Desperately Seeking Advice

[deeley_s]

Hi,

I have a problem that five electricians so far have been unable to track down.

I have a four bed detached house. Consumer unit is a wylex  16th edition, split - 100A switch and 30mA/80A RCD. We're getting truly random trips off the RCD. The source is the ring main. The RCD has been tested and cleared by all five electricians - if anything, it's on the slow side of acceptable rather than being twitchy.

We were advised that it must be an appliance, so we should try isolating all appliances and gradually re-introducing them to see which caused the trip. As best as we can (it's the ring main...) we've done that. No single device or pattern of devices causes the trip. Sometimes we can go weeks without a trip, other times we'll get a dozen in a single day. Last night it tripped when my wife turned the kitchen lights on, even though they are on the mains switch side of the box and not covered by the RCD at all. This morning it blew when I turned the kettle on (twice) but when I moved the kettle to another socket it was fine.

When the RCD fails, we flip the ring main 32A MCB off and the RCD will reset fine. The minute we try to flip the MCB on, the RCD trips AND the MCB fails. Yet if we persist with the MCB (yes, I know...), trying to set it occasionally  over the next 10-15 minutes, it will eventually turn on again. It will then run for anything from ten minutes to 10 weeks with no change whatsoever to the loads.

I'm not an electrician but do have C&G 2381 (16th edition) so I'm not a complete novice. However, this one ash me stumped. I don't even know who to call for help. Any suggestions?

 

[Murdoch]

Apart from RCD tests did any of the 5 sparkles do any other tests?

I would recommend changing the cu in the near future so that you don't have a single upfront rcd

 

[ProDave]

There is clearly a fault. As asked already, did any of the electricians do anything apart from test the RCD?

Insulation resistance testing is the big thing that's needed. with EVERYTHING unplugged. If that's clear then PAT testing each appliance then plugging them back in would be my next test.

Don't assume it can't be a fault on one of the non rcd protected circuits, though the fact the MCB trips suggests it is much more likely to be something on that circuit.

 

[deeley_s]

No. They just did a trip test  on the RCD.

We have a complex layout and a high load - kids with big computers, pond pump etc. The CU is laid out as:

100A switch covering:

32A cooker spur

6A lighting ring in ground floor extension

6 A d/stairs lighting ring

6A u/stairs lighting ring

6A spur to house alarm

Hager CD280U 80A RCD covering:

32A ring main

32A spur to loft and garage (which has its own CU with RCD)

16A Shower spur

32A spur to pond + IP68 garden socket (Pond pump chamber has its own IP68 CU with RCD)

20A spur to dishwasher and washing machine

I'm happy to change te CU to 17th Edition but don't ant to do that until we've found te problem - otherwise we'll lose the freezer in the garage as well when it trips.

 

[ProDave]

For a simple test I would disconnect the outside pond stuff, and by disconnect I mean disconnect at least L and N in the CU.

Outside stuff by it's very nature is more likely to have faults and really should be on it's own rcbo so if it has a fault it won't trip half your house.

Swap the cooker spur and garage sub main over.  then you will have an rcd on the cooker, and won't have two rcd's on the garage circuits.  again they could be an issue.

Interesting you have a Hager rcd in a wylex CU. I didn't think the busbar heights matched so some "adjustment" may have been needed. However that's not going to be the cause of your problem.

Do you have any test equipment for instance to do IR testing?

 

[binky]

random tripping is mostly fridges and freezers, which are the only appliances that switch on at random times of their own accord. However, if you have  a large number of electrical items, what you may be suffering from is a build up of earth leakage from a combination of appliances, especially if you have numerous IT products. So depending on what is being used, when you turn on the kettle which adds a few mA of earth leakage to what's already 'leaking' it might just be enough to trip RCD. 

My recommendation would be full testing of ccts, to ensure cabling is good, followed by conversion to RCBOs, so that you effectively have an RCD per cct, which in turn will give you far greater reliability, and limit effects of random tripping

 

[kerching]

This is one of the occasions where an earth leakage meter is a godsend!

Clamp it on the earth and start switching stuff on and note the readings.

It may well be accumulative leakage.....BUT with the MCB tripping as well it sounds more like a L/E fault

IR test is the next logical step,,as said before

 

[Murdoch]

I'm not an electrician but do have C&G 2381 (16th edition) so I'm not a complete novice. However, this one ash me stumped. I don't even know who to call for help. Any suggestions?

A competent, local sparky with a MFT and a clamp meter would be a good start - that said with such irregular tripping it could be tricky to track down.

 

[Blue Duck]

@ OP Do you have a pond?

 

[ProDave]

@ OP Do you have a pond?

Post #4 mentions

32A spur to pond + IP68 garden socket (Pond pump chamber has its own IP68 CU with RCD)

 

[Blue Duck]

Missed that.... Sorry!

 

[ProDave]

Why are you so interested in ponds Ducky?

 

[Andy™]

he's looking for a new home

 

[kerching]

Or he is looking to test his "poesterior water ingress protection rating"

No offence

 

[OnOff]

Did the electricians do the usual (and thorough) "dead tests" on the ring in question that should show up any loose connections, in the back of sockets etc? Just Google "ring circuit dead tests". If they had they might have given you a Schedule of Tests?

Or did they just test the RCD and give a quick nod to the sockets?

 

[Blue Duck]

MIP: Mallard Ingress Protection. :lol:

Don't get much time to read posts properly as I'm usually juggling 2 ducklings!

Funny you should mention a new home... New Tennant's downstairs like to smoke skunk and it stinks out the whole place! Have asked politely and not got them to stop! So a new pond may be on the horizon!

Enough waffle... The amount of time I have wasted looking into appliances tripping RCD's only to find a pond pump or some shoddy outside socket full of spiders is the cause.

 

[ProDave]

Ditto the above.

I spend hours chasing a low IR on on a ring final at our village hall, splitting it and testing legs etc.

That was until I found a spur from the boiler room fed the septic tank plant at the bottom of the field.

WTF that wasn't on it's own circuit?

 

[Evans Electric]

I'm thinking pond too , can you disconect it for a while and see how that goes ?  

Any spurs off the ring up into the loft for aerial booster etc ?  Cable nibled by squirrels  etc.

 

[deeley_s]

Hi Folks,

Thanks for all the good advice in here. My first thought was pond or Mice (we did have some, but got rid of them) or perhaps condensation from a damp problem. But the fact is that while some of the trips are completely random, others are closely associated with turning on an appliance.

This morning the trip went twice when I turned the kettle on. This evening  I plugged the same kettle into the same socket - and it was fine. I can't square that with a pond pump (which has a fast trip RCD on its own CU) or mice nibbling cables.

But I think Binky and Kerching have a point. I think we possibly have so much  normal leakage that we're close to the limit on the RCD and the next appliance we switch on, whatever it is, is enough to blow the RCD. And the MCB? I'm thinking inrush current when everything starts to switch back on again at the same time. I then go around yanking plugs out, and lo and behold the MCB and RCD both switch back in without a murmur.

To be sure, I need a sparky in North Wilts who can run a megger on the cables and if they have an earth leakage meter, so much the better. Any recommendations?

 

[ProDave]

Before you appoint a 6th electrician, be sure you both understand what you are investigating.  You don't want No 6 to just say "the rcd is fine" collect his money and depart do you?