In message ,
Pet wrote:


My concern arose after getting a 120v shock from our microwave oven
whilst leaning against the stainless drainer to remove some blinds.

The socket the Microwave was plugged into had a fractured Neutral (not
connected)

That's an interesting diagnosis. The case shouldn't be live under any
circumstances (well, not these days anyway), and unless you have a high
impedance supply and the neutral in the socket connected itself to Earth
I'm not sure how this happened... and why 120V?

so, the 120v from the microwave case screw could have been
far greater than a "tingle" if the sink had been bonded?


Possibly in this case because:

And, If it were bonded, would such a neutral fault/shock have tripped
the RCD? as on this occasion it didn't.



It seems to me as if as well as a neutral fault you also have suspect
earthing.

On a TN- supply if any part of an earthed object becomes live (as you
are implying the case of your microwave did) then enough current should
normally flow to blow the circuit fuse or trip the breaker. This is why
a whole-installation RCD is unneccessary for these supplies.

With a TT supply (local earth rod) the impedance of the earth path is
usually too high for this to happen, and so whole-installation RCDs are
required. We won't get into the types here, but the fact is that they
trip with minute amounts of current and your RCD *should* have
disconnected the supply the moment the case of your microwave became
live; it should not have had to wait for you to touch it.

You don't have a double-insulated microwave, do you? If so, it is highly
suspect and needs checking out.

The problem that bonding is designed to address is that although you
would think that your (say) cold water pipe shouldn't have anything to
do with your electrics, it is in fact probably quite well attached to
the electrical system through the earth. The bonding regulations are
there in order to ensure that any metal parts which could be "earthy"
are connected together. In that instance, if the case of your microwave
became live, then rather than the pipework being at (near) earth
potential, it would rise (due to the bonding) to the same potential as
the case. This protects you during the finite period it takes for a fuse
/ MCB to blow, and further protects you should circumstances conspire to
prevent them blowing.

Should you then happen to touch both live case and bonded metalwork,
rather than some current flowing, no current should be able to flow
because both are at the same potential. Sounds silly, but it works.

Supplementary bonding is only strictly required in a bathroom because it
is here where your body's resistance is lowest. Whether or not to
install it in a kitchen is a moot point; the OSG (guidance from the IEE)
suggests it isn't usually necessary, but the electrician's "bible" (John
Whitfield's book) insists that it is.

Where supplementary bonding *isn't* required is when the metalwork in
question is effectively isolated from the earth. This could be, for
example, a metal door frame or a metal radiator supplied by plastic
pipes: beyond a certain length of pipe (somewhere around 1m) the
resistance of the water is high enough to be able to consider that the
radiator isn't "earthy" at all.

--
Martin Angove: http://www.tridwr.demon.co.uk/
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