Nuisance RCD TRips

[puffernutter]

I have an ongoing problems with nuisance RCD trips.

The supply is a TT system with a "whole house" 100mA RCD on the incoming feeder. Downstream of the RCD are two consumer units:

1. House circuits (Shower, cooker, socket ring main downstairs, socket ring main upstairs, lighting ring main down stairs, lighting ring main upstairs)

1. Electric Heating and Garage (electric heating ring downstairs, electric heating ring upstairs, garage supply, immersion heater)

The garage supply is 2 x 2.5mm FTE and it also has a 30mA RCD at the garage end (approximately 50 yards away)

Over the last month we have been continuously experince random nuisance trips. (it quietened down over the last couple of weeks, but picked up again in the wet weather - significant?) We have tried narrowing it down, but with little success. Differentiation using RCBOs has been suggested, but that will require a new consumer unit. (The current one uses Wylex plug in mcbs.) It is interesting that sometimes just the "whole house" 100mA RCD goes, at other times the garage RCD (some 50 yards away) will also trip out.

Last night I turned off Consumer Unit 2 at the isolator and we suffered a trip at about 02:30. Therefore there is nothing on those circuits that can contribute to the trip. (Apart from the tails from the "whole house" RCD to the consumer unit). Tonight I plan to turn off consumer unit 1. I am hoping for a trip as that will rule out my house wiring as a cause.

If that is the case, what's the next step? I believe Western Power Distribution are the local distribution company in the South West, I assume that I aske them to check there supply side.

If it doesn't trip it means that it may be my problem. In which case, anybody in Wiltshire got a Megger I can borrow.....

Has anybody else had this sort of problem? Have people experienced trips due to "outside" influences? How have they been resolved?

Cheers

Peter

[Owain]

Quite possibly.

Is the 100mA RCD time-delayed? If it isn't, then a fault downstream of the garage RCD could trip either or both. Do not overlook the possibility you have two or more unrelated faults which are contributing to a total leakage enough to trip the RCD.

The only real way is to Megger all the circuits individually, measuring their resistances, and if you don't get a clear indication first time round re-test periodically looking for a varying resistance (possibly weather related).

Assuming you have meter tails into the 100mA RCD, then output tails into Henley blocks splitting to your two CUs, you could get another 100mA (time dleayed) RCD, and split the meter tails *before* the RCDs, so givign each CU a 100mA RCD without having to buy new CUs. Socket circuits on TT do need 30mA protection though, so you could use a 30mA RCD for the house CU 1 (if your electric heaters on CU2 all use FCUs), although whole-house RCD protection is contrary to Regs too. This may reduce nuisance tripping.

This work is probably liable to Part P.

Owain

[Christian McArdle]

The garage circuit should not be off the house 100mA RCD. Garage circuits are much more likely to trip than house ones and it is best that they have completely independent RCD protection.

Ensure that the 100mA unit has a time delay.

Christian.

[bigcat]

You really need to install RCBOs, and accept it means a new CU. The install design is a recipe for this kind of trouble. Also I'd put garage and house on different RCDs.

Until thats done, you may or may not resolve it by testing the insulation of all your appliances. You can do this with an ordinary multimeter, although not perfect it will pick up the problem in almost all cases, if not quite all.

I doubt its the power co's problem, hard to see how it could be.

NT

[puffernutter]

Thanks for all the replies.

As you may have gathered, this is what may be called a "legacy" system, so I need to spend some effort to get it up to meet the requirements of the 17th edition.

With regard to the comment "the garage should not be off the house RCD", I don't really have a lot of choice, both consumer units are "downstream" of that RCD, so even with new consumer units, the garage will be "downstream" of the "whole house" RCD.

Also, if I read the regs right, "whole house" protection using a 100mA RCDis mandatory on a TT supply.

Just to clarify the arrangement is as follows:

Incoming Mains (via a pole route) to meters and radio timeswitch. That inputs to the 100mA RCD. The output from the RCD goes into a tails "block" where these tails go into two separate consumer units (neither with RCDs in). Earth is via an earth rod in the front garden.

