RCD neutral fault.

[La Poste]

Greetings.
I am an electrician wannabee and am doing a spot of labouring for an electrician. Maybe not a wise career move at this time but it's too late to change now.
My electrician boss and I were having a discussion about why an RCD would trip even if all the MCBS connected to it were switched off and he said it was caused by an earth neutral fault.
I asked how can there be an earth neutral fault when all the circuits supplied by that RCD were switched off and hence no power was going through the RCD. He didn't know the answer and neither do I.
I was wondering if anyone could shed some light on this.
I thought RCD's measured the balance between phase and neutral and tripped if their was an imbalance IE a phase or neutral earth fault only if the circuits are on, so how can an RCD trip if all the circuit MCB's it supplies are in the off position?
The electrician removed all the neutral wires from each circuit one by one until he found the circuit that was causing the RCD to trip, all this was done with the MCB's in the off position.
The main switch to the fuse board was in the on position.
Thanks.

 

[bugsy]

Ever wondered why op's prefer RCBO'S ?

 

[pennywise]

If you read this entire thread through to the end you should get the answer.

http://www.electriciansforums.net/electrical-forum-general-electrical-forum/14963-fused-spur-faults.html

 

[La Poste]

Ok thanks for the replies I have read the thread.
The one thing that puzzles me is if you have an individul cable not connected to any other circuits just an RCD and mcb.
There is no possibility of leakage down the neutral from other circuits because this is just one wire supplying one circuit.
Let's say this wire supplies a lighting circuit.
The neutral and earth of this wire/ circuit are connected at the board as they should be but this is the only circuit supplied by this RCD.
You have the MCB switched off and you touch the neutral and earth wires of the cable/ circuit together.
Does this trip the RCD?
If it trips the RCD could someone please explain why?
It's very puzzling me.
Thanks.

 

[Jabbajaws]

Hi there, La Poste.

Although l cannot give you the answers, you are looking for, l am also eager to find out the answer, as l believe that l have activated an RCD in this way.

I was doing a job for a friend, the mcb was isolated, yet when l touched the neutral & earth wires together, of the circuit l was working on, l activated the RCD by doing so.

After putting this to my college tutor, he told me that this was impossible and that l must be mistaken.

I beg to differ with his explaination, as l activated the RCD with the circuit off on three different occasions. Please can anyone shed some light as to how this could happen?

 

[mogga]

its the same if you test with "test lamps" e-n will trip even without load or live present

 

[WayneL]

Ok thanks for the replies I have read the thread.
The one thing that puzzles me is if you have an individul cable not connected to any other circuits just an RCD and mcb.
There is no possibility of leakage down the neutral from other circuits because this is just one wire supplying one circuit.
Let's say this wire supplies a lighting circuit.
The neutral and earth of this wire/ circuit are connected at the board as they should be but this is the only circuit supplied by this RCD.
You have the MCB switched off and you touch the neutral and earth wires of the cable/ circuit together.
Does this trip the RCD?
If it trips the RCD could someone please explain why?
It's very puzzling me.
Thanks.

I think the simplest way to explain it is that the neutrals of an installation are all connected together at some point - whether it be the neutral bar, the main isolator, whatever.

On that basis, if any circuit is closed (ie current flowing), then the current is returning down the neutral and heading for the supply transformer.

Even, as you describe, if you have one line conductor through the RCD, through an MCB (open), up to the load and then the neutral back to the RCD, the other side of the RCD is still connected to the other neutrals of the installation.

If you then introduce a N - E fault on your RCD circuit, you are introducing a parallel path back to the transformer via earth.

Any current flowing in other circuits via the neutral could flow up your isolated circuit's neutral and down to earth - if this current was more than 25/30 mA, then the RCD could trip, as it is only flowing in the neutral and not in the isolated line conductor - an imbalance

 

[La Poste]

Thanks for the reply WayneL. That makes a lot of sense.

 

[martysparky]

I tripped the RCD the other day for the same reason..


Thats why you must remove live, neutral, cpc and re connect cpc, neutral, live.

as YOU CAN GET A SHOCK FROM THE NEUTRAL

I think WayneL explained this in another thread the other day..


EDIT pennywise already linked it. lol

 

[dixon9]

An RCD can also trip through an upstream fault.

 

[WayneL]

An RCD can also trip through an upstream fault.

You're going to have to explain that one.

 

[Jabbajaws]

I was just thinking that myself WayneL. What is an upstream fault?

Never heard of that before...

 

[WayneL]

What is an upstream fault?

He means an earth fault occuring on the supply side of the RCD - which is unlikely to be detected by the RCD(never say never), as what is flowing in and out of the RCD will be unaffected, so no imbalance.

 

[Jabbajaws]

Thanks for clarifying that for me WayneL. I was confused somewhat when l first read that...

 

[dlicheri]

The RCD works on the basic principle of electromagnetism. You have a live current, that goes through a coil on the RCD, generating a magnetic field (A), then to the load and returns on the neutral of the RCD producing a magnetic field (B) that is 180 degrees out of phase with the A. Therefore, A magnetic field is annulled by B magnetic field. Should there be a difference between the A and B, as it is an alternating current, it will produce a field that is proportional to the difference (C). If the current is big enough, it will produce a magnetic field C on a separate winding on the RCD solenoid that if big enough, it will cause the solenoid to activate or trip and cut the supply. Notice the statement big enough which is key to why your RCD is operating when you short N-E as there will always be a very small current present due to a potential difference between conductors or imbalances in transformer circuit.