Not to complicate Adrian's thread.. here's an earthing funny that lay
dormant for many years.
Victorian house - Granny Flat extension added to the rear - all is well
till a graphic artist takes over the flat.
On arrival installs extensive computer system, and at end of day takes a
shower. Two minutes into showering, adjusts the controls a 2nd or 3rd
time - and gets a shock.

Shower is not an electric one, earth bonding is spot on, shower tray and
outlet are plastic. Whole electrical installation seems faultless -
very professionally done with separate CU in the flat on the end of long
armoured cable from main board in the house. [No RCDs anywhere]

The armoured cable is neatly terminated in a proper gland at the
original board.... into a box which is not earthed. So no earth
connection to the armour, the earth conductor out to the flat.

The filters in the computer gear were all probably legal, but there was
a lot of it and enough capacitance to bring the floating earth up to
110V on an AVO.
When the water went out along the drain, presumably with its
conductivity raised by some expensive toiletry - one doesn't like to
enquire! - and reached actually soil-type earth via the cracks in the
Victorian drains.....

I can imagine the installation was tested with the earth clip to the new
box, as being the most accessible earth point: so however well-
intentioned the Govt's new Regulations are, I suspect they wouldn't have
made a haporth of difference in this case.
--
roger
delete x's to email

roger wrote:


The filters in the computer gear were all probably legal, but there was
a lot of it and enough capacitance to bring the floating earth up to
110V on an AVO.

Tell me about it.

We had laptop once, that had been fitted quite normally with capacitors
between live and chassis, and neutral and chassis.

And a mains cable that had no earth pin or even earth wire.
In fact, AFAICR the socket in the laptop into which it plugged was a two
pin affair.

Connecting the laptop to another PC via a serial cable blew the serial
board in the PC.

yup. 110V on the chassis...and a minor misaligment brought the serial
cable 'earth' on contact with an input pin.

As an occasional student of the wiring Regs, i've been wondering this
for a while - when there is a remote CU in an installation like in the
flat in this case, why is the connection made from the main CU to the
remore one made using armoured cable if its internal to the property.
Could the link be made with some hefty T&E?

"Welsh Bob" wrote in message
om...
As an occasional student of the wiring Regs, i've been wondering this
for a while - when there is a remote CU in an installation like in the
flat in this case, why is the connection made from the main CU to the
remore one made using armoured cable if its internal to the property.
Could the link be made with some hefty T&E?

It could be made with T+E but the armour gives a more secure earth bonding
because there is more of it and not just a thin inner core of bare copper.
Also, SWA Cable can be made more secure in any switch gear that is used to
isolate the remote mains supply. It is also more protected against damage.
But there is nothing stopping anyone from installing a sub main with heavy
enough T+E.

"BigWallop" wrote in message
...

"Welsh Bob" wrote in message
om...
As an occasional student of the wiring Regs, i've been wondering this
for a while - when there is a remote CU in an installation like in the
flat in this case, why is the connection made from the main CU to the
remore one made using armoured cable if its internal to the property.
Could the link be made with some hefty T&E?

It could be made with T+E but the armour gives a more secure earth bonding
because there is more of it and not just a thin inner core of bare copper.
Also, SWA Cable can be made more secure in any switch gear that is used to
isolate the remote mains supply. It is also more protected against
damage.
But there is nothing stopping anyone from installing a sub main with heavy
enough T+E.

Indeed, I have done it (yes it was inspected). There is also a separate
earth to the earth rod (in my case). Mine runs through the flat roof - I
guess if it was running exposed you might prefer armoured cable if it's
protected at 100A - you'd get a meaty flash off that in the right
circumstances.


--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

"roger" wrote in message
...
Not to complicate Adrian's thread.. here's an earthing funny that lay
dormant for many years.
Victorian house - Granny Flat extension added to the rear - all is well
till a graphic artist takes over the flat.
On arrival installs extensive computer system, and at end of day takes a
shower. Two minutes into showering, adjusts the controls a 2nd or 3rd
time - and gets a shock.

Reminds me of the one I had to look at a few months ago - Owner reported
getting a shock from the running water in his bathroom sink. With past
experience of customers I toddled along with the phone number of a good
psychiatrist in my pocket. Went in, turned on tap, put hand in water -
nothing. "Try taking your shoes off" says he. Took off shoes, turned on tap,
put hand in water and nearly landed on my ar*e! Checked usual things - no
fault showing. Stuck meter probe between running water and earth - best part
of 100v. Pulled bullet at meter - still 100v. Pulled meter bullet for flat
upstairs - voltage disappeared. Customer then said that when he first moved
in he used to get a shock off the walls (it was a refurbished old building
converted into upstairs / downstairs flats). Problem turned out to be a nail
through the live tail insulation to the upstairs flat. It had only nicked
the insulation and you couldn't see bare copper but it was obviously enough.
Why the voltage appeared only in the running water is beyond me - the shock
from the walls was presumably down to wet plaster and disappeared as it
dried out.
Richard.

BigWallop writes

"Welsh Bob" wrote in message
. com...
As an occasional student of the wiring Regs, i've been wondering this
for a while - when there is a remote CU in an installation like in the
flat in this case, why is the connection made from the main CU to the
remore one made using armoured cable if its internal to the property.
Could the link be made with some hefty T&E?

It could be made with T+E but the armour gives a more secure earth bonding
because there is more of it and not just a thin inner core of bare copper.
Also, SWA Cable can be made more secure in any switch gear that is used to
isolate the remote mains supply. It is also more protected against damage.

And, if the wind is in the right direction, SWA is cheaper - esp in the
larger sizes.
4-core SWA, 2 cores used for N and 2 for L, used to be about as cheap as
you could get for a long tough run.
--
roger