"Andy Wade" wrote in message
...
dennis@home wrote:

But not in a normal house, using normal appliances

Yes, in any electrical installation.

as they metal case will always be bonded to earth.

To speak this language properly you just say "earthed" - not "bonded to
earth." But yes, it will (or should, unless it's Class 2). So what? That
earthing and equipotential bonding are distinct and separate concepts
doesn't imply electrical separation.

I didn't ay they did.
I just object to being told bonding stuff to earth is wrong when in 99.999%
of homes that is exactly what is done.


[Towel rail example]
Its actually two faults, the earth has been disconnected and the live
insulation has failed, not actually very common.

I said that the o/c circuit protective (earth) conductor was a common
fault. If the case becomes live through an insulation failure that is
indeed a second fault. It's not an unknown situation though because the
first fault can go undetected for years.

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

The casing of the appliance is _earthed_ via its circuit protective
conductor (CPC) - typically the 'earth' wire in a twin-and-earth cable. In
a bathroom, ignoring the 17th ed. for a moment, the casing should also be
_bonded_ to the CPCs of circuits feeding any other equipment in the room,
and to any other extraneous-conductive-parts (E-C-P). The latter term
means anything that can import a potential into the room (usually, but not
necessarily, earth and with the ability to sustain the flow of a dangerous
current. So metal plumbing which might be earthed elsewhere counts as an
E-C-P, but floating items don't. The bonding might or might not be
earthed elsewhere.

That would be the main bonding to where exactly?

Main bonding is the connection of incoming metal services such as gas and
water to the main earth terminal, creating an equipotential zone. It's
absolutely fundamental to the safety of PME supplies where it's possible
for the suppliers combined neutral and earth conductor to come adrift,
leaving all your earthing up at 230 V.

So you agree that I was correct in saying you bond it to earth.


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

The supplementary bonding is connected to the earth terminal in the FCU.
This achieves bonding to the case via the CPC in the flex. There are
other situations where a green-and-yellow wire might be acting as both a
CPC and a bonding conductor.

And again.

I really don't see where you differ from what I said.


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.

Earth-free bonding is one of the measures recognised in BS 7671 for
special situations under an engineer's supervision. Green-and-yellow
identifies protective conductors, whether earthed or not, and should have
been used if this is something within the scope of BS 7671.

--
Andy