RCD Tripping

[DanMulhare]

I’ve a new self build house that I’ve moved into prior to the lockdown in the UK.
100amp supply from Scottish power.

Within my Consumer Unit I’ve an 80A RCD which is serving the following:
B40 MCB (garage, not connected)
B40 MCB (oven)
B32 MCB (upstairs ring sockets)
B32 MCB (outdoor and attic sockets, both separate radials)
B20 MCB (living room radial sockets)
B6 MCB (outdoor lights)
B6 MCB (kitchen and living room lights)
B6 MCB (rest of upstairs lights)

And I’ve a 63A RCD serving the other half of the CU:
B32 MCB (Dn Stairs ring socket 1)
B32 MCB (Dn Stairs ring socket 2)
B20 MCB (kitchen radial sockets)
B16 MCB (Central heating, boiler and UFH)
B6 MCB (Dn stairs lights)
B6 MCB (smoke alarms)

Just before the lockdown my mate who’s a qualified electrician fitted a new range master oven with 6mm 3 core and that’s the only addition to the house in months now. The oven has two separate ovens and a grill. The second oven is now not working but this happened a few days ago and if just came back on.

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.

The Issue I’m having is the 80A RCD keep tripping and it does so even if I use something on the other side of the CU, ive used an angle grinder on the down stairs ring and it works fine but It trips the 80A RCD which should be independent.

I’ve done a continuity test on all rings and everything is Ok.

The RCD is periodically tripping now with nothing plugged in.

This started three days ago and when I couldnt Get it to work I left it along and it worked fine again for 24 hrs and now faults again.

On occasion when I turn off the mcb’s, the rcd won’t reset.

Ive also traced all cables the lives and neutrals are all going to the correct RCD.

Any help would be appreciated. I’ve asked a few mates who are sparks but my mate who’s done all the wiring is self isolating with his mother who’s not well so I don’t want to bother him.

 

[pc1966]

Strange "unrelated" tripping of RCD is often down to a neutral to neutral fault, or more likely a neutral to earth fault. In both cases there my not be much voltage present so not much current flows, but if something large is used or an unfortunate combination of things, enough can flow to trip the RCD action (as it sees N != L).

Far more serious are cross-connected rings but probably not here (i.e. "ring 1 end 1" is accidentally paired with "ring 2 end 1or2" and not with "ring 1 end 2").
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If you test the RCD a couple of times with all load MCB off is it reliable to trip and reset?

A faulty RCD is always a simple explanation, but it tripping on an unrelated circuit sounds a lot like a neutral to earth fault somewhere.

 

[DanMulhare]

Far more serious are cross-connected rings but probably not here (i.e. "ring 1 end 1" is accidentally paired with "ring 2 end 1or2" and not with "ring 1 end 2").

Thanks for answer, any tips for carrying out some checks?

Surely the ring ends couldn’t be mixed up as you describe as the continuity test on all rings came back OK and I’ve been using heavy load equipment for months with no issues until three days ago.

 

[pc1966]

Tip No. 1 be very careful to isolate everything!

You have a separate isolator switch so you should not have to worry about exposed permanent live bits near the incoming switches.

If you can get the neutrals out (in pairs for ring circuits) then check if there is any resistance to earth, that is the most likely reason. Note the fault could be in something that is plugged in, in which case it would depend on the 13A socket being switched on.

Ideally you would test the house wiring independently by unplugging everything first, and if nothing found then check the appliances for earth faults.

 

[telectrix]

agree it's most likelya N-E fault, probably on the circuits protected by the RCD that isn't tripping. only occurring when a fair size load is applied. an exhaustive dead test on (disconnected) neutrals to E will find the fault.

 

[pc1966]

You don't need to remove the live wires as you can simply turn off all MCB to isolate that end. Otherwise you might be testing N1 -> L1 -> L2 -> N2 fault -> E by mistake.
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Also if your fault is a fixed appliance then it would depend on the state of any double-pole isolator switch. So best to turn all on (after powering off) and hopefully you would find at least roughly where a fault might be first, then to locate where along the cable(s) it has occurred.