The only additional RCD is in the garage and protects the garage sockets downstream of it.

I have had a quick look at Screwfix and TLC-Direct, neither of which specifically mention "time delayed" RCD's. but both mention that they are resilient to transients. Is that the same thing?

So, the plan could be as follows:

1. Fit two 30mA RCDs in separate boxes, downstream of the whole house RCD and upstream of the consumer units. This is probably the cheapest option and retains the exisiting CUs. The downside is that if there's a trip I lose lights as well (but we have that anyway and we are getting very adept at resetting the RCD in the dark!) I should also replace the exisiting 100mA RCD with a newer, time delayed version.

OR

1. Update both CUs to contain isolators and 30mA RCDs. This will also require new mcbs in both the CUs.

OR

1. Update both CUs and fit RCBOs throughout (a very expensive option). (1) has the same effect and is probably cheaper.

Last question - do I need the 30mA trip in the garage, or could I move it into the house (it is in a separate box, so it is easy to move).

Cheers

Peter

[Owain]

No, you do not have to use (and there are good reasons for not using) a whole house RCD.

What you do have to do is provide RCD protection of 100mA or better on all circuits, and 30mA or better on all socket circuits.

No, a time delayed RCD will specify that. You may have to ask in a local electrical wholesalers.

Which means buying three new RCDs :-(

If you have enough spare cable in the circuit cables, you could rearrange your circuits to have all the 30mA circuits on one CU and all the 100mA circuits on the other CU. Then you only need to buy one new

30mA RCD and enclosure, to add RCD protection to the second CU.

In the meantime, some emergency lighting units in the kitchen and over the stairs will help prevent accidents.

Or use a CU with a 100mA T/D RCD main switch, and 30mA RCBOs on the socket circuits.

You don't need it if the circuit is already protected by a 30mA RCD. However as it is the circuit most likely to suffer nuisance tripping it is best to have it on its own RCD.

Owain

[John Rumm]

That would be quite an effort, they have not published it yet! ;-) (still on 16th edition with amendments)

Your setup sounds not too far removed from my own...

I spent an uncomfortable day in a cupboard last week getting my old rewireable fuse CU replaced with a completely new setup (I have concluded I am the wrong size to be a sparks!). Anyway the plan I was following was pretty close to:

Also, if I read the regs right, "whole house" protection using a 100mA

it is...

That sounds pretty common for a 15th edition style setup. As you have found out it does have problems with unwanted trips, and also loss of lights etc.

The preferred way to do it now would be to have the 100mA RCD replaced with a 100mA with built in time delay, and then have a further 30mA RCD in the consumer unit that feeds the circuits that need more protection.

You have a potential discrimination problem there, in that a trip in the garage circuit could take out either its own RCD, the house one or both. You can't assume that since the house one is a higher trip current it will always trip second. (that is why time delay RCDs are used upstream of other RCDs)

No. You would need something like RS Stock number 264-9584 (a MK Sentry

6400S). TLC Can supply them, as well but they are not in the catalogue so you need to ask for them.

Could you move the circuits about so as to get all the things that want RCD protection onto one CU and the rest on the other? If so you could replace the 100mA main one with a time delayed version, and then add an extra 30mA one for the CU that needs it.

(failing that you may find replacing the CU is easier anyway)

I took the precaution of adding one of these in the meeter cupboard:

to the downstairs lighting circuit it can be used as an ordinary cupboard light, but should that circuit trip (or the whole house) it turns itself on.

Again, sorting circuits so as to not need a 30mA RCD for both would help.

The Rolls Royce solution, but as you say pricey. Note you would not need RCBOs on all circuits if you kept one upstream time delayed RCD in there.

Better in the garage since it will be closer to where you may be working when it trips.

[Peter Parry]

Before spending anything it might be worth looking for neutral to earth faults. To do this you need to isolate both neutrals and earths for each circuit which means disconnecting them at the CU. Short circuits between earth and neutral anywhere within the system will often lead to random seeming RCD trips.

