RCD questions with split-load consumer unit

[James]

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

 

[Andrew Gabriel]

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.

Can't completely answer without knowing what earthing system you
have -- see http://www.diyfaq.org.uk/electrical/electrical.html#system

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.

 

[Andrew Gabriel]

Make sure your earthing (including bonding) is up to current spec.

Not worth protecting:

That should be weighted/worded more towards "Worth not protecting".

o Fixed/stationary appliances with metal sheathed mineral insulated
heating elements, e.g. imersion heater, washing/dishwashing machines.

Add "electric oven", providing the oven control switch doesn't have a
combined socket outlet.

 

[James]

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.

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

 

[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?

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.

I will check the earthing around the house - thanks for the tip

Important points are service bonding (gas, water, electricity,
oil, etc) with 10mm² cable, and equipotential bonding in
bath/shower rooms, normally with 4mm² cable.

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...

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?

 

[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.

I will have to look up the different earthing systems and see which this
house fits best.

Important points are service bonding (gas, water, electricity,
oil, etc) with 10mm² cable, and equipotential bonding in
bath/shower rooms, normally with 4mm² cable.

ok, will do

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?

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 :-(

 

[James]

Can't completely answer without knowing what earthing system you
have -- see http://www.diyfaq.org.uk/electrical/electrical.html#system

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.

 

[steve]

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

 

[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.

Many thanks for pointing me at that very useful site
http://www.diyfaq.org.uk/electrical/electrical.html#system

I have taken the shower off RCD and now everything is ticketyboo.

Cheers
James

 

[Andrew Gabriel]

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 :-(

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.

 

[Andrew Gabriel]

Many thanks for pointing me at that very useful site
http://www.diyfaq.org.uk/electrical/electrical.html#system

I have taken the shower off RCD and now everything is ticketyboo.

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.