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Default Another RCD puzzler

Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with 2
* ring mains on the 30ma 80A RCD side. On the non-RCD side there are a
number of other circuits, including an outside socket, protected by a
"Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

....this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort of
appliance or wiring fault? FYI the house was rewired in 1997 along
with the CU replacement - I've had a lot of floorboards up following
CH install and it all looks in good nick.

I am contemplating chancing £30.00 on a new RCD and wanted to get the
group's general opinion.

Thanks

Luke
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Luke wrote:
Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with 2
* ring mains on the 30ma 80A RCD side. On the non-RCD side there are a
number of other circuits, including an outside socket, protected by a
"Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

...this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort of
appliance or wiring fault? FYI the house was rewired in 1997 along
with the CU replacement - I've had a lot of floorboards up following
CH install and it all looks in good nick.

I am contemplating chancing £30.00 on a new RCD and wanted to get the
group's general opinion.

Thanks

Luke


Yes, you should definitely suspect the RCD. Also, you may have an
insulation fault that is pushing the RCD very close to its trip level.

This means that the slightest imbalance or increase in humidity will set
it off. The vacuum cleaner sets it off because of the inrush on starting
(John Rumm might be along later to explain why).

You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT

http://www.tlc-direct.co.uk/Products/HGCD263U.html

Also, get your installation checked out at some point to see if you can
find where the leakage current is coming from.


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Dave Osborne wrote:
Luke wrote:
Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with
2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there
are a number of other circuits, including an outside socket,
protected by a "Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

...this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort
of appliance or wiring fault? FYI the house was rewired in 1997 along
with the CU replacement - I've had a lot of floorboards up following
CH install and it all looks in good nick.

I am contemplating chancing £30.00 on a new RCD and wanted to get the
group's general opinion.

Thanks

Luke


Yes, you should definitely suspect the RCD. Also, you may have an
insulation fault that is pushing the RCD very close to its trip level.

This means that the slightest imbalance or increase in humidity will
set it off. The vacuum cleaner sets it off because of the inrush on
starting (John Rumm might be along later to explain why).

You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT

http://www.tlc-direct.co.uk/Products/HGCD263U.html


ISTR Steeples are rebadged Volex sold by Denmans

Also, get your installation checked out at some point to see if you
can find where the leakage current is coming from.


If the RCD is not at fault then this is almost certainly a N-E fault or
possibly a L-E fault on the RCD side of the installation. I would suspect
that a N-E fault is more likely than a faulty RCD. I would start by
inspecting the back of all the sockets to check that a fixing screw is not
touching a neutral cable.

--
Adam


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Default Another RCD puzzler

In article
s.com, Luke scribeth thus
Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with 2
* ring mains on the 30ma 80A RCD side. On the non-RCD side there are a
number of other circuits, including an outside socket, protected by a
"Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

...this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort of
appliance or wiring fault? FYI the house was rewired in 1997 along
with the CU replacement - I've had a lot of floorboards up following
CH install and it all looks in good nick.

I am contemplating chancing £30.00 on a new RCD and wanted to get the
group's general opinion.

Thanks

Luke


Sounds like you may have a Neutral to Earth short somewhere.

Switch off at the main switch and pull the fuses or trip the MCB's and
do a high ohms test from the neutral of each circuit to Earth and you
may well find it there.

Whilst at it there should be no ohms from Live to earth either if there
is and you do find which circuit then trace it down from there .. easier
said then dun;!...
--
Tony Sayer



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In article , Dave Osborne
scribeth thus
Luke wrote:
Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with 2
* ring mains on the 30ma 80A RCD side. On the non-RCD side there are a
number of other circuits, including an outside socket, protected by a
"Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

...this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort of
appliance or wiring fault? FYI the house was rewired in 1997 along
with the CU replacement - I've had a lot of floorboards up following
CH install and it all looks in good nick.

I am contemplating chancing £30.00 on a new RCD and wanted to get the
group's general opinion.

Thanks

Luke


Yes, you should definitely suspect the RCD. Also, you may have an
insulation fault that is pushing the RCD very close to its trip level.

This means that the slightest imbalance or increase in humidity will set
it off. The vacuum cleaner sets it off because of the inrush on starting
(John Rumm might be along later to explain why).


