RCD questions with split-load consumer unit

Hi
I'm not an electrician but I'm fairly competent with household wiring and
after recently buying a new electric ceramic hob I realised my existing MCB
consumer unit did not have enough ways as it is quite old and only has 5.
So, I have just bought and installed a new Wylex 12 way split-load consumer
unit and I've got a couple of questions about the RCD which is a new area
for me. (I realise that I've been a bit naughty by installing this unit
myself as I don't have the necessary certificates in the UK, but I will get
the necessary council check done when the work is complete.)
So, Q1 - Which circuits should be RCD protected and which shouldn't? I live
in a standard 2 bedroom house, about 90 years old, which was rewired about
10 years ago at best guess.
And, Q2 - I've put the shower on a 45A MCB on the RCD side. I take a shower,
then switch it off with the shower controls. 5 minutes later, after drying,
I pull the isolator cord hanging from the ceiling to silence the extractor
fan, and ping, the RCD trips. Any ideas where the fault could be, and what I
should be looking for to find it?
Thanks in advance for knowledgeable/helpful answers :-)
Regards
James
Reply to
James
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Can't completely answer without knowing what earthing system you have -- see
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Must be protected: o Sockets which might be used to power outdoor portable equipment (typically taken as all ground floor sockets unless you have sufficient sockets installed outdoors, in which case those). o Sockets in a room (such as a bedroom) containing a bath or shower. (Sockets are not permitted in bathrooms/shower rooms, except for low power isolating transfer type for shavers which don't need RCD protecting.) o Fixed appliances in bathrooms/shower rooms in some zones in some cases.
Must be protected in a TT earthing system (but unless covered by one of the cases above, the RCD protection can be 100mA or more, as the purpose is not to protect against electrocution): o All socket outlets (except isolating transformer type). o All circuits where the earth fault loop impedance is too high to guarantee the fault current protective device will trip within 5 seconds of a ground fault.
Good idea to protect, but not mandatory: o All portable appliances, i.e. all socket outlets.
Not worth protecting: o Fixed/stationary appliances with metal sheathed mineral insulated heating elements, e.g. imersion heater, washing/dishwashing machines.
Bad idea to protect: o Lighting circuits o Central heating and any other frost protection o Fridge/Freezer o Burglar/fire alarm
Dedicated protection not to be shared with anything else: o Life support systems, e.g. tropical fish tank heater/airator.
Reply to
Andrew Gabriel
Make sure your earthing (including bonding) is up to current spec.
That should be weighted/worded more towards "Worth not protecting".
Add "electric oven", providing the oven control switch doesn't have a combined socket outlet.
Reply to
Andrew Gabriel
Many thanks for your prompt reply. I take it that a shower should be on the RCD given its proximity to water, even though it has a heating element akin to those in cookers and imersion heaters?
I will check the earthing around the house - thanks for the tip :-)
Have you any clue about my second question - why the shower trips the RCD? Actually, after some experimentation, the plot thickens. If the shower unit is switched off after use, then the isolator cord is pulled to halt the extractor fan and isolate the shower, the RCD trips. BUT, if the shower and fan are both isolated at the same time with the pull cord (ie, the shower is still running), the RCD does NOT trip. It seems if the isolator switch has the full shower load plus fan, no trip, but if it only has the extractor fan, then it trips. Can't immediately explain than one myself...
Cheers James
Reply to
James
Showers don't have to be on an RCD -- they are designed to be installed in a water spray environment. If you are on a TT earthing system, then you will almost certainly find they need RCD protection because earth fault loop impedance is too high without. If you don't have the test equipment to measure the earth fault loop impedance, then putting it on the RCD side is probably safer. If you do test the earth fault loop impedance and it's OK with respect to the fault current protection on the circuit, then it need not go on an RCD.
Important points are service bonding (gas, water, electricity, oil, etc) with 10mm² cable, and equipotential bonding in bath/shower rooms, normally with 4mm² cable.
How is the fan connected, and is it on a run-on timer? You don't normally connect a fan to a shower circuit (and it would need a separate fuse if you did). Have you got it cross-connected between the lighting and shower circuits somehow?