 

[radiohead]

Ideally you would test the house wiring independently by unplugging everything first, and if nothing found then check the appliances for earth faults.

I can never understand this approach.
Personally in this scenario I want to do a global insulation resistance test to earth with everything undisturbed and as it is when the tripping is happening. That will verify that there is or isn't an IR issue, if there is a poor reading then start disconnecting stuff.

 

[pc1966]

Good point by @radiohead with it isolated as-is you can first check ALL neutral to earth for signs of such a fault!

 

[pc1966]

I made the schoolboy error of assuming it was a N-E fault and then thinking of how to find it. Whereas @radiohead has the right approach of checking if that is a likely reason by doing an Insulation Resistance (IR) test first.

Then if it is found that there is a N-E fault you proceed to try isolating the location by unplugging/switching stuff off, and if that fails you start checking the fixed wiring of the mains distribution system to isolate which circuit it is one.

Now if you have a N-E fault tripping the RCD it is probably a low-ish resistance so should show up when testing with a multimeter or continuity tester. More generally though you would use a meter capable of testing insulation at 250V (in case electronics are still attached) or 500V (for proper verification of mains insulation), but that is unlikely to be in a DIY toolbox.

 

[James]

The above advice is great and if followed in a thorough and logical way, will undoubtedly find the fault.

one quick thing that is probably worth checking out before going to all the trouble of fault finding a random fault somewhere in the house.

OVEN, you have said that there is a fault with it, are you certain that it is fully disconnected from the board? Live and Neutral connections?

a N to E fault in one of the elements could cause the faults that you are experiencing even when the live is disconnected.

just a hunch but worth checking out.

 

[DanMulhare]

The above advice is great and if followed in a thorough and logical way, will undoubtedly find the fault.

one quick thing that is probably worth checking out before going to all the trouble of fault finding a random fault somewhere in the house.

OVEN, you have said that there is a fault with it, are you certain that it is fully disconnected from the board? Live and Neutral connections?

a N to E fault in one of the elements could cause the faults that you are experiencing even when the live is disconnected.

just a hunch but worth checking out.

The oven has two fan ovens and a separate grill. Throughout the last three days the main oven has always tripped the RCD but when I reset RCD power does go back to the main oven as the red led will light up just for a split second prior to tripping the RCD. The second oven however has not worked when the fault has been occurring but does work again some house later. This happened three days ago and has just happened now.

Everything appears to be working fine and I’ve not had an opportunity to undertake any asditional fault finding for the past number of hours.

The reason I didn’t think it was an issue with the oven was because I did fully disconnect the live and neutral wires, but not the earth and the issue of the tripping 80A RCD still kept happening earlier. Now it’s as if everything is working fine again which is really frustrating.

 

[pc1966]

Ah, intermittent faults really are the Devil's work!

Obvious thing is to check the condition of the cable at the oven outlet in case moving it has caused/released a fault. But more generally you should try tomorrow (if possible, and light so all power can go off safely, etc) to check for a N-E short. That is another reason for the high voltage insulation test, if you have a very small gap it might come and go but with a decent voltage it tends to show up once more.

If you suspect it is related to there and have some assistance then one can be looking at the test meter while the other gives the cable(s) a little wiggle to see if it provokes anything. If you are alone, then you might be able to do the same by checking for N-E at the oven outlet (again, carefully making sure the supply is isolated) while wiggling stuff.

But it might not be the oven, as a N-E style fault tends to trip RCDs when there is a lot of current flowing and ovens (and showers) are often the biggest power draw. Motors starting also cause a big surge in current, another typical trigger.

 

[DanMulhare]

Ah, intermittent faults really are the Devil's work!

Obvious thing is to check the condition of the cable at the oven outlet in case moving it has caused/released a fault. But more generally you should try tomorrow (if possible, and light so all power can go off safely, etc) to check for a N-E short. That is another reason for the high voltage insulation test, if you have a very small gap it might come and go but with a decent voltage it tends to show up once more.