Prolly thats causing the leakage to earth, via live or neutral to
alter..


You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT

http://www.tlc-direct.co.uk/Products/HGCD263U.html

Also, get your installation checked out at some point to see if you can
find where the leakage current is coming from.



Blimey .. they've dropped in price since I last bought one;!..
--
Tony Sayer




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On Nov 6, 2:01*pm, Luke wrote:
Hi everyone,

I've got a "Steeple" split load consumer unit installed in 1997 with 2
* ring mains on the 30ma 80A RCD side. On the non-RCD side there are a
number of other circuits, including an outside socket, protected by a
"Powerbreaker" 30ma integral RCD socket.

We moved into the house in July. Ever since we moved in, we've been
having nuisance RCD trips, seemingly following no pattern. Everything
would be fine for a while, then all of a sudden the RCD would trip -
usually when using the shower, or maybe switching the kettle on.
Lately the problem seems to have been getting worse - it tripped
during the night, a period of low activity, apart from the immersion
heater which is on the non-RCD side anyway.

I thought it might be mouse activity biting through the cable, but
then...

...this morning I went to switch on the vacuum cleaner, a double
insulated device, i.e. no path to earth. Immediately the RCD tripped.
The happens on the upstairs and downstairs rings. As I couldn't see
how there could possible be an earth fault in the vacuum cleaner, I
plugged it into the outside socket protected by its own RCD but NOT
protected by the RCD in the consumer unit. It worked fine and did not
trip the outside socket's own RCD.

Am I right to suspect the RCD itself in the CU rather than any sort of
appliance or wiring fault?


No, most likely is an electrically leaking appliance. Spend your notes
on a multimeter first.
http://wiki.diyfaq.org.uk/index.php?title=RCD
http://wiki.diyfaq.org.uk/index.php?title=Multimeter


NT
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On 06/11/2010 20:08, tony sayer wrote:

Whilst at it there should be no ohms from Live to earth either [...]


Hmm, I'd prefer to have quite a lot of ohms between line and earth :~)

--
Andy
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On 06/11/2010 15:46, Dave Osborne wrote:

Yes, you should definitely suspect the RCD.


I've never known an RCD fail by becoming over-sensitive. I've known a
few though that have become insensitive and failed to trip when they
should. The test button is there for a purpose...

For this case a N-E short or leakage, which could be in an appliance or
in the fixed wiring, is no. 1 suspect, as others have said. I'd put the
RCD itself a long way down the list.

--
Andy
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In article , Andy Wade
zen.co.uk scribeth thus
On 06/11/2010 20:08, tony sayer wrote:

Whilst at it there should be no ohms from Live to earth either [...]


Hmm, I'd prefer to have quite a lot of ohms between line and earth :~)


OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?..
--
Tony Sayer



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On 06/11/2010 23:12, tony sayer wrote:

OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?..


Milliohms, megohms, what's the difference :-)

If my meter indicated no ohms I'd think it needs a new battery.

--
Andy


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Andy Wade wrote:
On 06/11/2010 15:46, Dave Osborne wrote:

Yes, you should definitely suspect the RCD.


I've never known an RCD fail by becoming over-sensitive.


Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.

I've known a
few though that have become insensitive and failed to trip when they
should. The test button is there for a purpose...


The test button is of limited facility. There's no substitute for
testing with an RCD Tester. I've had RCD's that didn't trip on the test
button but did trip on a "proper" fault and vice versa.

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Dave Osborne wrote:

Andy Wade wrote:
On 06/11/2010 15:46, Dave Osborne wrote:

I've never known an RCD fail by becoming over-sensitive.


Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.


I wouldn't have thought so. Surely if there is a leakage (sorry, residual)
current of 17mA then there is something seriously wrong somewhere and it's
worth finding it and fixing it.


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On 07/11/2010 02:01, Dave Osborne wrote:

Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.


Fairynuff, but you shouldn't consider one that trips at 17 mA as
over-sensitive - the standard allows them to trip anywhere between 15
and 30 mA. In my experience though they all tend to trip at around the
20 mA mark, ±2 or 3 mA.

The test button is of limited facility.


