Another RCD puzzler
 

On Nov 8, 7:45*pm, Luke wrote:
I am pretty sure the cable is just lying under the floorboards,
no doubt with all its conductors haphazardly twisted together -

You may want to check under the floor just in case you have a small
leak (pinhole etc).

If your RCD has tripped many times (20+) you may want to go buy
another.

RCD/RCBO have a finite life, they can fail-off (will not turn on),
they can fail-on (trip button does not work), they can fail-time
(either too slow or not at all until cycled a few times), they can
fail-current (too high a current required to trip) and of course they
can be subject to recall because someone used a dodgy component inside
(sample component quality different to production supply quality :-)

You can buy a cheap RCD tester on Ebay, however I would suggest
converting to a dual RCD setup (with new RCD). Keep the old, as they
do fail and can leave you "freezer less & burglar alarm panel
sounding" overnight - been there twice hence all RCBO.

Another RCD puzzler
 

js.b1 wrote:
On Nov 8, 7:45 pm, Luke wrote:
I am pretty sure the cable is just lying under the floorboards,
no doubt with all its conductors haphazardly twisted together -


If your RCD has tripped many times (20+) you may want to go buy
another.

Why? Most RCDs suggest a test every 3 months. If 20 is the maximum number of
trips before the RCD breaks then you would have to change the RCD after 5
years.

A normal RCD test (not using the test button) would trip the RCD 4 times.

--
Adam

Another RCD puzzler
 

Quote
" 2) I have (from what I can gather) a TT system, although it is not an

overhead supply. The only earth provision to the property is via the
incoming water pipe. This may seem unusual, but when I first moved in,
I had an electrician check over the CU, and he confirmed that this was
the case. He said if he were rewiring he would convert to a TN-C-S,
but having spent a long time reading this newsgroup, I am not sure you
can necessarily do this without consultation with the DNO"

Any chance of a few piccies of your incoming supply and CU?

1. There is no way an electrician would pass an incoming water supply as a
means of earthing.
2. TT is very rarely from underground cables (I have seen it only once)
3. ALL circuits need RCD protection on TT supplies (not all at 30mA seehttp://wiki.diyfaq.org.uk/index.php?title=TT_Earthingandhttp://wiki.diyfaq.org.uk/index.php?title=Earthing_Types)

--
Adam

Hi again and thank you for your input.

I have posted some more photos to Facebook along with explanations of
what can be seen. I am pretty sure I have understood the situation
correctly in that the water pipe is the sole means of earthing.

I should have been more clear when I talked about the electrician who
visited - shortly after we bought the house I found an electrician
prepared to allow me to DIY a step-by-step rewire, and then sign off
the work as his own at the end. So it was more of a Q&A session, than
him "passing" the existing supply. He said it was unusual, and
actually went outside expecting to find an overhead supply, but of
course, it's underground.

It was rewired in 1997 so maybe the requirement for whole house RCD
protection a TT supply was not in force then? Or maybe I have
misunderstood the current situation completely.

Anyway - comments welcome!

Overall supply: http://www.facebook.com/photo.php?pi...f&id=600214934
Consumer unit: http://www.facebook.com/photo.php?pi...2&id=600214934
Waterpipe earth: http://www.facebook.com/photo.php?pi...5&id=600214934
Incoming: http://www.facebook.com/photo.php?pi...5&id=600214934

Luke

Another RCD puzzler
 

On Nov 8, 8:55*pm, "ARWadsworth"
wrote:
Why? Most RCDs suggest a test every 3 months. If 20 is the maximum
number of trips before the RCD breaks then you would have to change the RCD after 5
years.

No 20 is not the maximum, I recall Memera in 1990s stated a figure of
350-750.

My point was if his RCD has tripped 20+ times with nuisance trips,
plus quarterly tests, plus testing he has been subjecting it to, then
considering it is a single point of failure re whole-house RCD I would
pre-emptively replace it for £20. Another metric would be every 10
years, particularly as a TT setup.

My suspicion is RCDs are not well sealed - from external environmental
dust & humidity which gives a 7% failure rate, and internally from
debris created during a trip re arc suppression trap flash, carbon,
metal spray etc.

