Neutral - Earth faults

[Guest 004]

Had a quick look at a job which has it seems a neutal to earth fault.

Set up TNC-S feeding DB with 5 MCBs, and 1 RCBO feeding lighting circuits.

It was getting late and I was tired, so I suggested to the client you have a possible neutral to earth fault, and the circuit in question will need to be tested for this.

His reply was 'are you sure the neutral to earth fault is on that lighting ciurcuit?, as the all the neutrals are connected on one bar' This took me back a little as the guy obvioulsy had some knowledge. So I said I will need to test said circuit first, and if not conclusive test the rest of DB for said fault.

Just got me to thinking that if the N-E fault is somewhere else, would it in fact trip the RCBO for the lighting circuit?

 

[Hundleton1]

I take it the rcbo is tripping?

The part I don't like about them is you don't know if it trips with overload or earth leakage.

On the rcbo the neutral for the circuit should be going into it then it has a fly lead back to the neutral bar, its basically a separate circuit from the board so if their was an earth leakage yes the rcbo would trip but only if their was a problem on that circuit, problems on other circuits could not cause it as they are not connected to it, the neutrals are linked but behind the rcbo so its not looking for an imbalance there.

Is it constantly tripping or randomly, way forward would be test IR and continuity on all conductors,

If its a lighting circuit earth faults aren't that common, any work been done or any new additions recently ?

No lives or neutrals going into the rcbo that shouldn't be ?

 

[Octopus]

Not enough information to pass a helpful comment.

Why do you think there is a N-E fault? and what is tripping?

 

[Richard Burns]

If you have an RCBO then any fault occurring before the RCBO will not be detected as it is only measuring the supplying circuit and does not care what happens upstream of it. It is similar to saying will an MCB trip if there is a high current in another circuit, and yet your lights do not trip when you turn the oven on!

 

[Guest 004]

^ perfect, and beautifully explained

 

[Farmelectrics]

Richard were do you get your diagrams from and do you up load them

 

[Richard Burns]

Richard were do you get your diagrams from and do you up load them

I just draw them, here I (sorry) use powerpoint and adobe photoshop.
I do upload all the diagrams to the site, sorry mods, I know it consumes server space but I am not comfortable with using online storage, I hate signing up for things and prefer to have easy access to my stuff on the PC (too old fashioned). Though I did sign up for this forum! (I do wonder why sometimes:evil:!)