Weird earth leakage fault

[stilllearning]

Hi, been to a house today and customer complains that the RCD trips SOMETIMES when she uses the iron and almost always when she uses the hob. The RCD will not reset unless all the breakers are first switched off.
Apparently it's been like this for some years. Looks like the place was rewired maybe about 10 years ago and there was nothing obviously dodgy.

All circuits on the board are protected by one 30ma RCD. There are 2 rings, (1 for the kitchen and one for the rest of the house) 2 lights, 1 alarm, 1 outside light, 1 16a radial feeding one socket, 1 hob and 1 cooker circuit.

I tested the RCD in isolation and it was fine. I disconnected the hob and did a 1/2 times test and the RCD tripped.

I unplugged/switched off all the appliances and after isolating the various circuits and retesting the RCD I realised that it only fails the 1/2 test if BOTH ring circuits are switched on.

If just one ring is on (it doesn't matter which) and all the other circuits are on then the RCD passes the 1/2 test.

If just the 2 rings are on and everything else is off it still fails the test.

I did a ramp test on each ring in turn - one trips at 19ma the other at 23ma.

I did an IR test on the rings and they were both OK.

One sparky I spoke to suggested that one 30ma RCD may not be enough for the size of the house/number of appliances (it's a big property but as everything was switched off I can't see how this would matter?)

Would appreciate any ideas!
Thanks

 

[chivers67]

What's the actual current rating of the 30mA RCD.

I tend to have the hob on it's own (without the RCD protection) circuit to prevent nuisance tripping not sure why the iron is tripping the RCD from time to time though

 

[holmslaw]

..

 

[stilllearning]

Thanks for the replies, in answer to your questions:

The RCD is a 80a 30ma - I swapped it with a new just to double check but still had the same problem.

Pretty sure I unplugged everything.

It's a TT supply

I did the IR test between L&N (connected) and Earth (just in case something was plugged in) but only on the suspected circuits...

I thought of maybe swapping the board for a dual RCD split load or even RCCBO's - any thoghts?

Thanks again

[/quote]

 

[sparkyspike]

I did the IR test between L&N (connected) and Earth (just in case something was plugged in) but only on the suspected circuits...

What were the results?

When you tested ½ΔI on individual circuits, did you disconnect all of the neutrals of all the other circuits in the house as well as opening the breakers?

 

[stilllearning]

Hi Sparkyspike


The results on one ring were 48mohm (low but not low enough to trip an rcd?)

The other ring was 150Mohm

Thanks

 

[RF Lighting]

If the RCD covers all the circuits then you need to IR test ALL the circuits.

It could be any of them, with the iron tipping the leakage current that little bit too high.

Why are you testing L&N>E rather than L>E N>E & L>N?

 

[stilllearning]

Thanks RF lighting.

I tested L&N>E just in case i HAD left something plugged in - it's a very big house with more sockets than screwfix and I didn't fancy sending 500v up the plasma!

I appreciate what you say about testing every circuit but it's strange that it only trips when BOTH ring circuits are switched on with every appliance unplugged its as though there's a small leakage on both circuits and the added "leakage"from the RCD tester or the iron or hob is enough to take it over the edge.

If I test the RCD with either of the rings off and all the other circuits on or off everythings fine. The added problem is that with both rings on it doesn't always trip. Help!

 

[Spark123]

One theory to explore, see how many leaky appliances are plugged in i.e. with mains filters.
I'm thinking they will pass an IR test as it is DC but as they have a capacitor network between L, N and E the will have a bit of reactance between L, N & E which if enough current is allowed to flow could trip an RCD.

 

[comms]

First off if you are worried about something still being plugged in undertake the initial IR test at 250v between L/N l/E N/E you'll get an indication of a problem or if something is plugged in, then do same test at 500v.

Back to the problem, I had an exact same fault a while back on a TT system. Ended up removing all circuits from house cu to IR test in turn, eventually traced fault to a kids play house in the garden (very big country house) faulty appliance (pool heater) plugged into a trailing lead connected to a socket supplied via a cu run off a submain from house cu.

Not enough on its own to trip rcd but as soon as something else was turned on in house pop went the rcd.

 

[stilllearning]

Thanks Comms. I'm going back there today to take another look so will double check everything!

 

[bernardgreen]

I'm thinking they will pass an IR test as it is DC but as they have a capacitor network between L, N and E the will have a bit of reactance between L, N & E which if enough current is allowed to flow could trip an RCD.

The capacity between Live and Earth in the wiring will provide a route for AC current to flow and create some un-balance in the RCD, As Spark123 says IR testing with DC will not show this current. Hard to imagine enough wiring to create a 30 mA capacitive leakage but I have measured over 5 mA AC ( at 230 volt ) when the DC resistance was > 200Meg ( at 10 volt )

With care the AC current at 230 volts can be measured by bridging the MCB ( OFF ) with an amp-meter. Start with the meter set to a high amp range and then range down.
NOT RECOMMENDED FOR DIY testing

 

[RF Lighting]

Surely the safer idea would be to disconnect the outgoing wiring from the MCB, and connect the ammeter between the MCB and the wiring?

I don't like the idea of bridging an MCB at all

 

[stilllearning]

Thanks alot for all your replies and suggestions. I've been back today and found some other stuff plugged in that I missed first time round. I've IR tested ALL the circuits and they're all around 299Mohm.

As some of you suggested I think the problem is that one RCD is not sufficient for the amount of appliances and the organisation of the ring circuits. Basically they have one ring for FIVE bedrooms, 2 living rooms a huge kitchen and an even bigger conservatory. The kitchen alone has 2 American style fridge freezers, a wine chiller, 2 kickboard heaters, a rad with a built in immersion heater, a water heater, an ice maker, and an oven. In addition to that there are 3 computers numerous phone chargers, laptop tranformers etc etc used throughout the rest of the house. The CH boiler is also spurred off the ring.

There's an induction Hob on it's own circuit and another Oven on it's own circuit.

The other ring feeds a utility room with washer & Dryer, and a spur feeding a garage with 3 (yes 3) fridges/freezers.

The kitchen was only fitted 2 years ago (although the customer says there were tripping problems before but not as often) so I don't know why the CU wasn't changed at the time for a split load. Ideally the kitchen should have been on at least 1 ring of it's own.

I think replacing the CU with a split load or all RCBO board is the only way to sort it out..

Thanks again all.

 

[bernardgreen]

Surely the safer idea would be to disconnect the outgoing wiring from the MCB, and connect the ammeter between the MCB and the wiring?
I don't like the idea of bridging an MCB at all

Bridging the MCB is a quick way to locate the leaky circuit without having to take the wires out of the breaker. Obviously it is done with no load on the circuit and with a meter that has a fuse in the test circuit just in case.

NOT RECOMMENDED FOR DIY testing