dennis@home wrote:

"John Rumm" wrote
No such thing as "earth bonding"

Well you can argue about the term if you want, I am sure someone will
join in if you want.

John's absolutely correct and, to add to what he's already said,
earthing and bonding should be thought of as quite separate concepts:

- earthing (of Class 1 equipment) is an /active/ protection measure that
works by automatically disconnecting the supply to the faulty equipment;

- an alternative to earthing is the use of Class 2 ('double' insulated)
equipment which is constructed such that the chance of exposed metal
parts is close to zero. (The wiring regulations still require an earth
to be available in a circuit feeding a Class 2 appliance, so that it can
safely be replaced by Class 1 equipment.)

- Equipotential bonding is a /passive/ protection measure which prevents
dangerous 'touch voltages' appearing between different items simply by
connecting them together with low-resistance conductors of a size that
won't overheat or rupture for any current reasonably likely to flow.

No I just don't like earths when there is a small chance of getting a
shock because of them.

Things like water taps on plastic pipework (even with copper tails) are
not going to become live on their own - so they don't need to be bonded.
In the language of the wiring regs they are not
extraneous-conductive-parts because they don't import a potential from
outside the location.

Say a towel rail becomes live..
this indicates that there is a fault in the towel rail and that the case
*is not* connected to earth or the fuse would blow.

This is a good example, an open-circuit circuit earth (CPC) being a
fairly common fault.

Now all the other stuff like pipes are connected to earth and you get a
300V+ potential between them.. nasty shock.

With supplementary bonding in place there can be no significant touch
voltage, whether or not anything blows or trips. If the bonding
provides another path to earth, as it often will, the chances are that
the overcurrent protection or RCD will operate as normal and isolate the
fault. If the bonding is otherwise floating then you may end up with
230 volts on everything. With correctly done bonding there should be no
risk of a fatal or serious shock, but people are likely to feel tingles
when touching metal items and this will hopefully eventually lead to the
fault being diagnosed.

Under the 17th edition regs the supplementary bonding can be omitted,
but only if the main bonding is OK and all circuits feeding the bathroom
are 30 mA RCD protected. In this case there is the potential (no pun
....) for a serious shock between the live towel rail and other (earthy)
metalwork. However if the shock current is large enough to be really
dangerous the RCD will trip and cut off the supply within a few
milliseconds.

Of course if there were actual physical earth points on the towel
radiator rather than just the earth in the flex you might have a second
earth to connect to the pipes but that's not likely as the fuse hasn't
blown.

The regs allow the flex to be used as part of the bonding (you bond in
the fused connection unit). Clearly the committee considered that the
risk of an open circuit in the flex was low enough not to worry about.
If you disagree there's nothing to stop you bonding to the rail itself.

--
Andy