Another RCD puzzler
 

Luke wrote:

Quote
" 2) I have (from what I can gather) a TT system, although it is not an

overhead supply. The only earth provision to the property is via the
incoming water pipe. This may seem unusual, but when I first moved in,
I had an electrician check over the CU, and he confirmed that this was
the case. He said if he were rewiring he would convert to a TN-C-S,
but having spent a long time reading this newsgroup, I am not sure you
can necessarily do this without consultation with the DNO"

Any chance of a few piccies of your incoming supply and CU?
1. There is no way an electrician would pass an incoming water supply as a
means of earthing.

I have posted some more photos to Facebook along with explanations of
what can be seen. I am pretty sure I have understood the situation
correctly in that the water pipe is the sole means of earthing.

It isnt adequate.
Your pipes throughout the house will not be earthed, as you have a
plastic joint isolating them from the earthed section.
Get someone who knows about electrics to look at it. It needs a good
tidying up, and a good earth put in.

It could be a TT sytem, but where is your main earth connection?

You need a good earth spike fitting asap. Relying on a water pipe for
earth is not good.

After seeing these pics, I think you need to get this sorted pretty
quickly, it looks a mess, and is certainly not safe. Yes, people have
used stuff like this for years without incident, but you have taken out
a rcd, connected it loosely to a live terminal to test it. It is not
good practice, and to me, shows a disregard for safety, to both yourself
and family.

Alan.

--
To reply by e-mail, change the ' + ' to 'plus'.

Another RCD puzzler
 

On 08/11/2010 22:09, js.b1 wrote:

[...] and internally from debris created during a trip re arc
suppression trap flash, carbon, metal spray etc.

And damage by erosion of the contact surfaces.

That's the point though, surely? MCBs, RCDs (RCCBs) and RCBOs can all
have short lives if repeatedly breaking high fault current near to their
rated breaking capacity. But RCD trips caused by relatively small
residual currents - such as test button & RCD-tester tests, and N-E
shorts - don't involve breaking anything more than normal load current
and won't appreciably shorten the life. In damp conditions regular
testing is likely to extend RCD life.

--
Andy

Another RCD puzzler
 

Luke wrote:
Quote
" 2) I have (from what I can gather) a TT system, although it is
not an

overhead supply. The only earth provision to the property is via the
incoming water pipe. This may seem unusual, but when I first moved
in, I had an electrician check over the CU, and he confirmed that
this was the case. He said if he were rewiring he would convert to
a TN-C-S, but having spent a long time reading this newsgroup, I am
not sure you can necessarily do this without consultation with the
DNO"

Any chance of a few piccies of your incoming supply and CU?

1. There is no way an electrician would pass an incoming water
supply as a
means of earthing.
2. TT is very rarely from underground cables (I have seen it only
once)
3. ALL circuits need RCD protection on TT supplies (not all at 30mA
seehttp://wiki.diyfaq.org.uk/index.php?title=TT_Earthingandhttp://wiki.diyfaq.org.uk/index.php?title=Earthing_Types)

--
Adam

Hi again and thank you for your input.

I have posted some more photos to Facebook along with explanations of
what can be seen. I am pretty sure I have understood the situation
correctly in that the water pipe is the sole means of earthing.

I should have been more clear when I talked about the electrician who
visited - shortly after we bought the house I found an electrician
prepared to allow me to DIY a step-by-step rewire, and then sign off
the work as his own at the end. So it was more of a Q&A session, than
him "passing" the existing supply. He said it was unusual, and
actually went outside expecting to find an overhead supply, but of
course, it's underground.

It was rewired in 1997 so maybe the requirement for whole house RCD
protection a TT supply was not in force then? Or maybe I have
misunderstood the current situation completely.

Anyway - comments welcome!

Incoming:
http://www.facebook.com/photo.php?pi...5&id=600214934

Are these normal meter tails?

Your REC should be able to tell you what type of earthing system your supply
is. If it is TT then you would need an earth rod and you also have to change
the main switch to a 100mA time delayed RCD. If it is a TN supply then your
REC should be able to supply you with an earth block.