Agreed, but it will (usually) alert you to a device that isn't going to
trip at all. If the test resistor circuit has gone o/c you'll get a
false negative result, which errs on the safe side.

There's no substitute for testing with an RCD Tester.


Agreed, but, OOI, how many times have you found an RCD that failed on
elongated trip times but did actually trip on the test?

--
Andy
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Andy Wade wrote:
On 07/11/2010 02:01, Dave Osborne wrote:

Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.


Fairynuff, but you shouldn't consider one that trips at 17 mA as
over-sensitive


Given that Hager (for example) sell RCDs that consistently trip at
27-28mA, I can (and I do) consider one that trips at 17mA as over
sensitive.

To be clear, though I meant over sensitive in the sense of "10mA more
sensitive than it needs to be" rather than "failing the 1/2 Idn test".

- the standard allows them to trip anywhere between 15
and 30 mA.


Actually, the standard says that they must not trip at 15mA and must
trip at 30mA (within 200ms). It is entirely with the manufacturer to
decide what actual trip level they set their product at.

In my experience though they all tend to trip at around the
20 mA mark, ±2 or 3 mA.


Well, in my experience 'most' 30mA RCDs trip at 25-28mA.


The test button is of limited facility.


Agreed, but it will (usually) alert you to a device that isn't going to
trip at all. If the test resistor circuit has gone o/c you'll get a
false negative result, which errs on the safe side.

There's no substitute for testing with an RCD Tester.


Agreed, but, OOI, how many times have you found an RCD that failed on
elongated trip times but did actually trip on the test?


I do temporary power distribution for outdoor events. I don't know
exactly how many RCDs are in my care but it's in the region of 80~100
and of a variety of makes.

They all get fully tested at least twice (and often several times) a season.

I expect to get failures at the start of the season (bearing in mind
that they may have over-wintered un-powered in an unheated shed, garage
or container).

To answer your question, which I take to mean "how many times have I
found an RCD that tripped at the required fault current, but *not*
within the prescribed time" then I would say at least a couple each season.

Sometimes you get a duff batch and (usually after a season or two of
being fine) most or all of them fail to survive over-wintering.
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A clamp meter on the CUs earth wire can be useful (mA range).

A 30mA RCD will tend to trip at 23-25mA.
Several appliances with mains filters each leaking circa 1.5mA can
create a "background" leakage of 15mA making it very easy for another
appliance to push the total over the RCD trip threshold.

If the house does have a single RCD (TT supply or "whole house") then
conversion to a dual RCD CU would make life a lot easier when the
fault is found.


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In article , Andy Wade spambucket@maxw
ell.myzen.co.uk scribeth thus
On 06/11/2010 23:12, tony sayer wrote:

OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?..


Milliohms, megohms, what's the difference :-)

If my meter indicated no ohms I'd think it needs a new battery.


Well if there's no indicated 'omes then theres no problem..
--
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John Rumm wrote:
On 07/11/2010 10:00, Ronald Raygun wrote:
Dave Osborne wrote:

Andy Wade wrote:
On 06/11/2010 15:46, Dave Osborne wrote:

I've never known an RCD fail by becoming over-sensitive.

Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that
is is and has always been over sensitive.

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.


I wouldn't have thought so. Surely if there is a leakage (sorry,
residual)
current of 17mA then there is something seriously wrong somewhere and
it's
worth finding it and fixing it.


It depends a bit on what you have on the circuit. Lots of IT kit and
other electronic items for example will typically "leak" a small amount
due to the mains input filtering. With enough items on the circuit the
cumulative "normal" leakage can add up and take a lump out of your
available trip threshold.

And more to the point, if there is any step surge or interference on
the mains input, can transfer quite large currents resulting from the
harmonics higher than 50Hz.
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The Natural Philosopher wrote:

John Rumm wrote:
On 07/11/2010 10:00, Ronald Raygun wrote:
Dave Osborne wrote:

An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.

I wouldn't have thought so. Surely if there is a leakage (sorry,
residual) current of 17mA then there is something seriously wrong
somewhere and it's worth finding it and fixing it.


It depends a bit on what you have on the circuit. Lots of IT kit and
other electronic items for example will typically "leak" a small amount
due to the mains input filtering. With enough items on the circuit the
cumulative "normal" leakage can add up and take a lump out of your
available trip threshold.


