Supplementary Bonding time again

[The Jeep]

Well, there hasn't been a post about this thorny subject for a day or two, so here's mine...

Having looked at all the relevant guides, I still find the subject confusing. I have fitted an electric shower (with 45A DP isolator & pullcord), new zone 3 ceiling light and SELV fan (transformer and isolator in cupboard beyond zone 2) to a bathroom containing a metal (enamelled) bath with copper pipes, a sink with copper pipes and a radiator with copper pipes. There's also a copper gas pipe running through alongside the radiator pipes.

The existing bonding runs from the bath hot & cold (under the bath) to the radiator flow & return and on to the earth of a socket in the next room (the wiring of which is made from under the bath).

Any ideas what I need to do to bring this installation in line with current regs? I have read that all exposed metal parts (shower hose?) must be connected to all extraneous metal parts (radiator, bath, taps?), but this could easily become impractical. I don't understand why , under phase-earth fault conditions, the CPC of, say, the shower, would not be an adequate path for the fault current and trip the MCB (RCD).

I understand that, in a bathroom, you don't want extraneous metallic parts like a radiator and taps being at a different potential, but why would they be anyway? If one became live because of a fault and the other not, then a potentially fatal PD would esist between them, but...hmmm...this is where I get confused!

I understand that copper pipes may not be continuous back to the earthed water main due to plastic pipe additions, but, na, still confused! Is there a really user-friendly guide to all this anywhere??

 

[JohnD]

Is there a really user-friendly guide to all this anywhere??

yes

http://www.niceic.org.uk/downloads/NL139supp.pdf

http://www.niceic.org.uk/nonapproved/earthbond.html

 

[The Jeep]

Thanks John D. I get the idea, but still have a few niggles...

With regard to main bonding, as illustrated in the second of those linked articles, the idea seems to be that if you touched a faulty cooker (exposed conductive part) and, say, a metal tap (extraneous conductive part) simultaneously, the risk of electric shock to you would be reduced because the tap is at earth potential. But is this not offering the fault current yet another low resistance route to earth? And, if so, then it goes through you, too!

If that extraneous part had not been earthed then it could not act as a path to earth...

Also, is it assumed that the cooker CPC might not be adequate to allow the fault current to flow to earth by the designed route and thereby tripping the breaker? If so, why?

Maybe I'm getting too deep into this!

 

[The Jeep]

OK dokey...figured out what I'm going to do in the bathroom (no jokes, please!). Basically connect everything to everything else (well, not quite).

My question is this: if I use, say 4mm² PVC earth cable to connect, what kind of crimp terminals should be used? I already have the 10 & 16mm crimpers and connectors for main & equipotential bonding, but what is recommended for supplementary bonding? Are the warning label tags required and available in a smaller size?

Thanks.

 

[JohnD]

You don't generally need to crimp. The pipes clamps grip the earth core with a screw and nut. They come with their own metal labels. You should be able to join two ends at a clamp, though I prefer to use an uncut length and strip the inulation off enough to tuck under the screw head.

You will also be linking to the CPC in the lighting circuit and, if different, the shaver, and, if present, the immersion heater and electric shower circuit. If the pipes are copper with soldered joints you can often do a lot of the clamps in the airing cupboard if adjacent, otherwise, under the bath.

You are allowed to have joins in supplementary bonding, it doesn't have to be a continuous length like Main Earthing.

You can also use an earthing block if you have lots of wires to join.

 

[The Jeep]

Thanks John D. Will procede as suggested.