--
lier
Adam

Another RCD puzzler
 

On Nov 9, 8:30*am, (A.Lee) wrote:
you have taken out a rcd, connected it loosely to a live
terminal to test it. It is not good practice, and to me,
shows a disregard for safety, to both yourself and family.

I was a bit shocked too.

The simplest way to test if an RCD is faulty...
1) keep a correct replacement spare (£20-40), and
2) keep a Fluke Voltalert voltage tester ready
3) have an external isolator fitted so as to permit RCD change rather
than relying on touching the DNO cutout which is their property and
may be very unsafe to handle without proper protective equipment.

To dismount an RCD or RCBO and test it...
1) Buy a suitable enclousure, £8-11 re RCBO height
2) Supply it from a 16A MCB with 1.5mm cable
3) Connect any appliance by 1.25-1.5mm flex to trailing 1G socket
4) Screw some P-Clips or cable tie the cables for safety

To test for N-E, the most likely RCD tripping cause...
1) Leave RCD in place
2) Leave Busbar in place
3) Disconnect RCD-Neutral to CU-Neutral-Bar
4) Test RCD turns on ok with no neutrals connected
5) Connect a wire from RCD-Neutral to a terminal block
6) Connect each circuit's neutral in turn to the terminal block
7) Identify which circuit causes RCD tripping

A temporary N-E is best found by first examining any recently done
work, then every backbox, then any outside lights or supply (should be
DP switch indoors to isolate). Then if still transient tripping,
disconnect all appliances and leave the minimum connected to pin down
which is tripping. If still no joy move circuits to RCBO one by one
(£18-32), until tripping stops or the culprit is located.

Never do a lash-up on the floor.

If a supply is TT, never leave it without an RCD even for a brief test
- always have a spare RCD handy.
A TT supply has low L-N fault impedance (0.8ohm) so a L-N short will
trip a circuit breaker, but an extremely high L-E fault impedance
(earth rod is 200ohms) so can never trip a circuit breaker. That is
to say on a TT supply a L-E fault can create under 1A of fault current
which will mean a circuit breaker does not see a fault creating a fire
& electrocution risk. A 6A circuit breaker requires 10A fault current
to disconnect in minutes and 30A to disconnect instantly. This is why
a TT supply requires RCD protection to ensure disconnect of L-E faults
AND an earth rod with impedance 200ohms.

This is why you can not rely on a metal CW pipe as an earth rod 1) it
may be converted to plastic at any time 2) its impedance is not
guaranteed to be 200ohms.

The OP needs to update their education a little, go buy a Furse earth
rod (or two with coupler), suitable connector and 4mm 6491X & 20mm
flex conduit (contractors pack) or 16mm 6491X (no conduit required),
plus ideally get someone to fit it and test it. TLC Direct to the
earth rods etc, the 5/8" is better for tough ground and should last
longer.

Another RCD puzzler
 

On Nov 9, 8:47*am, Andy Wade wrote:
On 08/11/2010 22:09, js.b1 wrote:
[...] and internally from debris created during a trip re arc
suppression trap flash, carbon, metal spray etc.

And damage by erosion of the contact surfaces.

Yes.

That's the point though, surely? *MCBs, RCDs (RCCBs) and RCBOs can
all have short lives if repeatedly breaking high fault current near to their
rated breaking capacity. But RCD trips caused by relatively small
residual currents - such as test button & RCD-tester tests, and N-E
shorts - don't involve breaking anything more than normal load current
and won't appreciably shorten the life. *In damp conditions regular
testing is likely to extend RCD life.

Depends on the cause of repeated nuisance tripping.
N-E fault means disconnect of mere whole-house current (25A). L-E
fault means disconnect of higher fault current (up to 2100A although
the OC circuit breaker magnetic side will act instantly before current
can ramp, rewireables being slower).

Regular testing prevents stiction, maintaining low trip times.

I would replace an RCD at 10yrs on a TT supply due to critical
application.

Another RCD puzzler
 

On 2010-11-09 14:08:42 +0000, js.b1 said:


This is why you can not rely on a metal CW pipe as an earth rod 1) it
may be converted to plastic at any time 2) its impedance is not
guaranteed to be 200ohms.

