RCD Tripping

[DanMulhare]

It should not matter which way round for a simple short circuit style of fault. What you see with it initially low "resistance" and then rising is typical of charging capacitance (the meter measures the current flowing, so with a capacitor it is initially large and then drops as its voltage increases towards that of the source due to the accumulated charge).

Now all cable has capacitance, but I'm a little surprised to see enough of it to show up in your test but have no experience with your type of meter to know if that is reasonable or not.

The last thing you would try is a similar set of tests (both directions) using the diode check (arrow symbol) to see if anything electronic appears. Otherwise it looks as if the fault might be intermittent or not as simple as a E-N short (though it really looks like it).

Again if you had access to a proper insulation tester to check as 250V it would answer the question in a more certain manner.
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With the RCD off are you testing from the related strip of neutral terminals? (i.e. checking from RCD output's N to Earth)

Apologies I was testing all back to the corresponding N bar for the 80A RCD.
Further test below.

Test 5a and 5b
Testing at 2M
80A RCD in the off position
63A RCD in the on position
Red probe to E

(5a)
Black probe to N bar for the 80A RCD
Reading goes above 1 immediately and continues to rise
(5b)
Black probe to N bar for the 63A RCD
No reading at all.
I then switched to test at 200M and got a readings between 138 and 156 but only for a second before it would read 1 again.

Test 6a and 6b
Testing at 2M
80A RCD in the on position
63A RCD in the off position
Red probe to E

(6a)
Black probe to N bar for the 80A RCD
Reading goes above 1 immediately and continues to rise

(6b)
Black probe to N bar for the 63A RCD
No reading at all.
I then switched to test at 200M and got a readings between 138 and 197 but only for a second before it would read 1 again.

Conclusion when isolating and testing the separate N bars for both the 80A and 63A RCD’s the tests for the 63A RCD is the one I need to switch from 2M to 200M to see any reading and it’s not consistent or even going up.

Does this mean there is a N-E fault on one of the circuits fed off the 63A RCD?

 

[pc1966]

When you say "read 1 again" is that the maximum the display will show?

Unfortunately none of the readings are pointing to an obvious fault as it should show as only a couple of ohms so practically zero.

This does not mean there is no fault, as a barely touching cable might depend on temperature, or floor movement, etc, to make connection again and that is why usually a high voltage test is used to show cases of marginal contact/insulation.
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Going back to your original post:

I’ve unwired the oven from the CU and when I tried using a chop Saw on the upstairs ring socket it tripped the 80A RCD again.

Did you also unwire the neutral of the oven feed?

 

[DanMulhare]

When you say "read 1 again" is that the maximum the display will show?

Unfortunately none of the readings are pointing to an obvious fault as it should show as only a couple of ohms so practically zero.

This does not mean there is no fault, as a barely touching cable might depend on temperature of floor movement, etc, to make connection again and that is why usually a high voltage test is used to show cases of marginal contact/insulation.

Apologies,
Reading 1 again = what’s on the display screen prior to the test starting.

Would a very labourious but foolproof way to identify a fault be to take off every faceplate and manually check each socket outlet and fused spur?
If I can then identity whether an earth wire is touching a neutral it might make it more obvious.

I maybe should have mentioned previously but I don’t have all Earth wires terminated into backboxes for light fittings just yet as still painting so the light switches in some rooms are hanging off the wall with earth wires exposed.

If this could be the reason I’ll have cemented my status as an amature.

 

[pc1966]

If it is an E-N fault, which is likely but not actually established yet, then most likely is a back-box fault. Causes are typically things like:

• No grommet allowing the cable to be cut by the entrance hole of a metal box
• One of the switch/socket fixing screws crushing a wire and puncturing its insulation.
• No sleeving on earth wires someone managing to contact exposed conductors (which ought not to be possible if stripping length on L & N is correct, etc)

More serious but harder to find is a cable that has been punctured by, say a nail and has gone through the insulation between the N wire and central earth. But also possible are accessories with a fault caused by cable damage, stray bits of metal inside, etc, etc.

You would be much better to try and identify a circuit fault first as that will confirm what you are looking for to narrow down the number of places to look!

 

[DanMulhare]

When you say "read 1 again" is that the maximum the display will show?

Unfortunately none of the readings are pointing to an obvious fault as it should show as only a couple of ohms so practically zero.

This does not mean there is no fault, as a barely touching cable might depend on temperature, or floor movement, etc, to make connection again and that is why usually a high voltage test is used to show cases of marginal contact/insulation.
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Going back to your original post:

I’ve unwired the oven from the CU and when I tried using a chop Saw on the upstairs ring socket it tripped the 80A RCD again.

Did you also unwire the neutral of the oven feed?

Yes I unwired the N aswell from the oven
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If it is an E-N fault, which is likely but not actually established yet, then most likely is a back-box fault. Causes are typically things like:

• No grommet allowing the cable to be cut by the entrance hole of a metal box
• One of the switch/socket fixing screws crushing a wire and puncturing its insulation.
• No sleeving on earth wires someone managing to contact exposed conductors (which ought not to be possible if stripping length on L & N is correct, etc)

More serious but harder to find is a cable that has been punctured by, say a nail and has gone through the insulation between the N wire and central earth. But also possible are accessories with a fault caused by cable damage, stray bits of metal inside, etc, etc.

You would be much better to try and identify a circuit fault first as that will confirm what you are looking for to narrow down the number of places to look!

Ok
Thanks again for all the advice. Need to get to my work for the day now but is there any ways a novice can identify a circuit fault bearing in mind no conclusive results from the continuity test carried out or the E-N fault tests carried out.

 

[telectrix]

when you say you are decorating with switches hanging off, has any plastering been carried out? and have you recently painted walls? both could be a source of damp causing the fault.

 

[DanMulhare]

when you say you are decorating with switches hanging off, has any plastering been carried out? and have you recently painted walls? both could be a source of damp causing the fault.

All plastering was done last August / September with some Touching up still to take place.
Painting was done in November but still needed touching up to be done. So no damp since fault reared it’s head.

 

[pc1966]

Any chance of getting your sparky mate to come out with an insulation tester and at least establish if a fault can be found?

Also it might just depend on some switch setting if the fault is down-stream of a switch, so if you switch off the main isolator then turn on all sockets/switches it might just appear once more.

 

[OnlQQker]

I did fully disconnect the live and neutral wires, but not the earth

What is the Earth connected to on the cooker end?