Reply to
Andrew Gabriel
I will have to look up the different earthing systems and see which this house fits best.
ok, will do
The consumer unit has a thick cable running to the isolator pull switch. The fan and shower are connected to this so when on, they are both on, and when off both are off - ie there is no run-on timer and I assume it doesn't cross-connect with the lighting as the fan only operates when the shower isolater pull switch is on. Any further investigation will involve a very nasty crawl in the loft in a couple of inches of soot :-(
Reply to
James
From your article I would say I have the TN-S (The earthing conductor is connected to separate earth provided by the electricity supplier. This is most commonly done by having an earthing clamp connected to the sheath of the supply cable.)
I can see an earth cable come from the main 100A fuse unit along with the live and neutral supply tails. The tails go into the meter and the earth has its own terminal box.
Reply to
James
James
Q1: RCD protection firstly depends on your earthing arrangement going into your house.
If you live in a town or city, the supply authority will probably give you an earth terminal to connect to. This means that what should be protected by an RCD is all socket outlets expected to be used for equipment outdoors including socket outlets that are outdoors, (The NICEIC say that is all ground floor sockets in a two storey house!) and sockets or equipment in areas of increased risk of shock, (Equipment fitted within the various zones of a shower room for example). Lighting, first floor sockets and fixed equipment in normal locations, immersion heaters, e.t.c can be put on circuits that are unprotected by an RCD. (If you have a large freezer or alarm system downstairs and work away often, it's better to run in a dedicated, non RCD protected circuit with a switched spur (with flex outlet only), to the freezer or alarm system just in case of nuisance tripping of the RCD)
If you're in village, they probably won't go to the effort, and earthing will be your problem. This means that you will have to place an earth electrode in the ground and have ALL circuits protected by one or more RCD's (You can use different tripping ratings and time delay types to avoid nuisance tripping on some circuits)
Q2: We had the same thing when a microwave oven would ping at the end of cooking. After several years of some very intelligent engineers looking into the problem, we decided to buy a new one which didn't ping out! Look for condensation within the fan unit or more likely, within the pullcord switch after a shower. An easy test of this would be to use a rubber glove to operate the pullcord switch. If the RCD didn't trip on that occasion, a small current could have be finding it's way through your body, back to earth on the previous occasions. You could solve this by putting a better insulating cord on the switch and getting rid of the condense. If it's not that, there could be leakage or fault within the fan unit itself. Don't forget, where an RCD is fitted, it monitors the condition of both live and neutral conductors, so unlike an MCB, it will trip even if there is a very small amount of leakage between earth and neutral. (0.03 of an amp mormally)
You can download the shower and bath zones from the regulations pages of the iee website (iee.org), and useful info about RCD circuits can be found in the "wiring matters" newsletters on the same website. NICEIC has a few bits and bobs on it (niceic.org), and they also have a helpline you can ring (poop). Part P, the building regulation, can be found somewhere within the Office Deputy Prime Minister (odpm.gov.uk) website, which is about as useful as a chocolate fireguard, but worth a quick look I suppose. The book I recommend is the IEE Electricians Guide to the Building Regulations cost about £16 i think. (If not look on Ebay)
What else can I say, but, work safe, rub vinegar on the leads seals to get rid of any scratch marks you've made, and if the meter starts running backwards, let me know how you did it!! Cheers Steve
Reply to
steve
Many thanks for pointing me at that very useful site
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I have taken the shower off RCD and now everything is ticketyboo.
Cheers James
Reply to
James
I think you need to investigate the circuitry further. A fan protected only by the shower fuse is definately not acceptable. The fan would normally be in a lighting circuit, or ring circuit with a fused connection unit.
Reply to
Andrew Gabriel
That's not the right thing to do. You need to find the problem which causes the RCD to trip. There may well be a genuine fault still present.
Also, as I said before, you should test the earth fault loop impedance before running the shower without RCD protection. Also make sure all your bonding is up to spec as I said earlier.
Reply to
Andrew Gabriel

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