A common cause is nails through cables where they go over joists under floorboards.

[bigcat]

system,

Theres nothing wrong with a 15th edn system if you sort out the rcd situation.

garage

You get to wire it how you choose. The garage feed is much better not run off the same rcd as the house. Doing so would only cause you more problems.

Sort of. RCD protection on all circuits is needed, since TT supplies have inadequate earth condutance to clear earth to live shorts, and they would leave the whole earth system live if there were no RCD or ELCB. But that does not imply the RCD protection must be provided by one single 100mA delayed RCD, it could be via more than 1 RCD.

(neither

a recipe for trouble.

If you had an older 14th edition version with an ELCB there'd be no problem. Nuisance trips are rare with those, but they do offer significantly less protection. They will clear live earth shorts, but dont offer the additional touch electrocution protection of RCDs, and dont trip until large earth currents are flowing, so offer no short circuit fire protection. Their only real nuisance trip mode is they can occasionally trip on very close lightning strikes. Also they do permit very mild shocks to occur without clearance, since they only trip when the earth system reaches 50v, and this makes them a no-no with an electric shower.

cheapest

there's

important correction: when there are trips

yes, and freezer at 3am, and everything. And in some cases you may not be able to reset the rcd. A duff solution.

Setup 1 will give you the almost as much problem you have now, would be a waste of money.

same as 1 effectively, not much point. Neither of these solve the cause of the trip, youll still have trips.

option).

correct answer.

not at all the same.

Cost for option 3: split CU with 30mA RCD for lights, switch for skts and assotred MCBs: =A370

3xRCBOs: =A3100 (could reduce to 2 if money real tight) Labour to fit.

Run the lights on 6A MCBs on the RCD side of the CU, run the sockets on RCBOs on the switch side of CU. You dont need any RCD upstream of this, and can use the existing 100mA rcd for the garage feed only.

Now any future trip will only take out one circuit, not everything. Also since ever present leakage currents are now divided among several rcds, they will be much lses prone to nuisance trip. Plus your garage feed wont take out the house supply. Splash out an extra =A312 for 2x type C mcbs for the lights and blown bulbs are unlikely to trip the mcbs either.

This lot will make it far easier to find the source of the trips, and may eliminate them altogether. Any other approach just wont work.

If youre really stuck =A3wise, multimeter all your appliances, you may well find a faulty one with some L-E or N-E conductance thats causing the trips. Multimeter =A36.

NT

[Tony Williams]

Think about a Neutral-Earth short, somewhere in the house, either permanent or intermittant. It could be in the cabling or in an appliance.

A permanent short could do nothing for most of the time because your Neutral-Earth voltage is too low to generate the required trip current (through the resistance of your earthing rod, back to the supply transformer). But the N-E voltage may then rise at odd times, due to something happening externally.

An intermittant short (especially at 02:30) suggests maybe the fridge. This is what happened here, with the fridge (sometimes) doing an N-E short as it turned off.

[puffernutter]

Once again, thanks for all the replies. I shall consider the options posed and work on bringing the installation up to be compliant with the latest edition of the Wiring Regs and provide the most robust solution for the house.

With regard to emergency lighting (and based on recent experience!) I bought a rechargeable emergency light from Argos (can be set to turn on when the power fails). Twin fluorescent tubes, built in charger and by the weight of it, a fairly chunky lead acid battery. All for =A314.99

Anyway, the main issue is identifying the the cause, not mitigating the effect. On Sunday night I turned CU2 off (garage, electric heating etc.) at the isolator and CU1 was on and we had occasional trips through the night. Last night, I turned CU1 off at the isolator ("house" circuits) with CU2 on and we had no trips. However three minutes after turning CU1 back on I had a trip!

So, the next process is to isolate the circuit causing it and as it has been pointed out, with a TT system it is as likely to be a neutral/earth fault as a live/earth fault. The big problem is that it is intermittent! A hard fault will be a lot easier to solve.

So the next series of tasks is to start with the socket ring mains and turn off the MCB and remove the relevant neutrals to see what happens.

