Electrical funny

[Adrian Simpson]

I'm having a bit of "fun" with my electrical system at home, and would appreciate any advice.

The system has a RCD between the meter and the consumer unit (not sure why, it was there when I moved in, but it has proved useful). The earth is a spike in the ground at the back of the house.

Thursday night just as I turned the shower on, the RCD tripped out. So I found a torch and reset the trip in the dark. I tried the shower again, and off went the power again. I concluded that the shower had died (it is 10-12 years old, so that seemed a fair conclusion).

This evening (Saturday), I was busy cooking my evening meal (on an electric hob), and after about 10 minutes, off goes the power. Having been moving the wiring for the boiler earlier in the day, I wondered if I had got something wrong on that, so I turned off the RCD for that on the consumer unit, put the power back on, and went back to cooking. 5 minutes or so later (certainly not straight away), off goes the power again. I turned off the hob (which is 15 months old, so should have some life in it yet), reset the trip, then tried the other two rings, and within a minute or so, off goes the power again. The good news is that by now the spuds are done, so at least I can get something to eat.

Having eaten the spuds, and thought about it, I consulted my tame electrician (Dad). One thought that crossed my mind was that after a long dry summer, the earth spike could be making a bad contact (*), so whilst it was a long shot, it was worth a try. So I slowly poured some salty water round the spike, allowing it to soak in. I then tried the hob again for 20 minutes or so, and this time it was fine.

(*) memories of tales of old phone systems not working until a bucket of water was tipped over the earth lead.

So, is it a coincidence that watering the earth spike has got things working ?. Is there anything else I should do (getting it looked at professionally will be arranged next week) ?.

TIA

Adrian

[tony sayer]

In article , Adrian Simpson writes

The RCD has nothing to do with the earth as such. All it does is look at the current into the protected circuits, on the live and checks that the same comes back out on the neutral. Any imbalance i.e. leakage to earth then it will trip the level needs to be about 15 ma or so..

Usual causes are leakage between heating elements to earth especially cookers, immersion heaters, and anything which uses an enclosed heating elements inside a metal case.

Another cause sometimes is if the earth and neutral are inadvertently connected together anywhere. This will result in nuisance tripping when the load on the system increases. Some of the current will be carried by the earth rod and if you have made this a *better* earth then a part of the fault current will now be diverted via this route back to the substation and hence the imbalance between power in on the live and out on the neutral will cause the trip to drop....

[stefek.zaba]

This is an accurate description of a modern RCCD. However, the earlier type was voltage-operated - it would look to see when the house earth potential had risen to more than 50V above that from the earth rod (!). This type of voltage-operated earth breaker hasn't been fitted for 15+ years. (You can usually tell the difference by reading the case: the more recent current-balance ones will include an indication of the current sensitivity, usual values being 30mA or 100mA. The older voltage-operated ones tend not to have anything too detailed written on them, and will have their own relatively thin (4mmsq, say) earth wire going off to an earth rod).

But it's quite possible that the original poster has just such a unit. (It's also possible that it's a whole-house current-balance one). Whichever the exact type of the RCD, there *is* likely to be some sort of a fault. The whole-house RCD will have been fitted precisely because the supply can't provide a good earth - common in rural feeds, but also on some estates (our first house in Abingdon had such an arrangement). The dicking-about with the central heating wiring might have resulted in a small increase in earth leakage, such that the whole-house breaker now has less margin for other small leaks (e.g. from the cooker).

It's not a bad idea to get an intelligent and experienced electrician in at this point, who will have suitable test gear and the practice in using it. At our previous place I found just such a gem, who as part of the 50-quid inspection did some excellent fault finding on the earth leakage side of things (we had both a rather-too-sensitive nominal-30mA trip which went out at about 13-14 mA: the 30mA is a nominal value and is specced to mean "will pop at no more than 30, won't pop below 15". And we had both a grill and a cooker element which leaked a bit too much; this, added to the immodest number of computeren with their mains input filters each leaking 1-3 mAs, accounted for the excessive number of nuisance trips). So, in this case at least, it was well worth getting a Professional in!