If you suspect it is related to there and have some assistance then one can be looking at the test meter while the other gives the cable(s) a little wiggle to see if it provokes anything. If you are alone, then you might be able to do the same by checking for N-E at the oven outlet (again, carefully making sure the supply is isolated) while wiggling stuff.

But it might not be the oven, as a N-E style fault tends to trip RCDs when there is a lot of current flowing and ovens (and showers) are often the biggest power draw. Motors starting also cause a big surge in current, another typical trigger.

I’ve not been able to check anything today due to the day job but this morning I did just plug in a LAP MS6860D Socket Tester from Screwfix into sockets to see if there was any issue and nothing is showing up.

The condition of the cable at the oven is 100% as it’s only just been installed and all connection are good. I checked this as part of the first round of fault finding, thinking a good connection hadn’t been made a few week earlier and it had come loose.

 

[pc1966]

Unfortunately a socket tester will not find this sort of a fault. All they show up (reliably) is if the L & N are swapped, or if E is totally missing. Both are serious faults, but there are many problems they are no good at finding (high R earth, N & E swapped, for example). You can get better socket testers in the £40-60 range that do test earth R and so are better, but they still won't show up E-N swap (if you are on RCD though it will notice immediately).

My last point (on the previous post) was not very clear, but what I was trying to say is it might not be the oven circuit that is the fault, it might be that the oven is the only single load to cause enough current to trip via a fault in another circuit.

To test this the minimum you will need to check this is a multimeter or a continuity tester. If you plan on buying something then I would always advise that you get an instrument that is rated for mains use. This is very different from simply having a 600V scale!

Mains supplies have two things you have to be aware of:

1. The nominal 240V AC can have spikes to several kV on top
2. There is almost limitless energy available

So if something goes wrong (say to try measuring volts with the meter accidentally set to current, or an over-voltage causes a failure) then if it cant safely interrupt the fault current it will literally explode in flames in your face! Sadly the majority of electricians killed by electric accidents die from burns, not shock.

Any meter for mains use will have a CAT rating of III or (preferably) IV as covered here:

Measurement category - Wikipedia

en.wikipedia.org

One that seems well-liked here is the Testo 760-1 available, for example, here:

Testo 760-1 Digital Multimeter

Unique auto-test setup function Performs all basic electrical measurements Large backlit LCD screen with analogue bargraph

www.tester.co.uk

A cheaper and simpler tool for general use is a combined voltage tester and continuity tester. I personally don't like my voltage tester to sequel when it sees a short, but it is a different sound and many find it convenient. For example:

Martindale VT12 Voltage & Continuity Tester

GS38 compliant tips LED indication of voltage up to 690V Continuity testing with buzzer

www.tester.co.uk

If you want to check something is dead before working, that is the sort of tool you should use as there is far less to go wrong than a multimeter. Even so, you should prove the tester is working before and after you have "proved" the circuit in question is dead.
[automerge]1588089411[/automerge]
This multimeter is cheaper and if you find its data sheet then it claims to be CAT-III to 600V so is adequate for mains work:

Martindale MM39 Standard Digital Multimeter - Auto Ranging

Measures AC and DC voltages and currents, resistance and frequency Large dial for easy selection of test mode Automatically detects measurement range, requiring minimal input

www.tester.co.uk

 

[DanMulhare]

Unfortunately a socket tester will not find this sort of a fault. All they show up (reliably) is if the L & N are swapped, or if E is totally missing. Both are serious faults, but there are many problems they are no good at finding (high R earth, N & E swapped, for example). You can get better socket testers in the £40-60 range that do test earth R and so are better, but they still won't show up E-N swap (if you are on RCD though it will notice immediately).

My last point (on the previous post) was not very clear, but what I was trying to say is it might not be the oven circuit that is the fault, it might be that the oven is the only single load to cause enough current to trip via a fault in another circuit.

To test this the minimum you will need to check this is a multimeter or a continuity tester. If you plan on buying something then I would always advise that you get an instrument that is rated for mains use. This is very different from simply having a 600V scale!

