RCBO X5 trip time out, caused by washing machine

[Midwest]

I've used a particular socket in my kitchen to carry out monthly calibration checks (ZS & RCD) on my MFT, for several months with no issues. Last week I carried out my tests on my MFT and the x5 RCD test timed out at 40ms. Thinking I had a faulty RCBO, I replaced today, but the fault remained. Disconnected all loads & appliances, carried ou R1, Rn & R2 tests, insulation restance test & ZS. All good, so decided to carry out auto rcd rest after pluging back each appliance, and the washing machine stops the RCBO tripping at x5. Not PAT tested trained, I suspect some earth>live>neutral fault, but can someone explain why the fault would stop the RCBO tripping at x5

 

[Badged01]

Strange, I know some white goods can 'leak' some earth leakage and function perfectly without tripping an RCD --- i.e. the leakage is below 30mA. I would have thought that if the washer was 'leaking' a little (or its an accumalation of leakage from all you have plugged in) then the RCD test would actually trip earlier and your time would be quicker?

 

[Midwest]

Nope, tried it a couple of times and it caused the rcbo not to trip @ x5. I've experienced a neutral to earth fault before on a cable that stopped an RCD from completely working!

The washing machine is built in. So its turned off till I can take out to check, before I call out an engineer of might just check the heater element myself

 

[telectrix]

just fit a short croc lead across L and N on the plug of the machine, then IR test to E. ( take plug out of socket first though. :81

 

[Midwest]

just fit a short croc lead across L and N on the plug of the machine, then IR test to E. ( take plug out of socket first though. :81

I will when I have the time to remove the built in cupboard door, remove the plinth, remove the front decorative beading, remove closing mechanism, slide out WM......shouldn't have fitted the kitchen myself!

 

[Midwest]

Cancel that, Ill do an IR test from the isolator sw!

 

[Dizzy_Maskell]

I've seen loads of appliances cause issues, TBH normally fridges and freezers.
Now I know its a disputed issue but officially you only ever test RCD with loads disconnected.
I myself query this for this exact reason

I have done EICR and tested RCD at board then out of curiosity done it at a socket and they fail.
Obviously on the sheets I recorded the official times with disconnected loads but still made an advisory on the report.
This is also another reason it's a shame they removed Cat 3 further investigation from the codes

 

[Midwest]

Yep your right of course and I did on the previous tests I'd done on this socket/rcd, but for some reason I forgot to switch off my built in washer & diswasher. I would like to understand the reasons why the rcbo fails to do the x5. What is worrying is the the fact the rcbo is not functioning as it should. It makes you wonder what other devices are preventing rcd from working and perhaps good reason to carry at least one opeartion of an rcd with devices connected? Or should domestic properties be subject to PAT testing? Also perhaps a good reason to do regular MFT tests on RCD's, not just the push button.

 

[Lucien Nunes]

Appliances connected to the circuit under test can interfere with the operation of the tester and cause it to produce incorrect results, even though the RCD / RCBO is OK. For example, white goods often have an LC filter connected directly to the flex, energised even when the power switch is off. This appears to the tester as capacitors between all its leads, which can affect its test current and calibration.

In this regard, the MFT will produce different results compared to the RCD test button because the circuit is different. As an example, suppose there is an N-E fault downstream of the RCD. When the test button passes current from the load side line to the supply side neutral, the imbalance will be more or less unaffected or even slightly increased and the RCD will trip. But an MFT plugged into a socket outlet can't get at the supply side of the RCD, so it dumps the current to earth. The N-E short might now decrease the imbalance at the RCD because some of the current can return by the neutral, so the RCD might not trip.

I would check the washing machine IR etc as you have said, but if it turns out to be OK, there is a possibility that varistors or Y-capacitors internally connected to earth are causing a similar effect on the tester as above.

 

[Dizzy_Maskell]

Appliances connected to the circuit under test can interfere with the operation of the tester and cause it to produce incorrect results, even though the RCD / RCBO is OK. For example, white goods often have an LC filter connected directly to the flex, energised even when the power switch is off. This appears to the tester as capacitors between all its leads, which can affect its test current and calibration.

In this regard, the MFT will produce different results compared to the RCD test button because the circuit is different. As an example, suppose there is an N-E fault downstream of the RCD. When the test button passes current from the load side line to the supply side neutral, the imbalance will be more or less unaffected or even slightly increased and the RCD will trip. But an MFT plugged into a socket outlet can't get at the supply side of the RCD, so it dumps the current to earth. The N-E short might now decrease the imbalance at the RCD because some of the current can return by the neutral, so the RCD might not trip.

I would check the washing machine IR etc as you have said, but if it turns out to be OK, there is a possibility that varistors or Y-capacitors internally connected to earth are causing a similar effect on the tester as above.

This is my argument for why RCD should be tested with loads connected if is not going to operate when it's in use what's the point ?

It's great that it works with no load but then when in use fails to operate but this is a time old debate not for an overnight fix

 

[Flanders]

Appliances connected to the circuit under test can interfere with the operation of the tester and cause it to produce incorrect results, even though the RCD / RCBO is OK. For example, white goods often have an LC filter connected directly to the flex, energised even when the power switch is off. This appears to the tester as capacitors between all its leads, which can affect its test current and calibration.

In this regard, the MFT will produce different results compared to the RCD test button because the circuit is different. As an example, suppose there is an N-E fault downstream of the RCD. When the test button passes current from the load side line to the supply side neutral, the imbalance will be more or less unaffected or even slightly increased and the RCD will trip. But an MFT plugged into a socket outlet can't get at the supply side of the RCD, so it dumps the current to earth. The N-E short might now decrease the imbalance at the RCD because some of the current can return by the neutral, so the RCD might not trip.

I would check the washing machine IR etc as you have said, but if it turns out to be OK, there is a possibility that varistors or Y-capacitors internally connected to earth are causing a similar effect on the tester as above.

I gather from what you saying is that the appliances are interfering with the MFT but in a real fault situation the RCD would still trip within the 40ms (As long has when the RCD was tested it operated correctly without any load connected).

 

[tony mc]

What is the earthing arrangement for the premises ? Its not TT by any chance.

 

[Midwest]

I gather from what you saying is that the appliances are interfering with the MFT but in a real fault situation the RCD would still trip within the 40ms (As long has when the RCD was tested it operated correctly without any load connected).

That's my point, with the WM inadvertently connected, my MFT test showed the RCD test x1 56ms, x5 timed out at >40ms. So are we saying the RCD would trip? No it's TN-C-S.

 

[Badged01]

....shouldn't have fitted the kitchen myself!

Careful the kitchen fitters dont find out you're doing that. Will add more weight to their argument for them fitting the electrics! Nah, forget it, they're gonna do the electrics anyway. Fit kitchens to your hearts content ......

 

[telectrix]

to over simplify the situation, your MFT is basically trying to bung 150mA from L-E or N-E through the RCD. if you have a load connected, that is a parallel path down which a proportion of that 150mA can flow.add some capacitance and other electronic wizardry, and it will screw up your test. that's why you should perform RCD tests with loads disconnected- even with the circuit dissed from the RCD., although it's much easier to test at a socket, esp. if using auto-test.