This was an interesting one!

Got a call from my Mum, who is currently having a fair amount of
building work done on her place (large Victorian semi, built late
1800s), repairing damage caused by tree roots (settlement cracks etc).

She wanted me to disconnect some unwanted surface mounted sockets in a
couple of rooms before the builders get to them. So I said I would pop
round and sort it out. Then in passing she mentioned that a plasterer
was complaining of getting electric shocks from a wall!

I figured this was worthy or more urgent investigation!

Apparently, the plasterer was just finishing off a small patching
operation on the interior wall above a second floor window. This had
involved hacking off existing plaster, resin bonding / rebricking the
wall, covering with expanded metal lath and finally replastering inside.
While doing the final polish on the plaster. he was getting a tingling
sensation though his trowel. As he applied more water to the finish
coat, it got worse, to the point he was getting a reasonably significant
belt off it!

I thought some basic tests and inspections would be a good start. She
has a "whole house" 30mA RCD that feeds two CUs via a Henley block. Both
CUs are Wylex rewireable types - a 8 way doing most of the house, and a
two way that feeds an electric shower in a cloakroom extension. The
earthing has been updated relatively recently to PME (probably when a
new meter was installed recently). There is also good cross bonding to
gas and water services. Over the years we had eliminated all the
remaining PBJ coated wire, and what remains is all PVC. Some bits of
stranded T&E remain, but I think most of the sections of twin with
separate earths have been replaced.

My first thought was that if he was getting a shock like that, it was
surprising the RCD did not trip. So I did an RCD test, but it looked
like it was working fine. At 15mA it did not trip in the two second
window my tester measures. At 30mA it tripped in 20ms. I did a earth
loop impedance test and that also tripped the RCD (which it is not
supposed to do!). I may go back and repeat that.

The bit of wall he was plastering is not noticably near anything
electric (although to be fair I did not investigate the loft above).
Although one of the wires that was to be removed did run along the base
of the wall and into the adjacent room, passing close to some disused
lead pipes that were boxed into the corner of the adjacent room. This I
thought might be a possible source since the pipe would have been close
to where he was working. However, having taken the cable out there was
no sign of damage to it. I could not get a voltage reading between the
newly plastered wall and a water pipe - even on a very high impedance
DMM. Needless to say the wall had dried out a fair bit by the time I was
looking at it.

The only other possible cause I think of, was if he threw enough water
about the place, he could have got the carpet wet along with a socket,
junction box, and FCU on the skirting board, perhaps enough to make his
metal ladder "live" so that as he trowled the wall he was actually
getting an earth reference from the damp wall.

Any thoughts?

--
Cheers,

John.

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I did a earth loop impedance test and that also tripped the RCD (which
it is not supposed to do!). I may go back and repeat that.

Could you explain how drawing 20A from live to earth doesn't trip the RCD?
(Or do you have one of those fancy ones that claims to work with an RCD in
circuit?)

Christian.

John Rumm wrote:

Got a call from my Mum, who is currently having a fair amount of
building work done on her place (large Victorian semi,

Remember that last word:

.................................... she mentioned that a plasterer
was complaining of getting electric shocks from a wall!



My first thought was that if he was getting a shock like that, it was
surprising the RCD did not trip. So I did an RCD test, but it looked
like it was working fine.


Any thoughts?

The RCD seems to work OK. The "significant belt" you say the plasterer
got with a wet wall may've been below the tripping threshold. But a
thought which runs through my fevered brain is that the source of the
belt might be from next door - t'other side of the party wall - which
need not have an RCD in place, but might have sthg in contact with (say)
the old lead pipes you mention.

Just another confusing possibility to add to your mystery - Stefek

I suspect capacitive coupling to a buried cable would do it.
With the plaster all wet and conductive, the capacitance to
something like a light switch drop with both wires live would
be higher than normal.

