Erratic RCD

[lavillegour]

I have a Wylex consumer unit with 6 mcb's protected by one RCD.

In the past month or so this RCD has been tripping out at variable times of the day or night.

Periods between trips is also variable - sometimes a week other times within days.

Unfortunately there is no pattern whatsoever !

I have posted on the 'rated tradesmen' website but did not receive any response from local electricial tradesmen.

So ....? As a reasonably competant DIY er thought I'd try to ascertain some REASONS/SOLUTIONS/ PARTIAL SOLUTIONS.

Would very much appreciate some experienced advice such as :

Should I replace all MCB's with MCBRO's (?) so that only one circuit trips rather than the whole house?

Should I replace the RCD in the hope that it is malfunctioning ?

Will a really competent Electrician (once I find him) suggest a complete house re-wire or (hopefully) have some better solution ?

 

[ericmark]

The RCD tripping will be caused by something but not necessary in the house. Thunderstorms can take mine out. Not all RCD's are the same. Reading this it seems the X-pole RCD is specially designed to not trip as often as the standard type. There are even auto resetting

Untitled

• ericmark
• 6 Dec 2008
• 34

but the price is rather silly.
But as to best method around the problem it really does depend on why it's tripping. It would be unusual to have whole house on one 30ma RCD and the question must be asked is it a 30ma or is your house on a TT supply and the RCD is a 100ma or larger type?

For tripping due to faults in the house we would be looking at something with a timer that will do something at odd times. Favourite is a frost free freezer as these switch into de-frost mode from time to time automatically. As to if faulty is another question as just switching can produce enough of a spike to trip a RCD. Same applies to central heating system with not only boiler but motorised valves etc.

The only way to really isolate some rouge piece of equipment is to swap the RCD which it runs through. If only one in the house then this presents a problem. The regulations don't say more than one should be used but it does say it should be split to reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced
by equipment in normal operation. So in the main we would fit more than one RCD at 30ma.

The taking account of danger that may arise from the failure of a single circuit such as a lighting circuit will often mean either emergency lights or RCBO's but the RCBO is in the main a single pole device so can't replace the RCD with TT supplies. Also the quality of a RCBO is not as good as the X-Pole type I linked to so it may in fact trip out more than one of the X-Pole type.

There are 10ma plug-in RCD's but although less milliamp the trip time is the same so may not help in working out what is causing the tripping.

However before advising on way forward you need to state what the RCD is for? There are three main reasons to fit a RCD.
1) Personnel protection must be 30ma or less.
2) Fire must be 300ma or less.
3) Earth fault detection size dependent on earth loop impedance in the main 100ma is used.

With 2) and 3) normally there is also a time delay refereed to as S type. There are also some old types called ELCB-v that were used for earth loop impedance problems but are now no longer used.

So question one is what type have you got?

 

[lavillegour]

ericmark- how very kind of you to reply in such a comprehensive manner..Thanks !

I have a wylex consumer unit (installed some time ago) which has what I call 6 basic MCB's (2- 6 amp (lights) 1- 16 amp (water heater & shower) 2 -32 amp (sockets) and 1 - 32 amp for cooker.
The unit is fitted with a 100amp load RCD (with test switch) and 30Ma trip marked WSES100/2.
There have not been any alterations to the wiring etc for 1 year +

My research leads me to believe that to reach even safer standards I should upgrade (to maybe RCBO's - even a 17th edition consumer unit etc and my cheque book is ready for this eventuality - BUT am wondering whether this form of upgrade will solve the problem ?

(had a brief conversation with an electrician at B&Q but received the usual sharp intake of breath - usually used when the problem could be enormous and costly (complete rewire ? -very scary).
Would appreciate more advice please...

 

[ban-all-sheds]

Have you bought any new electrical items in the last month?

You say you've done no electrical work, but have you done anything in the house- drilled holes in the walls? Fixed flooring? Moved furniture? Moved stuff in the loft?

Do you have any outside lights or sockets?

 

[333rocky333]

Should I replace all MCB's with MCBRO's (?) so that only one circuit trips rather than the whole house

You probarly mean Rcbo.s they are better but would only be any use if the Main rcd thats tripping was replaced with a main switch.
Which would most likely mean a new board.
Would be worth the expense as it will help locate the problem both now and any future ones, as well as reduce inconvenience to you as they will not all trip.
You could be looking at a few hundred pound but IMO worth it

 

[lavillegour]

Well............ No (not to my knowledge)

When this first started happening we went on holiday so I switched OFF most things in the house.

I believe the only ones still operating at that time was The fridge freezer and the SKY + box (to catch the rugby !)
....and a couple of bedside clock/radios.

When we returned the RCD had tripped (Overnight 1st night). We lost the whole of the Freezer food.
eric martin mentioned fridge/freezer as a likely contender and (unless I misunderstood) buying and trying a stand-alone RCD on the fridge socket wouldn't really help
Am waiting to hear whether a new dual RCD consumer unit COULD solve..