The nightmare scenario is that I have a "partial" fault on the garage and a "partial" fault on the house. Neither (individually) is enough to trip consistenly, but both together......

Anyway, when I've identified the circuit, I can explore further to determine if it's the circuit or something attached to it.

I'll keep you posted.

Cheers

Peter

[tony sayer]

Immersion heater if you have one?.

Prime suspect....

[Dave Liquorice]

Nightmare indeed. Known leaky things are mineral insulated water heater elements. You appear to heat by electric so one assumes you have an immersion heater. These aren't normally wired into the E7 supply as you made need to "boost" it during the day when the E7 circuit is dead. Have a good check of that it may have become leaky due to corrosion.

[puffernutter]

That's a good thought and something I tested early on. The immersion is relatively new and it is also on CU2 - the heating circuit. That ran all night last night with no trips. So by itself it seems ok, but as I said earlier, if its slightly leaky.....

Our "E7" is through a radio? timeswitch and the whole house goes over to the cheap rate.

It also switches back at 07:30 - how do I know, the pulse (or whatever) interferes with the electronic door bell (battery powered totally separate) such that it rings !

Cheers

Peter

[Ian White]

Ditto the kettle, the washing machine, the dishwasher, or anything else that heats water and might have an unswitched neutral.

Only problem is... when these things corrode through and start leaking, the fault is usually not intermittent.

Another diagnostic tool might be a mains electric clock that will show when it stopped. Keeping a log of the times might correlate with something. Also, plotting it out can sometimes show things you might miss from reading a list of numbers.

Couldn't be a gnawing rodent, could it?

[Dave Liquorice]

How do you "boost" the HW during the day or can't you? The boost sometimes feeds a second immersion heater near the top of the tank so you only heat a little water rather than the whole tank. The E7 would feed an immersion at the bottom of the tank thus heating all the water.

All the radio unit does is replace the mechanical clock. AFAIK all E7 supplies operate as you describe.

Electronic as in radio or some synthesized bell thing? Almost certainly due to an arc when the E7 contactor drops out. Says something about both the bell and the E7 switching...

[puffernutter]

I can't! There is only one feed out of the "Board" meter/timeswitch. It is an "all or nothing" set up.

Cheap (and nasty!) electronic. The bell push looks like it feeds into a transistor switch, so it's probably RFI as the load switches (must be break before make).

Cheers

Peter

[Dave Liquorice]

That must be very inconvient. What happens if you use all your stored hotwater during the day? Pans on the stove, kettle? Yuk...

I'd be surprised if anything other than the heating CU is switched. Of course there still could be a load there even after 7hrs if your storage heating etc hasn't replenished all it's lost heat or is not properly set up on the heat input/output controls.

Switching the normal house CU strikes me as a bad idea, even with a make before break switch, from reliabilty of the switch and connected equipment POV. The meter is normally fed a "sense" wire from the timeswitch to tell which unit counter to increment.

[bigcat]

Indeed. But I dont think youve seen the light yet. Let me attempt to explain:

1. There will almost inevitably be acceptable earth leak curents in most or all circuits, this will make determining where the fault is very difficult indeed.
2. As you corectly describe, youre in for a nightmare if you try to patch up the existing setup. Its a horror, and will take day after day after day. During which time you may lose your freezer contents, and will suffer ongoing inconveniences as parts of the house are unpowered.
3. And at the end of it all, youll still have an unstable install thats bound to repeat the whole process in future.

Its just not a sensible way to go.

OTOH if you rewire the CU/RCD arrangement:

1. You immediately will have no further full loss of power problem, you'll only ever lose one circuit.
2. beacuse of 1. you wont lose your frozen food
3. Your acceptable leakage currents will be divided among several RCDs, which will either: a) eliminate all your troubles, present and future, or b) make fault finding miles easier (Realise that a is as likely as b)
4. Your future upgrade issue will already be done, meaning no more work to do.

Its the only sensible course to take.

this is more or less inevitable

NT