When working in the loft on some earthed metalwork, which meant
laying on my side to access it, I noticed I was getting a strong
50Hz hum feeling when brushing against it. Checked with a neon
screwdriver, and it lit dimly. Then I realised I was proping
myself up on one arm, which was laying along a lighting cable.
Lift arm off cable, and the 50Hz hum feeling and lit neon both
stopped. Cable was in perfect condition -- it was just capacitive
coupling, helped by me being rather moist in a hot loft.

However, you should check this out thoroughly, remembering the
death of the MP's daughter just recently after people had noticed
something similar with a metal cutlery rack, which was in fact
live.

--
Andrew Gabriel

"John Rumm" wrote in message
...
This was an interesting one!

Got a call from my Mum, who is currently having a fair amount of
building work done on her place (large Victorian semi, built late
1800s), repairing damage caused by tree roots (settlement cracks etc).

She wanted me to disconnect some unwanted surface mounted sockets in a
couple of rooms before the builders get to them. So I said I would pop
round and sort it out. Then in passing she mentioned that a plasterer
was complaining of getting electric shocks from a wall!

I figured this was worthy or more urgent investigation!

Apparently, the plasterer was just finishing off a small patching
operation on the interior wall above a second floor window. This had
involved hacking off existing plaster, resin bonding / rebricking the
wall, covering with expanded metal lath and finally replastering inside.
While doing the final polish on the plaster. he was getting a tingling
sensation though his trowel. As he applied more water to the finish
coat, it got worse, to the point he was getting a reasonably significant
belt off it!

I thought some basic tests and inspections would be a good start. She
has a "whole house" 30mA RCD that feeds two CUs via a Henley block. Both
CUs are Wylex rewireable types - a 8 way doing most of the house, and a
two way that feeds an electric shower in a cloakroom extension. The
earthing has been updated relatively recently to PME (probably when a
new meter was installed recently). There is also good cross bonding to
gas and water services. Over the years we had eliminated all the
remaining PBJ coated wire, and what remains is all PVC. Some bits of
stranded T&E remain, but I think most of the sections of twin with
separate earths have been replaced.

My first thought was that if he was getting a shock like that, it was
surprising the RCD did not trip. So I did an RCD test, but it looked
like it was working fine. At 15mA it did not trip in the two second
window my tester measures. At 30mA it tripped in 20ms. I did a earth
loop impedance test and that also tripped the RCD (which it is not
supposed to do!). I may go back and repeat that.

The bit of wall he was plastering is not noticably near anything
electric (although to be fair I did not investigate the loft above).
Although one of the wires that was to be removed did run along the base
of the wall and into the adjacent room, passing close to some disused
lead pipes that were boxed into the corner of the adjacent room. This I
thought might be a possible source since the pipe would have been close
to where he was working. However, having taken the cable out there was
no sign of damage to it. I could not get a voltage reading between the
newly plastered wall and a water pipe - even on a very high impedance
DMM. Needless to say the wall had dried out a fair bit by the time I was
looking at it.

The only other possible cause I think of, was if he threw enough water
about the place, he could have got the carpet wet along with a socket,
junction box, and FCU on the skirting board, perhaps enough to make his
metal ladder "live" so that as he trowled the wall he was actually
getting an earth reference from the damp wall.

I have experienced shocking plaster once. It was caused by a connector block
buried in the wall where a shaving light had been replaced. It was OK until
the wall was retiled. The wet tile cement caused the area being tiled to
become live, the voltage dropping as you moved away from the source - quite
interesting really. I discovered it while tiling behind the bath taps as my
hand was touching the bath tap as my finger touched the wet tile cement.
Nothing serious, just a tingle, but somewhat unexpected.


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

Christian McArdle wrote:

I did a earth loop impedance test and that also tripped the RCD (which
it is not supposed to do!). I may go back and repeat that.


Could you explain how drawing 20A from live to earth doesn't trip the RCD?
(Or do you have one of those fancy ones that claims to work with an RCD in
circuit?)

