makes sense if the two are different you would need a resister of some kind between them one that could disapate a large amount of energy a tank of water would do the trick.

But then again it could be just the same BS you get told at school in science.
 

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