Mains supplies have two things you have to be aware of:

1. The nominal 240V AC can have spikes to several kV on top
2. There is almost limitless energy available

So if something goes wrong (say to try measuring volts with the meter accidentally set to current, or an over-voltage causes a failure) then if it cant safely interrupt the fault current it will literally explode in flames in your face! Sadly the majority of electricians killed by electric accidents die from burns, not shock.

Any meter for mains use will have a CAT rating of III or (preferably) IV as covered here:

Measurement category - Wikipedia

en.wikipedia.org

One that seems well-liked here is the Testo 760-1 available, for example, here:

Testo 760-1 Digital Multimeter

Unique auto-test setup function Performs all basic electrical measurements Large backlit LCD screen with analogue bargraph

www.tester.co.uk

A cheaper and simpler tool for general use is a combined voltage tester and continuity tester. I personally don't like my voltage tester to sequel when it sees a short, but it is a different sound and many find it convenient. For example:

Martindale VT12 Voltage & Continuity Tester

GS38 compliant tips LED indication of voltage up to 690V Continuity testing with buzzer

www.tester.co.uk

If you want to check something is dead before working, that is the sort of tool you should use as there is far less to go wrong than a multimeter. Even so, you should prove the tester is working before and after you have "proved" the circuit in question is dead.
[automerge]1588089411[/automerge]
This multimeter is cheaper and if you find its data sheet then it claims to be CAT-III to 600V so is adequate for mains work:

Martindale MM39 Standard Digital Multimeter - Auto Ranging

Measures AC and DC voltages and currents, resistance and frequency Large dial for easy selection of test mode Automatically detects measurement range, requiring minimal input

www.tester.co.uk

Neillian

Unfortunately a socket tester will not find this sort of a fault. All they show up (reliably) is if the L & N are swapped, or if E is totally missing. Both are serious faults, but there are many problems they are no good at finding (high R earth, N & E swapped, for example). You can get better socket testers in the £40-60 range that do test earth R and so are better, but they still won't show up E-N swap (if you are on RCD though it will notice immediately).

My last point (on the previous post) was not very clear, but what I was trying to say is it might not be the oven circuit that is the fault, it might be that the oven is the only single load to cause enough current to trip via a fault in another circuit.

To test this the minimum you will need to check this is a multimeter or a continuity tester. If you plan on buying something then I would always advise that you get an instrument that is rated for mains use. This is very different from simply having a 600V scale!

Mains supplies have two things you have to be aware of:

1. The nominal 240V AC can have spikes to several kV on top
2. There is almost limitless energy available

So if something goes wrong (say to try measuring volts with the meter accidentally set to current, or an over-voltage causes a failure) then if it cant safely interrupt the fault current it will literally explode in flames in your face! Sadly the majority of electricians killed by electric accidents die from burns, not shock.

Any meter for mains use will have a CAT rating of III or (preferably) IV as covered here:

Measurement category - Wikipedia

en.wikipedia.org

One that seems well-liked here is the Testo 760-1 available, for example, here:

Testo 760-1 Digital Multimeter

Unique auto-test setup function Performs all basic electrical measurements Large backlit LCD screen with analogue bargraph

www.tester.co.uk

A cheaper and simpler tool for general use is a combined voltage tester and continuity tester. I personally don't like my voltage tester to sequel when it sees a short, but it is a different sound and many find it convenient. For example:

Martindale VT12 Voltage & Continuity Tester

GS38 compliant tips LED indication of voltage up to 690V Continuity testing with buzzer

www.tester.co.uk

If you want to check something is dead before working, that is the sort of tool you should use as there is far less to go wrong than a multimeter. Even so, you should prove the tester is working before and after you have "proved" the circuit in question is dead.
[automerge]1588089411[/automerge]
This multimeter is cheaper and if you find its data sheet then it claims to be CAT-III to 600V so is adequate for mains work:

Martindale MM39 Standard Digital Multimeter - Auto Ranging

Measures AC and DC voltages and currents, resistance and frequency Large dial for easy selection of test mode Automatically detects measurement range, requiring minimal input

www.tester.co.uk

[/QUO

Unfortunately a socket tester will not find this sort of a fault. All they show up (reliably) is if the L & N are swapped, or if E is totally missing. Both are serious faults, but there are many problems they are no good at finding (high R earth, N & E swapped, for example). You can get better socket testers in the £40-60 range that do test earth R and so are better, but they still won't show up E-N swap (if you are on RCD though it will notice immediately).