The regs do allow for a metal CW pipe to be used as an earth rod in a
TT supply so long as it has been considered for the purpose and is from
a private water supply. The OSG says that this is not applicable to
domestic installations, although I have failed to spot this restriction
in the regs.

Another RCD puzzler
 

On Nov 9, 2:08*pm, "js.b1" wrote:
On Nov 9, 8:30*am, (A.Lee) wrote:

you have taken out a rcd, connected it loosely to a live
terminal to test it. It is not good practice, and to me,
shows a disregard for safety, to both yourself and family.

I was a bit shocked too.

The simplest way to test if an RCD is faulty...
1) keep a correct replacement spare (£20-40), and
2) keep a Fluke Voltalert voltage tester ready
3) have an external isolator fitted so as to permit RCD change rather
than relying on touching the DNO cutout which is their property and
may be very unsafe to handle without proper protective equipment.

To dismount an RCD or RCBO and test it...
1) Buy a suitable enclousure, £8-11 re RCBO height
2) Supply it from a 16A MCB with 1.5mm cable
3) Connect any appliance by 1.25-1.5mm flex to trailing 1G socket
4) Screw some P-Clips or cable tie the cables for safety

To test for N-E, the most likely RCD tripping cause...
1) Leave RCD in place
2) Leave Busbar in place
3) Disconnect RCD-Neutral to CU-Neutral-Bar
4) Test RCD turns on ok with no neutrals connected
5) Connect a wire from RCD-Neutral to a terminal block
6) Connect each circuit's neutral in turn to the terminal block
7) Identify which circuit causes RCD tripping

A temporary N-E is best found by first examining any recently done
work, then every backbox, then any outside lights or supply (should be
DP switch indoors to isolate). Then if still transient tripping,
disconnect all appliances and leave the minimum connected to pin down
which is tripping. If still no joy move circuits to RCBO one by one
(£18-32), until tripping stops or the culprit is located.

Never do a lash-up on the floor.

If a supply is TT, never leave it without an RCD even for a brief test
- always have a spare RCD handy.
A TT supply has low L-N fault impedance (0.8ohm) so a L-N short will
trip a circuit breaker, but an extremely high L-E fault impedance
(earth rod is 200ohms) so can never trip a circuit breaker. That is
to say on a TT supply a L-E fault can create under 1A of fault current
which will mean a circuit breaker does not see a fault creating a fire
& electrocution risk. A 6A circuit breaker requires 10A fault current
to disconnect in minutes and 30A to disconnect instantly. This is why
a TT supply requires RCD protection to ensure disconnect of L-E faults
AND an earth rod with impedance 200ohms.

This is why you can not rely on a metal CW pipe as an earth rod 1) it
may be converted to plastic at any time 2) its impedance is not
guaranteed to be 200ohms.

The OP needs to update their education a little, go buy a Furse earth
rod (or two with coupler), suitable connector and 4mm 6491X & 20mm
flex conduit (contractors pack) or 16mm 6491X (no conduit required),
plus ideally get someone to fit it and test it. TLC Direct to the
earth rods etc, the 5/8" is better for tough ground and should last
longer.

Hi again, OP here.

I've been in touch with Western Power Distribution who have told me
that they do not supply an earth to my property and that PME (TN-C-S)
is not available because my supply is arranged as a "wall loop".
Googling didn't enlighten me but the gist is that my supply cable is
shared between too many other properties to be eligible for PME
(something about the supply cable looping round one garden wall and
onto the next house!).

They also said that they do not install TN-S earthing anywhere in my
area.

So the thing to do is to convert to TT properly, including the whole
house 100ma RCD, and install an earth rod and provide myself with a
proper earth connection for my supply, which is currently missing. As
I mentioned, I do have an electrician on side who is happy for me to
do the work, so I will carry it out with his consultation.

In the short term, I will address the break in continuity caused by
the push-fit plastic fitting, by adding a "bridge" of 10mm earth cable
with a clamp each side.

I know that my means of isolation/testing would not win any safety
awards, and I do appreciate the feedback from members of the group.

Luke

Another RCD puzzler
 

Luke wrote:

I have posted some more photos to Facebook along with explanations of
what can be seen. I am pretty sure I have understood the situation
correctly in that the water pipe is the sole means of earthing.

