On Fri, 5 Sep 2003 13:06:03 +0000 (UTC), "NC"
wrote:

excuse my ignorance - but what do you mean by 'bonded' ?? earthed ??

Bonding is like earthing, but without the earth !

All metal parts (pipes, shower rails, bathtubs) are connected together
with green & yellow, and this is connected to the earth terminal of
the supply to the room. There is no need to run a separate earth
connector for this metalwork, back to any centralised earth. Nor does
this bonding conductor need to be as large as many earth conductors
do.

The difference between bonding (or equipotential bonding) and earthing
is in what they're trying to achieve.

Earthing is there so that when a live-case fault develops, enough
current flows through the earth terminal to blow the fuse or breaker
supplying the equipment. Impedances must be low, or there won't be
enough current to blow the fuse - which is why it's important to test
earth loop impedance, not just rely on a neon tester.

Equipotential bonding is there so that no two metal objects can
develop a high voltage across them. This is typical from a high
impedance fault condition - maybe a heater with failing insulation,
not enough to cause an earth fault, but still enough to make the case
give you a shock. Humans are normally high impedance, so won't suffer
such shocks - but wet humans are much more conductive, so bonding
becomes especially important in bathrooms.