Downstairs lighting circuit MCB randomly tripping since electrician's visit last week.

[John D v2.0]

not enough current was being created to trip the RCD

very high fault current

damaged cable going straight to earth

Unless it was going via an isolating transformer or possibly has a cleverly balanced tuned circuit connecting to earth, I struggle to understand how a non faulty RCD could pass even 30mA to earth.

Another possibility is there was a fault that only showed up with 500v DC and not normal mains.

 

[JohnW2]

Unless it was going via an isolating transformer or possibly has a cleverly balanced tuned circuit connecting to earth, I struggle to understand how a non faulty RCD could pass even 30mA to earth.

I can understand your struggling, but personally would probably have used stronger language ...

.... it is surely quite impossible that a correctly wired non-faulty RCD could fail to operate in the presence of a L-earth short on a circuit it was protecting?

Kind Regards, John

 

[John D v2.0]

Yeah, I suppose that's the only purpose of an RCD

 

[ban-all-sheds]

.... it is surely quite impossible that a correctly wired non-faulty RCD could fail to operate in the presence of a L-earth short on a circuit it was protecting?

No, it is not quite impossible.

It is just incredibly unlikely.

 

[JohnW2]

No, it is not quite impossible. It is just incredibly unlikely.

I suppose that depends upon what one regards as 'non-faulty'. I would be inclined to take it to refer to an RCD which, when correctly wired, would operate in response to a massive current imbalance (due to an L-E 'short') in a circuit it was protecting - in which case it would be, by definition, impossible for a 'non-faulty' RCD not to operate in that situation (since, if it didn't operate in that situation, it would not be 'non-faulty').

However, this is obviously just semantics, and I'm sure we actually agree about the practical situation.

KInd Regards, John

 

[ban-all-sheds]

No, it is not "semantics". (In the incorrect sense in which you are inclined to use the term.)

A device which is otherwise working but is prevented from doing so by an unknown agency using unknown methods is not faulty.

 

[JohnW2]

A device which is otherwise working but is prevented from doing so by an unknown agency using unknown methods is not faulty.

Maybe, but I don't think that adds anything useful to this discussion.

I have a (large) lawn to mow!

Kind Regards, John

 

[undistilled]

My supposition at the time was that the circuit current was being entirely diverted away from returning back to the RCD hence there was no imbalance for the RCD to register in the sense that the outgoing side of the RCD never sees a return current to measure against. In any case, there was a complete circuit, a live to earth dead short and nothing to trip the RCD. Those pesky electrons must be going somewhere and my opinion was straight down an alternative route which probably also supposes a neutral to earth fault. In the event of a plumber disconnecting a water pipe my assumption was that sh/he could be holding a conductive part of negligible impedence and a juicy current wanting to become part of said plumbers life. Anyway, perhaps just fantasy but I still did a good arse covering exercise.

 

[undistilled]

Edit = complete circuit but not where it is supposed to be.

 

[JohnW2]

My supposition at the time was that the circuit current was being entirely diverted away from returning back to the RCD hence there was no imbalance for the RCD to register in the sense that the outgoing side of the RCD never sees a return current to measure against.

Perhaps you're not expressing yourself clearly enough, but that statement appears to make no sense. If the 'return' (neutral) current through the RCD is zero (or near zero), which is not impossible with a very low impedance L-E fault, where exactly is the L current is coming from? If it is going through the RCD, then there is a massive imbalance, which would cause the RCD to operate. If it's not going through the RCD, then the RCD is not 'correctly wired', which is one of the conditions I mentioned.

Kind Regards, John

 

[undistilled]

So in ccjjcc1 case perhaps a cable melted onto a heating pipe or some other such damage.

 

[JohnW2]

So in ccjjcc1 case perhaps a cable melted onto a heating pipe or some other such damage.

Something like that is obviously possible, although it's fairly unlikley that it would then ever be possible to reset the MCB. In any event, that doesn't explain why his RCD did not operate (assuming it is non-faulty and correctly wired).

Kind Regards, John

 

[333rocky333]

I can understand your struggling, but personally would probably have used stronger language ...

.... it is surely quite impossible that a correctly wired non-faulty RCD could fail to operate in the presence of a L-earth short on a circuit it was protecting?

Kind Regards, John

what if it was an old voltage operated one and the fault was going to earth via the pipework extraneous conductive part

 

[JohnW2]

what if it was an old voltage operated one and the fault was going to earth via the pipework extraneous conductive part

Well, yes, that could explain it - what you describe was the big problem with VOELCBs, which, for the reason you mention, were useless in the presence of parallel paths to earth (i.e. via extraneous-c-ps).

I have to confess that when the OP (and others) referred to an RCD, I assumed that they meant RCD - but any or all of them may possibly have used the wrong term. A VOELCB is, of course not a Residual Current Device - since it detects the actual earth leakage current (IF it is going to earth via the installation's 'earth'), not the 'residual current' (L-N imbalance).

Kind Regards, John

 

[333rocky333]

I just knew you would say its not an RCD
Is that was Residual current refers to, the imbalance, i never realised that