HTH, Stefek

[BigWallop]

You could try putting another metre length on the earth spike to take it further in to the ground. :-))

One thing I've noticed on new installation I'm visiting, is that the RCD's which are fitted in the mains supply like yours, are only rated at 30mA imbalance fault signal. This is really not adequate for a whole house supply. This, to me, is erring on the "to safe to be properly functional" side of things and is now causing a lot of nuisance breaks in whole house supplies that are being installed these days.

A split load consumer unit seems to be a better design for a whole house supply system and this is the road I'd advise you to take, if not already installed, in your property. The split load CU separates low Wattage loads like lighting and ventilation fans etc. from heavier loads like heating circuits and larger domestic appliances fitted to other ring circuits throughout the house. The heavier loads are controlled by an RCD with individual MCB's to each circuit and depending on the load you may be putting on the supply, can be rated up to 100mA fault signal before a trip is activated. The lighter loads are controlled through a standard double pole mains switch. This is safer because it leaves the lighting on so you're not stumbling around in the dark to find torches and things, and if the it is the lighting that has tripped off, then you can still plug in a table to see your way around.

I still think designers of these electrical installations are not considering that many people are actually drawing a lot more power from the supply than they think and they are not allowing for heavier heating elements in showers and washing and cooking appliances, and the fact that a lot more dwellings can have more than one of each of these items installed today.

The only place I now err on the side of "better safe than sorry" is in the earthing continuity I install. If I can't get below 2 ohm impedance to earth on any of my installations, then I feel that I have failed to provide an adequate safety measure, even though an allowance of 10 ohms or below is required to pass current testing approval.

These are just my thoughts on things. So. End of ramble. :-)

[Wanderer]

Must admit, this was the first thing that crossed my mind when I read this thread. First thing, get someone who knows the difference to see if you have a voltage operated trip.

[Andy Dingley]

No, chances are that it's a voltage-activated RCD. Is it old ? 15-20 years ? These things were a bit prone to this problem.

-- Blind drunk - Please ignore all postings I make, until I sober up enough to notice the .sig file that has been attached to them. Thank you.

[ARWadsworth]

It could be labelled ELCB if it is one of the older types. Also as you say, the old units frequently are connected to earth whereas an RCD is not.

Adam

[Andrew Gabriel]

Some RCD's have earth connections too. A unique feature of a Voltage Operated ELCB is that it has two separate earth connections -- one to the house wiring, and the other to the earth rod. It operates by measuring the voltage between these two connections.

[BigWallop]

I was thinking of it being an RCD (ELCB) with an active earth connection, or so I would have thought due to the age of the installation as the OP mentions.

[BigWallop]

[The Natural Philosopher]

Hear Hear!

I replaced my 30mA with a 100mA and haven't had a nuisance trip since.

By all ,means protect sockets or outsode rings witha sperate 30mA, but teh amount of leakage off electronic equipment is enough to regurarly trip 30mA IME.

[tony sayer]

Well FWIW our house and the rented ones we own all have 30 ma RCD's and none have any nuisance tripping problems except when a fault occurred on an immersion heater when it warmed up a bit.

The only common problems I've seen over the last few years are where a lot of computers and electronic equipment's are connected to the same protected circuits. IIRC there are new reg's to cater for this sort of thing in offices etc now...

[The Natural Philosopher]

Yerrs Tony. But I have - lets see.

Two computers. Two printers Scanner Two monitors PC loudspakers. PABX Cisco router Labgear distribution amplifier.

6 TV's 3 radios

...all of whach are permanently pluugedd in, and, because the remotes work, probablly have RF filters across the incoming mains.

Plus a random selection of battery chargers and so on. Not to mention an immersion heater and two electric cookers, two fridges and a deep freeze, all also permamently on.

Plus 9 extrenal lights and a klargester down the garden, and 2 external power sockets, that get a leetle moist occasionally...

30mA was nusiance tripping on load for a long time - no pattern to it - just general earth neutral leakage it would seem. 100mA is rock solid.