I should have been more clear when I talked about the electrician who
visited - shortly after we bought the house I found an electrician
prepared to allow me to DIY a step-by-step rewire, and then sign off
the work as his own at the end. So it was more of a Q&A session, than
him "passing" the existing supply. He said it was unusual, and
actually went outside expecting to find an overhead supply, but of
course, it's underground.

It was rewired in 1997 so maybe the requirement for whole house RCD
protection a TT supply was not in force then? Or maybe I have
misunderstood the current situation completely.

Anyway - comments welcome!

Overall supply:
http://www.facebook.com/photo.php?pi...f&id=600214934
Consumer unit:
http://www.facebook.com/photo.php?pi...2&id=600214934
Waterpipe earth:
http://www.facebook.com/photo.php?pi...5&id=600214934
Incoming:
http://www.facebook.com/photo.php?pi...5&id=600214934

As you say in the caption to the first photo, there are two heavy-ish earth
wires from the CU, one to the water pipe, the other to the now redundant
fuse box on the Economy 7 side.

Are you able to remove the cover of that box? It would be interesting
to see what's inside. I take it the hole in the wall just above the
skirting board in the corner (bottom right of photo) and the dark streak
up the wall is where the cable to the water heater used to be, which was
connected to the incoming supply inside this box. It's worth looking to
see whether there is an internal link inside this box between the Neutral
and Earth connections, similar to that illustrated in the diy faq:

http://wiki.diyfaq.org.uk/index.php?...g_Types#TN-C-S

If there is, it would identify your installation as TN-C-S, and not TT as
you thought. In that case your water pipe connection is not supplying
your earth, but is just a bonding connection. If it is, I don't think
that plastic section ought to be there, because as it is your internal
water pipes are apparently not earthed.

Another RCD puzzler
 

Ronald Raygun wrote:
Luke wrote:

I have posted some more photos to Facebook along with explanations of
what can be seen. I am pretty sure I have understood the situation
correctly in that the water pipe is the sole means of earthing.

I should have been more clear when I talked about the electrician who
visited - shortly after we bought the house I found an electrician
prepared to allow me to DIY a step-by-step rewire, and then sign off
the work as his own at the end. So it was more of a Q&A session, than
him "passing" the existing supply. He said it was unusual, and
actually went outside expecting to find an overhead supply, but of
course, it's underground.

It was rewired in 1997 so maybe the requirement for whole house RCD
protection a TT supply was not in force then? Or maybe I have
misunderstood the current situation completely.

Anyway - comments welcome!

Overall supply:
http://www.facebook.com/photo.php?pi...f&id=600214934
Consumer unit:
http://www.facebook.com/photo.php?pi...2&id=600214934
Waterpipe earth:
http://www.facebook.com/photo.php?pi...5&id=600214934
Incoming:
http://www.facebook.com/photo.php?pi...5&id=600214934

As you say in the caption to the first photo, there are two heavy-ish
earth wires from the CU, one to the water pipe, the other to the now
redundant fuse box on the Economy 7 side.

Are you able to remove the cover of that box? It would be interesting
to see what's inside. I take it the hole in the wall just above the
skirting board in the corner (bottom right of photo) and the dark
streak up the wall is where the cable to the water heater used to be,
which was connected to the incoming supply inside this box. It's
worth looking to see whether there is an internal link inside this
box between the Neutral and Earth connections, similar to that
illustrated in the diy faq:

http://wiki.diyfaq.org.uk/index.php?...g_Types#TN-C-S

If there is, it would identify your installation as TN-C-S, and not
TT as you thought. In that case your water pipe connection is not
supplying your earth, but is just a bonding connection. If it is, I
don't think that plastic section ought to be there, because as it is
your internal water pipes are apparently not earthed.

Do internal waterpipes need bonding? Are they extraneous conductive parts?


--
Adam

Another RCD puzzler
 

On Nov 9, 3:43*pm, Piers Finlayson wrote:
On 2010-11-09 14:08:42 +0000, js.b1 said:
This is why you can not rely on a metal CW pipe as an earth rod 1) it
may be converted to plastic at any time 2) its impedance is not
guaranteed to be 200ohms.

