RCD tripping. TT. where could be the fault? (FIRE ALARM SENSOR)

[JohnW2]

I was using your wording.

It was benard's wording, not mine. I don't recall ever having heard the phrase before - which is why I put his words in quotes when I referred to them.

Yes but the Neutral current from the load is not increased because there is a leak elsewhere - so no sneak current at the bottom RCD; just the load current.

Exactly - and that's the reason why (unless I'm going mad), despite what bernard suggested, the bottom RCD will not trip I was merely seeing if I could think of a reason why he had shown his ('dotted green') "sneak path" going through the bottom RCD ....

Agreed, but that small proportion is part of the load return current to the joint which is then missing from the back to tranny return path upstream of the joint.

Exactly, and again my point - and the reason why I can't see why the bottom RCD could trip.

Kind Regards, John

 

[flameport]

Tomorrow, I will have the faulty section determined, after removing the ring wires from consumer unit

The test is not valid if the wires are still connected to the consumer unit, as you will be testing most of the other connected circuits at the same time.

That's why the first step is to

Disconnect the wires for that circuit at the consumer unit,

 

[JohnW2]

..... I have now removed all 8 double sockets leaving bare, exposed wires. .... Tested all wiring, all ends, with multimeter: continuity ~0.5 Ohm and no shorts between L/N/E ...except... for the first and last ends which both read 40 Ohm between N and E.
...... Fault is somewhere between the consumer unit and either first or last socket point. ..... Tomorrow, I will have the faulty section determined, after removing the ring wires from consumer unit and will take it from there.

As flameport has implied, IF your testing so far has been with the 'first and last ends' still connected at the CU, then all bets are off - and low measured N-E resistance in those first/last bits of cable is very probably not indicative of any sort of 'fault'. Indeed, if you have a TN-C-S installation, one would expect that measured N-E resistance to be much closer to zero than 40Ω, since the N and 'E' feeds to the CU are joined together close to the CU in that situation.

Kind Regards, John

 

[bernardgreen]

Am I missing something?

No.... I think my reasoning is flawed,

 

[mrmisior]

i think that this is TT. plastic sheath on the incoming cable which comes from overhead pole. earth provided by a rod.

 

[JohnW2]

i think that this is TT. plastic sheath on the incoming cable which comes from overhead pole. earth provided by a rod.

Your photo is certainly consistent with that.

In that case, if you measured the N-E resistance of the two end bits of cable whilst they were still connected to the CU, then I would think that 40Ω is quite a credible ('normal') result to get - as I suggested, not indicative of any fault.

If your previous measurements were with the cables connected to the CU, then the simple answer to the above will be determined by seeing what results you get when you disconnect the cables from the CU.

Kind Regards, John

 

[mrmisior]

Update on my troubles:
Dropped the wires for the ring in question 'downstairs sockets'.
There is no N-E fault within this circuit - it's open according to 9V multimeter.

Regardless whether the 'downstairs sockets' is wired to CU or not, there is a consistent ~40 Ohm between Neutral bus and Earth bus at the CU - that is when either RCD is ON (otherwise when both RCDs are OFF - open).

I was going to keep looking for that N-E fault

Since the reading is correct, i am still puzzled why this particular circuit is/was tripping the one RCD.
However, i found a little bit of moisture and corrosion at a far end single socket in a corner of the house. Socket itself was never used.
Luckily, this was the last/first socket in the ring.
I am going to turn this ring into a radial with a B16A MCB (had a spare in the CU). Blank and isolate the 'suspect' socket. Pull the cable out of CU.
Should I do anything else before calling electrician with a megger to certify?

 

[JohnW2]

Update on my troubles: Dropped the wires for the ring in question 'downstairs sockets'. There is no N-E fault within this circuit - it's open according to 9V multimeter.

Thanks. That's as I expected.

Regardless whether the 'downstairs sockets' is wired to CU or not, there is a consistent ~40 Ohm between Neutral bus and Earth bus at the CU - that is when either RCD is ON (otherwise when both RCDs are OFF - open).

As I've explained, that it what one would expect. With a TN installation, it would be much lower,but for TT, 40Ω is credible.

I was going to keep looking for that N-E fault

Although it remains possible that the RCD tripping is due to an N-E fault (one which would on;ly be detected by testing at a much higher voltage), there is no longer any specific reason to think that there is necessarily any N-E fault to find.

Since the reading is correct, i am still puzzled why this particular circuit is/was tripping the one RCD.

Unfortunately, that's often the case with RCD trips. Are you absolutely certain that nothing connected to the circuit has been 'overlooked'?

As you are suggesting, I think you're getting very close to the point at which the only option left will be to call an electrician. Sorry about that!

Kind Regards, John

 

[mrmisior]

Are you absolutely certain that nothing connected to the circuit has been 'overlooked'?

a fire alarm sensor spurred within ceiling. not mentioned on periodic. all other fire alarm equipment is on its own ring, so i could have not suspected that.
anyway, 5 minutes fix.
again. thank you everyone for help!

 

[JohnW2]

a fire alarm sensor spurred within ceiling. not mentioned on periodic. all other fire alarm equipment is on its own ring, so i could have not suspected that.
anyway, 5 minutes fix.

Are you saying that, having found this, you have determined that it was the cause of your RCD trips, and that you therefore have "fixed" the problem?

Kind Regards, John

 

[mrmisior]

Are you saying that, having found this, you have determined that it was the cause of your RCD trips, and that you therefore have "fixed" the problem?

Kind Regards, John

yes, i swapped the smoke sensor with another one to test and no more trips as well as no more trips without one. there is a ceiling plug-in rose for these in each room - it has no exposed live blades so it is safe to leave one empty until i get a replacement.
i follow the routine when reassembling the circuit: http://www.sparkyfacts.co.uk/Inspection-and-Testing-Ring-Circuit-Testing.php
would be nice to also do an actual loop test but don't have the equipment.

 

[JohnW2]

yes, i swapped the smoke sensor with another one to test and no more trips as well as no more trips without one. there is a ceiling plug-in rose for these in each room - it has no exposed live blades so it is safe to leave one empty until i get a replacement.

It's obviously great if you have cured your problem, but I have to say that I don't really understand how this can have occurred.

I thought (maybe wrongly?) that mains-powered smoke sensors/detectors were 'Class II' ('Double-Insulated') and therefore did not have an earth connection (although they may well have an 'earth' terminal, connected to nothing, in which to 'park' a cable's earth wire).

If that were the case with yours, then the only ways in which there could be an L-E or N-E fault/'leak' that could trip an RCD would be for it to either be seriously mechanically damaged or, say, 'filled up with water' - thereby creating an electrical path from L or N to a 'parked' earth wire.

What is the make/model of the smoke sensor in question?

Kind Regards, John