Is the Electrician going over the top in conformance


Postby beamer » Sat Oct 11, 2008 11:50 am

My kitchen is being revamped and the electrician has advised that I need a new consumer unit to conform to 17th edition. I have no issue with this but on top he is quoting for a consumer unit that incorporates a 100ma RCD in addition to the two 30ma zones.

Presently I have an external 100ma RCD that protects the power to a 1.5KW sewage pumping station. The RCD connects via a 6 inch length of cable to the fuse box CB but the electrician is insisting that current regulations do not allow any power to leave the consumer unit without being earth leakage protected! The additional cost is significant versus simply having a 3 zone consumer unit with 2 zones protected within the box and the other protected outside the box.

Can anyone clarify the regulations for this.
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Postby ericmark » Sat Oct 11, 2008 8:35 pm

RCD protection (30ma) is required for:-
1) Everything in a bathroom.
2) All sockets under 20 amp.
3) Cable not conforming to set standards and buried less than 50mm in a wall.
RCD protection (300ma) is required for:-
Fire protection not normal in Domestic
RCD protection (500ma) is required for:-
Sockets over 32 amp on construction sites.
Also with a TT supply a RCD will be required the size is by calculation normally 100ma.
One can fit a smaller RCD so TT supplies would normally have 30ma now.
If except for bathroom you wired your lights in Ali-tube cable to BS 8436 then you would not need an RCD unless it was required due to low earth loop impedance.
If the pump would likely trip a 30ma RCD and you are on a TT supply then maybe it would require a 100 ma RCD I have never inquired as to if RCBO are available in other than 30ma I think there is an add on type Vigi I think but not sure.
So question has to be what supply do you have? Is there an earth rod?
Eric
ericmark

Postby beamer » Sat Oct 11, 2008 9:10 pm

[quote="ericmark"]RCD protection (30ma) is required for:-
1) Everything in a bathroom.
2) All sockets under 20 amp.
3) Cable not conforming to set standards and buried less than 50mm in a wall.
RCD protection (300ma) is required for:-
Fire protection not normal in Domestic
RCD protection (500ma) is required for:-
Sockets over 32 amp on construction sites.
Also with a TT supply a RCD will be required the size is by calculation normally 100ma.
One can fit a smaller RCD so TT supplies would normally have 30ma now.
If except for bathroom you wired your lights in Ali-tube cable to BS 8436 then you would not need an RCD unless it was required due to low earth loop impedance.
If the pump would likely trip a 30ma RCD and you are on a TT supply then maybe it would require a 100 ma RCD I have never inquired as to if RCBO are available in other than 30ma I think there is an add on type Vigi I think but not sure.
So question has to be what supply do you have? Is there an earth rod?
Eric[/quote]

My supply is single phase with the earth is taken from the incoming armoured cable shield. 30ma RCD's trip on the pump transient start up, hence I need 100ma for the pump station consisting of two 1.5KW pumps.

Since the 100ma RCD is already in place and working just fine, I would like the electrician to fit a 3 zone consumer unit, the 3rd zone would simply be a single unprotected 30 amp CB that connects to my present pump RCD via the 6 inch length of external cable. That's where the electrician is stating that it would not meet current regulations. i.e any wiring leaving the consumer unit MUST be RCD protected even though it never leaves the mains power supply board (the RCD is next to the consumer unit).

Seems a little extreme to me but I welcome informed input on this.
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Postby ericmark » Sun Oct 12, 2008 1:45 pm

You must appreciate we can only go on what you say and there may be something we are unaware of.
But if the length of cable (That is wires covered with sheaf i.e. a double insulation of some sort) is on the surface where it can be seen then there is no need for it to be protected by an RCD. So he seems to have miss read the regulations.
Only where the cable is buried at less than 50mm in a wall is either RCD protection (At 30ma so the 100ma trip will not count) or one of a range of cables or protective shields complying with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 the tin capping will not comply but there is some similar capping which would need hack saw or angle grinder to cut rather than tin snips does. There does seem a few problems where SY cable does not have required BS number but does really do what is required which is that if someone not realising there are cables below tries to knock in a nail either it will fail safe or stop the nail from entering the cable. 522.6.5 to 522.6.8 are the regulations.
So to the RCD I can’t see a problem but from the RCD as not 30ma there maybe.
Eric
ericmark

Postby beamer » Sun Oct 12, 2008 4:49 pm

[quote="ericmark"]You must appreciate we can only go on what you say and there may be something we are unaware of.
But if the length of cable (That is wires covered with sheaf i.e. a double insulation of some sort) is on the surface where it can be seen then there is no need for it to be protected by an RCD. So he seems to have miss read the regulations.
Only where the cable is buried at less than 50mm in a wall is either RCD protection (At 30ma so the 100ma trip will not count) or one of a range of cables or protective shields complying with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 the tin capping will not comply but there is some similar capping which would need hack saw or angle grinder to cut rather than tin snips does. There does seem a few problems where SY cable does not have required BS number but does really do what is required which is that if someone not realising there are cables below tries to knock in a nail either it will fail safe or stop the nail from entering the cable. 522.6.5 to 522.6.8 are the regulations.
So to the RCD I can’t see a problem but from the RCD as not 30ma there maybe.
Eric[/quote]

Eric many thanks for your explanation. After reading your note it seems clear to me that the electrician has miss understood / got confused over the external cable regulations. In my case the cable really is no longer than 6 inches and is rated at 30 amp, hence it would be difficult to miss.

With regard to the RCD being 100ma and not 30ma, how do I get over that requirement? Flygt the manufacturer of the pumps recommends a 100ma RCD to avoid nuisance tripping. Initially I did have a 30ma RCD in place but it tripped once or twice per month!
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Postby sparx » Sun Oct 12, 2008 6:51 pm

Hi, the electricians statement is factually incorrect!
There is no requirement for all wiring to be RCD protected.

If the wiring is surface run either clipped direct or in trunking or conduit it does not require an RCD.
Therefore if a lenght of cable either tails or tw/E comes from an MCB on surface to an existing RCD no other protection is needed!
To go further it would be within the regs to wire the entire house in minitrunking and only the sockets would need rcd protection (which could be by using RCD sockets), plus possibly other circuits in a bathroom, even this could be achieved with an rcd FCU for the light/fan ect.
Also as to your pumps if they are wired with say SWA cable and are not connected with plug/sockets they may well not require any RCD protection under regs although it is a good idea
as ERICMARK says it's always difficult without seeing set up personally to be 100% certain but from your account I feel your instincts are right.

For the sake of correctness the regs that apply are 522.6.6 & 7,
"A cable concealed in a wall......"ie NOT SURFACE!

Page 100 in the RED BOOK.

regards SPARX
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Postby ericmark » Mon Oct 13, 2008 6:55 am

If the cable to the pump is not buried in wall at less than 50mm and any plug and socket is marked as to what it is for i.e. Pump than there is no problem with 100ma trip.
Eric
ericmark

Postby beamer » Mon Oct 13, 2008 4:00 pm

Many thanks Sparx and Ericmark. The pump supply is wired direct from the RCD through to an industrial grade control box via plastic conduit. Cables from the control box are sent some 20 meters under ground to the pumps.

I feel confident thanks to your valuable input that the Electrician has simply miss understood the regulations and will push him to review the situation.

Thanks again to you both.
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Postby ericmark » Tue Oct 14, 2008 9:05 am

Assuming conduit visible and underground cable is SWA then no RCD is required other than that recommended by pump manufacturer.
ericmark

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