In message j3l6c.284$W11.194@newsfe1-win,
"Doctor D." wrote:


"Christian McArdle" wrote in message
et...
I need to cross bond two sinks fed by an instant electric water heater
in
a
cloakroom (no bath or shower.)

No you don't. Supplementary bonding is not required.

Christian.


Thanks.
Interesting.

I have a failure certificate from an NICEIC member electrician stating that
this needs to be done before we get a pass!

Would it make a difference that this installation is in a public building
and not a domestic property?



Quite the opposite. BS7671 specifically isn't statutory (yet) for
domestic properties, but through the Electricity at Work Act etc. is for
just about everywhere else.

I've always found this aspect of supplementary bonding puzzling, and I
now think I've found the reason. John Whitfield's excellent "The
Electrician's Guide to the 16th Edition of the IEE Wiring Regulations
BS7671" quite emphatically states that all extraneous conductive parts
should be bonded anywhere where there is a chance of a person
simultaneously touching an earthed electrical device and the
aforementioned extraneous conductive part (section 5.4.3). This book
is in many ways the jobbing electrician's Bible, and I'm sure that
this is why you see bonding in kitchens, loos and laundry rooms.

But the purpose of this bonding is to ensure that a person doing so
doesn't experience dangerous level of exposure. John Whitfield does some
needlessly complicated multiplication and division, based on some
rules of thumb about safe levels of current and the average resistance
of the human body and comes to the conclusion that unless your extraneous
conductive part has 25kR or more of resistance to the system earth it
*must* be bonded. If this is really the case then why the heck don't we
bond radiators next to sockets in living rooms and bedrooms?

The safety requirement is enshrined in BS7671:2001 413-02-04 where as
far as I can tell it actually states that the requirement for TN systems
is a 0.4s disconnection time (413-02-08) and for TT systems that
(basically) there should never be more than 50V between an exposed
conductive part (e.g. the metal case of an electrical item) and an
extraneous conductive part (your water pipes) (413-02-20). There's
something in there about 5s disconnection too, but the 50V is the
important bit for this discussion.

So, apart from "locations containing a bath or shower" which have their
own over-riding supplementary bonding rules (section 601) and certain
other applications (part 6), this means that if you can prove that a
phase-earth short will trip your MCB in 0.4 seconds (TN systems) or not
give rise to more than 50V (TT systems) you are ok.

The complication is that the 50V requirement is almost impossible to
meet for a TT system as the impedance of the earth rod is usually too
high. In this case an RCD is fitted which of course will trip with a
much lower current and hence makes meeting the 50V test easy.

So is supplementary bonding in locations outside a bathroom ever
neccessary? Let's go back to 413-02-04:

"Where the conditions for automatic disconnection [discussed above]...
cannot be fulfilled by using overcurrent protective devices, then
either:

(i) local supplementary equipotential bonding shall be applied... but
the use of such bonding does not obviate the need to disconnect the
supply for reasons other than protection against electric shock, such as
thermal effects, or

(ii) protection shall be provided by means of a residual current
device."

To summarise, the way I see it is this:

Supplementary bonding is *always* required in a bathroom.

If your circuits meet the 0.4s / 50V tests then it is *not* required in
kitchens, downstairs loos, places with sinks or laundry rooms etc. Point
the electrician who failed you in the direction of 413-02.

The reason you see such a lot of bonding in places like this, even on
new installations is not that the circuits don't meet the tests but
rather that the electricians in question have read John Whitfield's book
:-)

fx: sits back and waits to be shot down in flames

HTH

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Dessert? I'll take a piece of cherry ã.