I was using one of those fancy ones that is supposed not to ;-)

(Megger LT5 - seems to do what it says on the tin when I try it here
(although I am on a TT setup so hence a lower max fault current))

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Stefek Zaba wrote:

Got a call from my Mum, who is currently having a fair amount of
building work done on her place (large Victorian semi,


Remember that last word:

Ah, yes see where you are going...

The RCD seems to work OK. The "significant belt" you say the plasterer
got with a wet wall may've been below the tripping threshold. But a

Not being there at the time it is idfficult to quantify the level
here... he still managed to get a decent finish on the plaster - so it
could not have been that bad ;-)

thought which runs through my fevered brain is that the source of the
belt might be from next door - t'other side of the party wall - which
need not have an RCD in place, but might have sthg in contact with (say)
the old lead pipes you mention.

Just another confusing possibility to add to your mystery - Stefek

Good thinking... alas one potential flaw in this theory is that the wall
in question is a far from the party wall as it is possible to get -
oposite side of the house in fact. Also the adjoining house was split
into three flats ten years ago or so, and in the process I think they
upgraded the electrics to include RCDs on each flat.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Andrew Gabriel wrote:

I suspect capacitive coupling to a buried cable would do it.
With the plaster all wet and conductive, the capacitance to
something like a light switch drop with both wires live would
be higher than normal.

That was one of my thoughts... although I am not aware of any cables
there (above a window, opposite side of the room from the door). Also
that area of plaster had been hacked back to brickwork and lath without
revealing anything. There is a possibility that there is something
routed above it in the loft - but again that seems less likely since it
is right over to the side of the house away from any lights etc.

I ran a metal detector over the wall and could not find evidence of any
buried metalwork (some parts of the house still have buried pipes from
the old gas lighting!

There is a wall mounted electric radiant heater about 7 foot away that
gets power from a cable drop from the loft though. So that may warrant
further investigation.

However, you should check this out thoroughly, remembering the
death of the MP's daughter just recently after people had noticed
something similar with a metal cutlery rack, which was in fact
live.

Yes point noted! I want to get to the bottom of what was going on
anyway, if even just out of curiosity.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article ,
[snip]
Yes point noted! I want to get to the bottom of what was going on
anyway, if even just out of curiosity.

Turn the logic upside down?

Any chance he was standing or touching something vaguely
live so the wet plaster was actually acting as the Earth?

--
Tony Williams.

Tony Williams wrote:

In article ,
[snip]

Yes point noted! I want to get to the bottom of what was going on
anyway, if even just out of curiosity.


Turn the logic upside down?

Any chance he was standing or touching something vaguely
live so the wet plaster was actually acting as the Earth?

Trn the logic sideways.

Does the plasterer have a friend who is an electrician?

"John Rumm" wrote in message
...
... I could not get a voltage reading between the newly plastered wall and
a water pipe - even on a very high impedance DMM. Needless to say the wall
had dried out a fair bit by the time I was looking at it....

I wonder whether you would have got a different result if you had dug down
to the expanded metal mesh and tested that?

Colin Bignell

On Fri, 5 Nov 2004 16:16:11 -0000, "nightjar"
wrote:


"John Rumm" wrote in message
...
... I could not get a voltage reading between the newly plastered wall and
a water pipe - even on a very high impedance DMM. Needless to say the wall
had dried out a fair bit by the time I was looking at it....

I wonder whether you would have got a different result if you had dug down
to the expanded metal mesh and tested that?

Colin Bignell


Knowing little about electrics I didn't want to say but I thought some
mention of the metal mesh would have been made before now?

If it ended up near or touching some old cabling/pipework etc. Then
when the wet plaster was applied over it and now it's dry? Or am I
still not understanding electrics... ;-)

Mark S.

Tony Williams wrote:

In article ,
[snip]

Yes point noted! I want to get to the bottom of what was going on
anyway, if even just out of curiosity.