 

[lavillegour]

Yes Rocky I am thinking along those lines (replacement consumer unit) but of course am trying to examine all options first so as to become better placed to discuss/decide with an expert on site.

It does seem that electricians aren't too happy with this type of problem but I do realise that the cost of an experienced one would be well worth the effort.

Trouble is where and how do I find a well qualified experienced one ?

 

[333rocky333]

A stand alone plug in rcd adapter temporary on the freezer may pinpoint it but there always a likelhood the Mainrcd trips first before it though.

The adapter would need to be a LATCHING one that means if theres a power failure it does not trip, therefore if the adapter thing trips you know it may be the freeezer, yet if the main trips the adapter remains untripped indicating it may not be the freezer although not infallable. as the main trip could react faster than the adapter.
Hope that makes sense.

Rcbos wont clear your fault just help you locate it.
The way forward is to have your wiring insulation tested, though if you have the board changed, this should be done as part of the work involved, so if you can afford it go for the board change but ensure that the cost involves the relevant testing of all circuits and a written report.

 

[333rocky333]

Ive done fault finding most of my career and that is the hardest to find,due to the iregularity of it, you could spend days and get nowhere or be lucky,most electricians id expect do shy away from this as its hard to allocate a time and cost and most clients dont want to pay the time.
In a way you want the fault to get worse so you can pinpoint the damn fault.
Dont get me wrong there is people out there that can do that work, but many out there that go for the just rewire it option, which would be no help at all if its a faulty appliance

 

[ricicle]

>Damp/water in an outside light or socket
>Rodent damage to a cable
>A nailed cable that is only just starting to fault (wall or floorboard)
>A crushed/damaged cable under a floorboard (between board and joist)
>Even a cobweb in an internal socket or switch
>Damp/condensation inside a metal back box of an accessory.

 

[ban-all-sheds]

Don't know what sort of property you live in, but if you have somewhere secure outside, then at this time of year you can stop using the fridge and store things in (fox & vermin proof) crates/boxes outside.

Eat up all your frozen food and/or borrow space in friends/family freezers, and you can turn off the F/F and see if the tripping stops. Although as rocky says it's a perverse shame that the problem isn't worse, as you might have to wait a while to be sure you'd found it.

But...

Finding and replacing the culprit won't change the fact that you don't have a very good CU design, with 1 RCD covering everything, so there would be benefits in having that replaced. And in not having the F/F on an RCD/RCBO.

How old is the wiring? It's unlikely that you'd need a rewire.

 

[lavillegour]

Good helpful advice Rocky.
My (genuine) thanks to you, ericmark and shedsy for making the time to help.

Am looking into the Linked RCD idea (maybe the digital RCD eric ?) but am tending towards merely 'instructing' a qualified electrician to carry out rudimentary tests with the possibility/probabilty of replacing the consumer unit.

Probably be cheaper than labour costs for extensive testing etc ? ?

 

[lavillegour]

bl**dy 'ell ricicle you'd brought out my worst thoughts !

Maybe I'll just have to burn the house down.

Like the idea though of trying dual RCD's (would one use these with 'bog standard' mcb's or RCBO's as well ?)

-also keeping fridge/freezer off RCD protected circuits - or is this 'legal' ?

 

[ericmark]

If RCBO's will fit the box you have, as most RCBO's are longer than MCB's and old boxes are not tall enough for them to fit, then RCBO's is likely easy option changing the existing RCD for either a 300ma S type version if TT or isolator if TN system.

Personally if I was up grading I would use an old split box and have sockets on RCBO's but the items less likely to cause problems on a RCD.

What is needed is the ability to swap items from one RCD device to another to work out which are causing problems.

At the moment my house is using two RCD's both feeding separate old consumer units and it's been that way for last 25 years. I find every now and again I seem to get a series to trips to both RCD's maybe for a couple of months and then without doing anything we will get another 12 months trouble free. I will guess some one in the road on same phase has something faulty sending spikes down the line. In the end it stops working completely and we get trouble free again.

I am sure if I fitted the X-Pole RCD's it would cure the problem but lived with it now for 25 years. We have a cat so when on holiday always have some one to visit to feed cat and if it should trip re-set RCD.

 

[ban-all-sheds]

also keeping fridge/freezer off RCD protected circuits - or is this 'legal' ?

It can be done - not always the easiest of things to do, but there's always a way.

The biggest issue is whether the cable to the socket is, or could be, surface mounted. If so, then as long as the socket is labelled as non-RCD protected and for use of the F/F only, it's fine, or it could be wired to an FCU and not use a socket.

If the cable cannot be clipped to the surface of a wall, then it would have to either be run in steel conduit or be buried at least 50mm deep or be of a type which doesn't need RCD protection.

• http://www.voltimum.co.uk/files/gb/attachments/niu/l/attachments/c5-43.pdf
  
  
• http://web.archive.org/web/20061110111450/http://www.niceic.org.uk/downloads/concealedcables.pdf
  
  
• IEE article
  
  
• Wiring Regulations requirements for concealed cables.