Nuisance RCD Tripping - Diagnosis Advice PLEASE

[2nd Life]

Issue one is the Aga CU. That has a normal main switch (not RCD) and an MCB. So a live-earth fault would not trip the MCB as less that 2 amps of fault current would flow. It would stay on waiting for someone to touch it and that someone would become the path to earth.


Does the test button on the RCD itself work?
If not, then this all becomes even more serious as things won't turn off under fault conditions.

Which make/model MFT is it? It could be a faulty RCD, faulty tester, or the tester being blinded by DC earth leakage.

On the Aga question the plan is to integrate it back into the CU once replaced.

 

[2nd Life]

Issue one is the Aga CU. That has a normal main switch (not RCD) and an MCB. So a live-earth fault would not trip the MCB as less that 2 amps of fault current would flow. It would stay on waiting for someone to touch it and that someone would become the path to earth.


Does the test button on the RCD itself work?
If not, then this all becomes even more serious as things won't turn off under fault conditions.

Which make/model MFT is it? It could be a faulty RCD, faulty tester, or the tester being blinded by DC earth leakage.

Could isolate the Aga for now and swap out the main switch for an RCD to cover us until the CU swap?

 

[2nd Life]

Issue one is the Aga CU. That has a normal main switch (not RCD) and an MCB. So a live-earth fault would not trip the MCB as less that 2 amps of fault current would flow. It would stay on waiting for someone to touch it and that someone would become the path to earth.


Does the test button on the RCD itself work?
If not, then this all becomes even more serious as things won't turn off under fault conditions.

Which make/model MFT is it? It could be a faulty RCD, faulty tester, or the tester being blinded by DC earth leakage.

Or swap the Aga MCB for an RCBO?

 

[DPG]

Or swap the Aga MCB for an RCBO?

Might as well just connect it to the main CU when you swap it. Unless it's going to be a good while before you can get this done?

 

[2nd Life]

Might as well just connect it to the main CU when you swap it. Unless it's going to be a good while before you can get this done?

Hi DPG, no we'll be getting on with it. Particularly given all the problems that the current setup can and is causing. Need to get rid Asap. But it could be weeks. And the RCD issue is daily so really want to check that nut if I can

 

[timhoward]

So bizarrely the ramp test at the socket did not trip the RCD, just get a reading of greater than 33mA where the tester I assume stopped.

Test button works on the RCD in question it looks to be a newer one, but not on the other one after investigation.

I find myself wondering if these two things are linked....
It might be worth double checking that the socket you used to test loses power when you turn the RH RCD off. There are a few cables that look bigger than typical lighting wires the left hand end.

It's possible to test the ramp test function of an MFT by putting a multimeter in series between tester earth wire and earth, set to the mA function, but it helps if the multimeter has a peak hold function.

 

[pc1966]

So bizarrely the ramp test at the socket did not trip the RCD, just get a reading of greater than 33mA where the tester I assume stopped. I tried the speed test too and same result. No trip. For an RCD that seems on the verve of tripping constantly not sure what to make of that. Could be the tester at fault?

It could be any of a number of problems:

• The MFT might be faulty
• The RCD might be faulty
• There might be some leakage that is either desensitising the RCD ("blinding" by DC) or is in opposite direction to the L-E test used, cancelling some of the test current.

So you can look at trying:

• For the 1st point can you check the MFT on someone else's socket? Or get a plug-in RCD adaptor and see if it tests/trips normally.
• Once you know the MFT is working OK then test the RCD at the CU with all MCBs off, and see if that is any better.
• Get a basic clamp meter that allows you to check for leakage, and see what is happening on the tails with all off, and then one MCB at a time on.

 

[2nd Life]

I find myself wondering if these two things are linked....
It might be worth double checking that the socket you used to test loses power when you turn the RH RCD off. There are a few cables that look bigger than typical lighting wires the left hand end.

It's possible to test the ramp test function of an MFT by putting a multimeter in series between tester earth wire and earth, set to the mA function, but it helps if the multimeter has a peak hold function.

Hey Tim. No I mapped every circuit in the house going round with a socket tester etc it's Definitely on the tripping RCD. I'll try the multi meter trick that would help to know the MFT is working. Cheers again

 

[2nd Life]

It could be any of a number of problems:

• The MFT might be faulty
• The RCD might be faulty
• There might be some leakage that is either desensitising the RCD ("blinding" by DC) or is in opposite direction to the L-E test used, cancelling some of the test current.

So you can look at trying:

• For the 1st point can you check the MFT on someone else's socket? Or get a plug-in RCD adaptor and see if it tests/trips normally.
• Once you know the MFT is working OK then test the RCD at the CU with all MCBs off, and see if that is any better.
• Get a basic clamp meter that allows you to check for leakage, and see what is happening on the tails with all off, and then one MCB at a time on.

Thanks for that PC! I think the clamp meter is a must I'll be picking one up asap. At very least this is a fantastic fault finding theory session to build up my knowledge. Thanks to all for the help it's much appreciated

 

[DPG]

Thanks for that PC! I think the clamp meter is a must I'll be picking one up asap. At very least this is a fantastic fault finding theory session to build up my knowledge. Thanks to all for the help it's much appreciated

Make sure it can read accurately at low mA figures.