My last point (on the previous post) was not very clear, but what I was trying to say is it might not be the oven circuit that is the fault, it might be that the oven is the only single load to cause enough current to trip via a fault in another circuit.

To test this the minimum you will need to check this is a multimeter or a continuity tester. If you plan on buying something then I would always advise that you get an instrument that is rated for mains use. This is very different from simply having a 600V scale!

Mains supplies have two things you have to be aware of:

1. The nominal 240V AC can have spikes to several kV on top
2. There is almost limitless energy available

So if something goes wrong (say to try measuring volts with the meter accidentally set to current, or an over-voltage causes a failure) then if it cant safely interrupt the fault current it will literally explode in flames in your face! Sadly the majority of electricians killed by electric accidents die from burns, not shock.

Any meter for mains use will have a CAT rating of III or (preferably) IV as covered here:

Measurement category - Wikipedia

en.wikipedia.org

One that seems well-liked here is the Testo 760-1 available, for example, here:

Testo 760-1 Digital Multimeter

Unique auto-test setup function Performs all basic electrical measurements Large backlit LCD screen with analogue bargraph

www.tester.co.uk

A cheaper and simpler tool for general use is a combined voltage tester and continuity tester. I personally don't like my voltage tester to sequel when it sees a short, but it is a different sound and many find it convenient. For example:

Martindale VT12 Voltage & Continuity Tester

GS38 compliant tips LED indication of voltage up to 690V Continuity testing with buzzer

www.tester.co.uk

If you want to check something is dead before working, that is the sort of tool you should use as there is far less to go wrong than a multimeter. Even so, you should prove the tester is working before and after you have "proved" the circuit in question is dead.
[automerge]1588089411[/automerge]
This multimeter is cheaper and if you find its data sheet then it claims to be CAT-III to 600V so is adequate for mains work:

Martindale MM39 Standard Digital Multimeter - Auto Ranging

Measures AC and DC voltages and currents, resistance and frequency Large dial for easy selection of test mode Automatically detects measurement range, requiring minimal input

www.tester.co.uk

Brilliant thanks, I understand now. The fault is most likely with another circuit as when I completely disconnected the oven wires and tried another high usage power tool it also tripped the RCD.

I’ve already done a continuity test on all Neutral and live wires on all Ring mains.

I’ve a multimeter alread. https://www.tlc-direct.co.uk/Technical/DataSheets/TestMeters/TMTMTM01_Instructions.pdf

However as I’m a novice, I’m not well versed in how to check for a neutral earth fault etc, I’ve searched online for tutorials etc but can’t find anything.

 

[pc1966]

While I would be reluctant to use that meter on an energised system as it has no CAT rating given (though it has some protection) you will be checking with the power off so it is perfectly safe.

The first check (again, with the main isolator off so whole system dead) is to look at the resistance between all the N and E circuits with everything as-is. You can get to this easily in the CU as there will be a terminal strip for earth and probably two for neutral (but all neutrals are connected, just they route through different RCD). Just in case an RCD is tripped, you can check from each N strip to the common E strip.

You should see infinite resistance from N to E with the input isolated. If you see any resistance reading below 1M ohm then there is a fault.

To make life easier, manually trip the RCDs and check if that has isolated the N-E fault to one RCD's set. If present, it will probably be one of the 80A's set as it is tripping.

If you have established there is some fault between N and E then you can proceed to disconnect all appliances (realted to the RCD set where there is a fault) that it is practical do do, in order to establish if the fault is in one of them. If you still see an E-N resistance below 1M then the fault must be in the fixed wiring or a non-detached appliance.

At this point you can open all MCB and try lifting each circuit's neutral(s) at the CU to check those lifted wire(s) to E. When you disconnect the circuit with the fault you should see the remaining N to E go open (above 1M) and see the fault resistance on the wire(s) you have disconnected (so from them to E). For each that checks out OK replace the neutral(s) at that point to the hole they came from in the N terminal block to avoid later confusion.