And more to the point, if there is any step surge or interference on
the mains input, can transfer quite large currents resulting from the
harmonics higher than 50Hz.


I don't find either of these two answers particularly illuminating.

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?

Doesn't in all circumstances "what goes in must come out" apply? In
other words why should the live current ever (at any instant) differ
from the neutral current (except in direction)?

Or is it that there is never a real leak, but the breaker is fooled into
thinking there is, in which case what is the mechanism for fooling it?

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Ronald Raygun wrote:

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?


No idea about the harmonics (that was skipped over during my course),
but IT equipment has a measurable line to earth leakage.
It is something to do with the transformers in them, which leak a
little. Add 10 computers in one room, and there is potential to trip an
RCD.

There are special rules in the 17th Ed. Regs. for rooms containing IT
equipment, especially concerning their earthing - not earthed properly,
and you then touch the casing of a computer, you can get a shock, so all
earth connections have to be in separate terminals,and must be in a
ring, even if the line circuit is a radial.

Alan.
--
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On Nov 7, 12:12*am, John Rumm wrote:
On 06/11/2010 15:46, Dave Osborne wrote:

Yes, you should definitely suspect the RCD. Also, you may have an
insulation fault that is pushing the RCD very close to its trip level.


This means that the slightest imbalance or increase in humidity will set
it off. The vacuum cleaner sets it off because of the inrush on starting
(John Rumm might be along later to explain why).


Covered he

http://wiki.diyfaq.org.uk/index.php?...nsitising_RCDs

Also, get your installation checked out at some point to see if you can
find where the leakage current is coming from.


Following some of the tests described here can find many causes:

http://wiki.diyfaq.org.uk/index.php?...cate_the_cause....

--

Hi everyone and thanks for the input so far.

This morning I connected the vacuum cleaner directly up to the RCD in
the consumer unit. Nothing else connected. Immediately I turned on the
vacuum cleaner, the RCD tripped. So, I have reproduced the fault
having isolated the wiring.

Bearing in mind the above, and the fact that I have seemingly random
trips occuring on all RCD protected circuits, at inexplicable times
(i.e. no load), I am inclined to suspect the RCD as being over-
sensitive, and am minded to buy a replacement.

I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?

Luke


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Luke wrote:
On Nov 7, 12:12 am, John Rumm wrote:
On 06/11/2010 15:46, Dave Osborne wrote:

Yes, you should definitely suspect the RCD. Also, you may have an
insulation fault that is pushing the RCD very close to its trip level.
This means that the slightest imbalance or increase in humidity will set
it off. The vacuum cleaner sets it off because of the inrush on starting
(John Rumm might be along later to explain why).

Covered he

http://wiki.diyfaq.org.uk/index.php?...nsitising_RCDs

Also, get your installation checked out at some point to see if you can
find where the leakage current is coming from.

Following some of the tests described here can find many causes:

http://wiki.diyfaq.org.uk/index.php?...cate_the_cause...

--

Hi everyone and thanks for the input so far.

This morning I connected the vacuum cleaner directly up to the RCD in
the consumer unit. Nothing else connected. Immediately I turned on the
vacuum cleaner, the RCD tripped. So, I have reproduced the fault
having isolated the wiring.

Bearing in mind the above, and the fact that I have seemingly random
trips occuring on all RCD protected circuits, at inexplicable times
(i.e. no load), I am inclined to suspect the RCD as being over-
sensitive, and am minded to buy a replacement.

I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


If your facts are correct,, and your assumption that the vacuum cleaner
is blameless is true.

My washing machine did this. 3k ohms showed between a motor frame and
its windings. Even stripped down. New motor fixed it.


Luke

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Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?


Its usually capacitors connected between Neutral and earth and Live and
earth. Not a lot of current is intended to flow but some does. These are
mainly used to stop or alleviate interference (radio frequency) problems
often referred to as EMC compatibility. That's it in brief..

Doesn't in all circumstances "what goes in must come out" apply? In
other words why should the live current ever (at any instant) differ
from the neutral current (except in direction)?