The regs do allow for a metal CW pipe to be used as an earth rod in a
TT supply so long as it has been considered for the purpose and is from
a private water supply. *The OSG says that this is not applicable to
domestic installations, although I have failed to spot this restriction
in the regs.

542.2.4 ... with precautions taken against its removal (warning sign I
guess).

Basically for all "water company grid" connected people you can not
use the metal CW pipe as an earth rod.

Another RCD puzzler
 

Is a plastic push-fit joint connector too short to count as electrical
isolation?

The plastic fitting as non-conductive. However, the water inside it is
conductive - 22mm is 65k-ohm for 1m length and 15mm is 100k-ohm for 1m
length. Generally something is an extraneous conductor when its
resistance to MET is 22k-ohm.

Another RCD puzzler
 

Piers Finlayson wrote:
On 2010-11-09 14:08:42 +0000, js.b1 said:


This is why you can not rely on a metal CW pipe as an earth rod 1) it
may be converted to plastic at any time 2) its impedance is not
guaranteed to be 200ohms.

The regs do allow for a metal CW pipe to be used as an earth rod in a
TT supply so long as it has been considered for the purpose and is
from a private water supply. The OSG says that this is not
applicable to domestic installations, although I have failed to spot
this restriction in the regs.


I was trying to keep it simple. But you are correct a privately owned water
supply can be used as an earth rod.

--
Adam

Another RCD puzzler
 

js.b1 wrote:
Is a plastic push-fit joint connector too short to count as electrical
isolation?

The plastic fitting as non-conductive. However, the water inside it is
conductive - 22mm is 65k-ohm for 1m length and 15mm is 100k-ohm for 1m
length. Generally something is an extraneous conductor when its
resistance to MET is 22k-ohm.

http://www.theiet.org/publishing/wir...s.cfm?type=pdf

or http://preview.tinyurl.com/kwajjp if the above wraps.

Another RCD puzzler
 

On Nov 9, 4:59*pm, Ronald Raygun
wrote:

As you say in the caption to the first photo, there are two heavy-ish earth
wires from the CU, one to the water pipe, the other to the now redundant
fuse box on the Economy 7 side.

Are you able to remove the cover of that box? *It would be interesting
to see what's inside. *I take it the hole in the wall just above the
skirting board in the corner (bottom right of photo) and the dark streak
up the wall is where the cable to the water heater used to be, which was
connected to the incoming supply inside this box. *It's worth looking to
see whether there is an internal link inside this box between the Neutral
and Earth connections, similar to that illustrated in the diy faq:

http://wiki.diyfaq.org.uk/index.php?...g_Types#TN-C-S

If there is, it would identify your installation as TN-C-S, and not TT as
you thought. *In that case your water pipe connection is not supplying
your earth, but is just a bonding connection. *If it is, I don't think
that plastic section ought to be there, because as it is your internal
water pipes are apparently not earthed.- Hide quoted text -

Hi,
I removed the cover from the old fuse box and found that the 10mm
earth wire from the CU directly above it simply connects to the earth
wire that used to feed the old electric boiler, i.e. there is no
internal link between Neutral and Earth as per the photo on the Wiki.
The "dark streak" up the wall is historical and predates the
installation of the electric boiler, (probably pre-1997) as the cable
to the electric boiler is still in place and runs in surface mounted
trunking vertically up the wall to the first floor.

Definitely looks like it's a TT installation then, which ties in with
what Western Power Distribution told me, that they do not supply an
earth to this property.

Luke

Another RCD puzzler
 

Dave Osborne wrote:
js.b1 wrote:
Is a plastic push-fit joint connector too short to count as
electrical isolation?

The plastic fitting as non-conductive. However, the water inside it
is conductive - 22mm is 65k-ohm for 1m length and 15mm is 100k-ohm
for 1m length. Generally something is an extraneous conductor when
its resistance to MET is 22k-ohm.

http://www.theiet.org/publishing/wir...s.cfm?type=pdf

or http://preview.tinyurl.com/kwajjp if the above wraps.

http://www.voltimum.co.uk/files/gb/a...nts/e169-9.pdf



--
Adam