Hello

I noticed that a light switch back box measured around 100V ac w.r.t. the
cold water tap in our recently-purchased house. I checked the earth bonding
on the metal pipework for the hot and the cold tap and the radiator; all are
at the same zero volt potential. I checked the earth of a nearby mains
socket and saw no p.d. between it and the pipework mentioned. I looked
around the house and I see lots of earth bonding that has been carried out
within the last 3-4 years.


Measuring a current flow to ground from this "dodgy" earth, I see a couple
of hundred microamps flowing (actually about 275uA). Not enough to hurt but
you can just about feel a "buzz" on your finger tip.

I shall start trying to trace the lighting ring main back, but I am puzzled
as to why this lighting ring earth should float when a nearby socket ring
main earth doesn't. I am yet to carry out furthr investigations i.e. other
looking at other ring circuits.

Maybe this is a simple fix but I'd appreciate some guidance before I get up
loads of floorboards etc.

thanks in advance

DDS

Duncan Di Saudelli expressed precisely :
I noticed that a light switch back box measured around 100V ac w.r.t. the
cold water tap in our recently-purchased house. I checked the earth bonding
on the metal pipework for the hot and the cold tap and the radiator; all are
at the same zero volt potential. I checked the earth of a nearby mains socket
and saw no p.d. between it and the pipework mentioned. I looked around the
house and I see lots of earth bonding that has been carried out within the
last 3-4 years.

You really need to make all measurements from one reference point and
that should be the consumer unit earth, so you probably need a long
cable of some sort to do that.

The sort of numbers you are quoting suggests the voltage is simply
induced from the live running adjacent to the earth. I would probably
be prudent to check the resistance of the earth from all lighting
switches and lights, to gain a better idea of what is wrong.

If the wiring dates back to the 1960's, it was not unusual for the
lighting circuits not to be provided with an earth. Later mods and
improvements to the circuits might mean metal boxes installed, but with
no earth.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

The sort of numbers you are quoting suggests the voltage is simply induced
from the live running adjacent to the earth. I would probably

D'oh! Yes, an obvious contributor. Thanks - I shall pick up on your other
points (common reference via long lead etc.) and start afresh tomorrow. I
tyake your point regarding 1960s wiring and I shall look into the earthing
of the lighting rings.

thanks for the feedback. Time for some sleep!

DDS

Duncan Di Saudelli wrote:
The sort of numbers you are quoting suggests the voltage is simply induced
from the live running adjacent to the earth. I would probably

D'oh! Yes, an obvious contributor. Thanks - I shall pick up on your other
points (common reference via long lead etc.) and start afresh tomorrow. I
tyake your point regarding 1960s wiring and I shall look into the earthing
of the lighting rings.

thanks for the feedback. Time for some sleep!

Another thing to consider is the sensitivity of the volt meter, The more
ohms per volt, the more you are going to get confused.

Dave

On Sat, 13 Mar 2010 22:19:02 GMT, Harry Bloomfield
wrote:

If the wiring dates back to the 1960's, it was not unusual for the
lighting circuits not to be provided with an earth. Later mods and
improvements to the circuits might mean metal boxes installed, but with
no earth.

I've seen early 60's wiring with metal backboxes for the light
switches and no earth, the wiring being double insulated either in
singles or zip connected doubles.

Makes things interesting when you contact a switchplate fixing screw
with a finger and the backbox has been made live from a missed stray
conductor (stranded not solid conductors)


--

In article ,
Duncan Di Saudelli wrote:
Measuring a current flow to ground from this "dodgy" earth, I see a
couple of hundred microamps flowing (actually about 275uA). Not enough
to hurt but you can just about feel a "buzz" on your finger tip.

I shall start trying to trace the lighting ring main back, but I am
puzzled as to why this lighting ring earth should float when a nearby
socket ring main earth doesn't. I am yet to carry out furthr
investigations i.e. other looking at other ring circuits.

For a start, lighting circuits ain't rings - they are radials.

How old is the house? Earths haven't always been required on lighting
circuits. Even although TW&E may have been used to wire them.

Most likely cause is it either isn't connected or is broken somewhere.
Especially where a ceiling rose has been replaced with an alternative
fitting which doesn't have the required terminals.

--
*Give me ambiguity or give me something else.

Dave Plowman London SW
To e-mail, change noise into sound.

I noticed that a light switch back box measured around 100V ac w.r.t. the
cold water tap in our recently-purchased house.

Thanks everyone for your advice and a few fault-finding pointers. I did
quickly realise that the voltage measured was probably sourced from a high
impedance and that measuring with a DVM with a 20M input Z would giuve
unrealistic readings - hence my decision to measure the current and see it
as only a few hundred uA i.e. a "leakage" current.

I'll start working round the ring - sorry, the radial wiring, thanks for
putting me straight - and we'll see where we get to. Hopefully I'll find a
floating "ned" which can then be tied down.

Thanks all.

DDS

On Sun, 14 Mar 2010 13:38:09 -0000, Duncan Di Saudelli wrote:

Hopefully I'll find a floating "ned" which can then be tied down.

Would need to be tied down if it was floating face down in the Clyde,
best place for it.

--
Cheers
Dave.