Turn the logic upside down?

Any chance he was standing or touching something vaguely
live so the wet plaster was actually acting as the Earth?

What you mean like what I said in the last paragraph of my original
post? ;-)

I expect that is one of the most likely possibilities....

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"John Rumm" wrote in message
...

... I could not get a voltage reading between the newly plastered wall and
a water pipe - even on a very high impedance DMM. Needless to say the wall
had dried out a fair bit by the time I was looking at it....


I wonder whether you would have got a different result if you had dug down
to the expanded metal mesh and tested that?

You mean like a thick ear from one's mother for messing up her nice new
plaster? ;-)

I figured that since that was inserted by hand, plugged and screwed into
place, and then plastered over with wet bonding plaster; had it have
been live he would have got at least a tingle sooner. It seems it was
only when he was throwing lots of water about when polishing the top
coat that it got interesting.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Mark S. wrote:

On Fri, 5 Nov 2004 16:16:11 -0000, "nightjar"
wrote:

I wonder whether you would have got a different result if you had dug down
to the expanded metal mesh and tested that?

Knowing little about electrics I didn't want to say but I thought some
mention of the metal mesh would have been made before now?

If it ended up near or touching some old cabling/pipework etc. Then
when the wet plaster was applied over it and now it's dry? Or am I
still not understanding electrics... ;-)

That would be plausable, although I would expect him to have got a shock
sooner if that was the case. The other problem is that it seems an
unlikely place to have any wires - other than the metal lath I could
find nothing with my metal scanner and there are no electrical fittings
in the area.

The only pipes around are some disconnected ones that run floor to
ceiling in the corner of the adjacent room in a boxed in section. These
used to be the rising main and feed to the hot water cylinder to/from
the cistern in the loft which is made out of slate and still there!. (I
went through all the plumbing and eliminated all the remaining lead pipe
that was in use when I was about sixteen IIRC - but never shifted the
slate tank since it must weigh a ton all by itself!).

Perhaps I ought to go back and megger the upstairs light and power
circuits...

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Fri, 05 Nov 2004 12:46:23 +0000, John Rumm
wrote:

Andrew Gabriel wrote:

I suspect capacitive coupling to a buried cable would do it.
With the plaster all wet and conductive, the capacitance to
something like a light switch drop with both wires live would
be higher than normal.

That was one of my thoughts... although I am not aware of any cables
there (above a window, opposite side of the room from the door).

Can you try a cable detector to see if there are any live cables in
the vicinity?

sPoNiX

On Fri, 05 Nov 2004 13:21:17 +0000 (GMT), Tony Williams
wrote:

Any chance he was standing or touching something vaguely
live so the wet plaster was actually acting as the Earth?

Maybe his metal stepladder was standing on an extension lead and
damaged the insulation?

sPoNiX

In article , John
Rumm writes

"John Rumm" wrote in message
...

... I could not get a voltage reading between the newly plastered wall and
a water pipe - even on a very high impedance DMM. Needless to say the wall
had dried out a fair bit by the time I was looking at it....


I wonder whether you would have got a different result if you had dug down
to the expanded metal mesh and tested that?

You mean like a thick ear from one's mother for messing up her nice new
plaster? ;-)

I figured that since that was inserted by hand, plugged and screwed into
place, and then plastered over with wet bonding plaster; had it have
been live he would have got at least a tingle sooner. It seems it was
only when he was throwing lots of water about when polishing the top
coat that it got interesting.


Nothing coming UP through the floor perchance, and the wall is
"earthing" him?....
--
Tony Sayer

sPONiX wrote:

Can you try a cable detector to see if there are any live cables in
the vicinity?

Did that, but could not find anything. I need to look in the loft to be
sure though.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Old unearthed gas pipes? I've seen this effect in a Victorian house
back in the past.

Regards
Capitol