But if you find a disconnected N that is going to E, double-check that the remaining N are now isolated in case you are very unlucky and have a double fault!

 

[DanMulhare]

While I would be reluctant to use that meter on an energised system as it has no CAT rating given (though it has some protection) you will be checking with the power off so it is perfectly safe.

The first check (again, with the main isolator off so whole system dead) is to look at the resistance between all the N and E circuits with everything as-is. You can get to this easily in the CU as there will be a terminal strip for earth and probably two for neutral (but all neutrals are connected, just they route through different RCD). Just in case an RCD is tripped, you can check from each N strip to the common E strip.

You should see infinite resistance from N to E with the input isolated. If you see any resistance reading below 1M ohm then there is a fault.

To make life easier, manually trip the RCDs and check if that has isolated the N-E fault to one RCD's set. If present, it will probably be one of the 80A's set as it is tripping.

If you have established there is some fault between N and E then you can proceed to disconnect all appliances (realted to the RCD set where there is a fault) that it is practical do do, in order to establish if the fault is in one of them. If you still see an E-N resistance below 1M then the fault must be in the fixed wiring or a non-detached appliance.

At this point you can open all MCB and try lifting each circuit's neutral(s) at the CU to check those lifted wire(s) to E. When you disconnect the circuit with the fault you should see the remaining N to E go open (above 1M) and see the fault resistance on the wire(s) you have disconnected (so from them to E). For each that checks out OK replace the neutral(s) at that point to the hole they came from in the N terminal block to avoid later confusion.

But if you find a disconnected N that is going to E, double-check that the remaining N are now isolated in case you are very unlucky and have a double fault!

Thanks,
Another novice question, to test the resistance do I need to set what ohm level to test, I’ve tried to send a photo of the multimeter but it won’t upload for some reason but you can see on the manual that I put the link up of what options are available. Also on page 9 in the manual it says to select the desired range.

Apologies for the novice question. Thanks.

 

[pc1966]

Another novice question, to test the resistance do I need to set what ohm level to test,

Probably you will see the fault on most of the ranges, but as the conventional limit for an insulation fault is 0.5M or 1M (but at high test voltages) you could initially set it to the 2M range, if it reads very low then check again on the 200 Ohm range.

 

[DanMulhare]

Probably you will see the fault on most of the ranges, but as the conventional limit for an insulation fault is 0.5M or 1M (but at high test voltages) you could initially set it to the 2M range, if it reads very low then check again on the 200 Ohm range.

Does it matter if the red or black lead goes to N or E, are they interchangeable? I’ve done one test to check if they were interchangeable and it would appear so.

Test 1
Testing at 2M
Both RCD in the on position
Red probe to N
Black probe to E
I’m getting readings that continue to go up starting well below 1 ( 0.3) up to 1.3 if I hold the probes long enough but always going up with some fluctuations.

Test 2 (probes swapped)
Testing at 2M
Both RCD in the on position
Red probe to E
Black probe to N
I’m getting readings that continue to go up starting well below 1 (0.4) up to 1.3 if I hold the probes long enough but always going up with some fluctuations.

Test 3
Testing at 2M
80A RCD in the off position
63A RCD in the on position
Red probe to E
Black probe to N
I’m getting readings that continue to go up starting at 1.1 and go up and past 1.2 within a matter of seconds compared to the minutes it took for the pervious two test, again always going up with some fluctuations.

Test 4
80A RCD in the on position
63A RCD in the off position
Red probe to E
Black probe to N
I’m getting readings that continue to go up starting at 0.9 and go up and past 1.2 within a matter of seconds compared to the minutes it took for the pervious two test, again always going up with some fluctuations.

Conclusion:
There doesn’t appear to be any difference in the results of any of the four test and so I’m still confused as to what the problem might be as all were reading above 1 and continuing to rise.

I’ve tried uploading photos of the test results but not had any luck again

 

[pc1966]

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)