"Andy Wade" wrote in message
...
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:

But not in a normal house, using normal appliances as they metal case will
always be bonded to earth.


- 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.

Its actually two faults, the earth has been disconnected and the live
insulation has failed, not actually very common.


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.

As I said unless you are going to modify the appliance you are bonding to
earth.


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.

That would be the main bonding to where exactly?


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.

So where do you not bond to earth within the FCU?

Like I said for domestic wiring it is bonding to earth, there is very little
alternative if you have any sort of appliance.

And as I said if you want to discuss bonding in an industrial context it can
be done.

We bonded huge amounts of metal together but we didn't use green and yellow
earth cable to do it as it wasn't earth.