I think 30mA is fine for a small house, but not a large one really.

[Adrian Simpson]

Today, the power stayed on, so I got a proper meal tonight :-)

To follow up on the various comments made so far (and many thanks for them) :

The main RCD was fitted new last December (the previous one had a broken casing, so it was changed on safety grounds) 30mA trip, 80A capacity. It is situated on the tails between the meter and the consumer unit, so only has Live/Neutral connections to it. As the house is a low occupancy two bed bungalow, then the total consumption is not that high (one 4 ring ceramic hob, a double oven and 8.5Kw shower being the major items), everything else could be run off a 13A socket. Not sure why the earthing is done the way it is, I'm not rural, but I am on the edge of town. So far as I can tell, none of my neighbours has earthing arrangement that I have, but I doubt that any of them could tell me what they do have. The good news is that at least I do have an earth now (it was not connected up when I moved in).

This morning, I rechecked the wiring that has been disturbed recently, and all appears to be in order. The change in the central heating wiring was to moved the switched fused spur unit along the wall a few inches to make for a better location, the cabling involved is the same as before (which has been in for 6 weeks or so).

Adrian

[stefek.zaba]

Good - can't have you Blaining out on us through lack of food ;-)

OK, this gives us more information to work with. Your whole-house RCD is definitely a Modern current-operated one, then. It's no longer thought Good Practice to fit a single 30mA RCD as a whole-house fitment, actually: split-load arrangements which leave the lights on and put the more-likely- to-have-benign-leakage circuits (immersions and cookers) on the non-RCD side, and the more-likely-to-fault-with-danger-to-human-life circuits

- especially sockets rings "likely" to be used to supply tools used outside the house - on the RCD side. So whoever fitted the 80A/30mA trip (you? a sparks?) was following last decade's accepted practice ;-)

IF you're confident in your own competence, you could go split-load at minimal kit cost by running the tails which now go to the RCD into a Henley block (fat-arse junction box ;-) from which you'd run tails into the existing RCD-and-on-to-current-CU, and also out to a new small CU with say 6 or 8 non-RCD-protected ways, for your lights, cooker, and immersion/CH circuits. This is *only* safe if you have a decent earth for these non-RCD circuits, mind! If you're not sure of that, you really should get a real sparks in. S/he might end up recommending you fit a time-delayed 100mA-trip RCD to give short enough disconnection times if your existing earth isn't low resistance enough; downstream of that you could (and should) still run a split-load arrangement, whether in a single new CU or the Henley-block-and-two-CUs stylie outlined above. The difference in both trip ratings and time-to-disconnect between the

100mA time-delayed trip and the 30mA faster-acting trip preserves good "discrimination", i.e. an earth leakage fault on the 30mA side should make that trip, and that trip only, disconnect, leaving power on to your lights and other circuits.

Hope that helps (and thanks for bottom-posting; now move further up the style scale by trimming the post you're replying to more aggressively

- OK? :-) - Stefek

[Andy Dingley]

You won't be, it just feels like it.

-- Smert' spamionam

[Dave Liquorice]

A whole house RCD should be a 100mA time delayed device. It is there for circuit protection because the earth loop impedance is (or maybe) to high for prospective fault currents to be large enough to trip overload devices within the specified time(s). It is not there for shock protection.

Quite. For that you need a 30mA non-delayed device, even then a belt through one of those hurts. The delayed/non-delayed action is to provide discrimination between the devices, the current rating alone does not guarantee that.

[The Natural Philosopher]

I don't. I rely on common sense. ;-)

[tony sayer]

No argument there then:-))...

[Adrian Simpson]

In article , [e-mail] writes

I'm trying to get hold of a sparks this week to have a look at it (I won't touch that end of the circuitry). Who knows when s/he will get around to looking at it (if my experience of local builders/plumbers is anything to go by, it could be weeks).

I'm happy working on the load side of the consumer unit, but I don't touch the CU or the supply side.

I _never_ top post !. Snipping is always a difficult one, how much context to leave, how much to remove.

Thanks again.

Adrian