Thats the idea .. what goes into the "house" on one conductor must come
out on the other, any discrepancy then thats "leakage" sometimes caused
by capacitors in equipment, sometimes caused by insulation breakdown and
sometimes caused by YOU when you might grab hold of a live conductor and
the leakage makes its way to Earth by you .. not a good thing then the
RCD trips out and disconnects you from Earth..


Or is it that there is never a real leak, but the breaker is fooled into
thinking there is, in which case what is the mechanism for fooling it?


No theres no fooling at all as such, what comes in must be the same as
what goes out more then 30 milliamps difference - then trip out time.

Sometimes an Earth Neutral short can be rather difficult to comprehend
as normally with no current flowing then nothing will happen as Earth
and Neutral are bonded together at the incoming point on most supply
instances. However when some current starts to flow then as the current
increases what is going in on the live isn't all coming out on the
Neutral thru the RCD as the current thru that increases due to load then
there will come a point when it gets past the trip value then out will
go the trip!..


If no current is being consumed in the connected circuits then the
current imbalance isn't enough to fire off the trip. Switching anything
on that will cause sufficient current to flow will .. after the tripping
current point is reached then out it goes.


Put that in a simplistic way with nothing switched on then nothing will
be flowing thru the live to come out on the neutral so no imbalance.

However switch something on then the current flowing Into the RCD on the
live it should see an equal current flowing OUT on the Neutral which it
won't as its flowing away through the Earth system, not necessarily to
earth as such, just the earth part of the incoming supply connected to
the neutral now flowing around and past the RCD on the wrong conductor

When there is sufficient current flowing thru the live conductor and not
out of the neutral side then once that reaches the tripping current then
it will trip out...
--
Tony Sayer



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In article , A.Lee
scribeth thus
Ronald Raygun wrote:

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?


No idea about the harmonics (that was skipped over during my course),
but IT equipment has a measurable line to earth leakage.
It is something to do with the transformers in them,


Capacitors I'll think you'll find..

which leak a
little. Add 10 computers in one room, and there is potential to trip an
RCD.

There are special rules in the 17th Ed. Regs. for rooms containing IT
equipment, especially concerning their earthing - not earthed properly,
and you then touch the casing of a computer, you can get a shock, so all
earth connections have to be in separate terminals,and must be in a
ring, even if the line circuit is a radial.

Alan.


--
Tony Sayer


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I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?

Luke


No its not if its a Neutral Earth short, then that can cause that with
no problems at all as can a lamp or anything else that causes currents
to flow.

The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to Earth
that can trip it.

Way to find out is to tray another load such as an electric fire on the
same circuit, tripping then Earth Neutral short very likely, no tripping
then prolly faulty caps in the vacuum cleaner..


--
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On Nov 7, 8:40*pm, tony sayer wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


Luke


No its not if its a Neutral Earth short, then that can cause that with
no problems at all as can a lamp or anything else that causes currents
to flow.

The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to Earth
that can trip it.

Way to find out is to tray another load such as an electric fire on the
same circuit, tripping then Earth Neutral short very likely, no tripping
then prolly faulty caps in the vacuum cleaner..

--
Tony Sayer


Yes, but the vacuum cleaner has no earth cable and in any case I wired
it directly to the CU. There was no earth wire involved at all.


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Luke wrote:
On Nov 7, 8:40 pm, tony sayer wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents
no path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


Luke


No its not if its a Neutral Earth short, then that can cause that
with no problems at all as can a lamp or anything else that causes
currents to flow.

The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to
Earth that can trip it.

Way to find out is to tray another load such as an electric fire on
the same circuit, tripping then Earth Neutral short very likely, no
tripping then prolly faulty caps in the vacuum cleaner..

--
Tony Sayer


Yes, but the vacuum cleaner has no earth cable and in any case I wired
it directly to the CU. There was no earth wire involved at all.


Was this wired directly into the RCD with all the other connections to the
RCD removed (apart from the incoming L and N supply) or was it wired into
the RCD with the outgoing neutral still connected. It does make a
difference.

--
Adam


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Ronald Raygun wrote:
The Natural Philosopher wrote:

John Rumm wrote:
On 07/11/2010 10:00, Ronald Raygun wrote:
Dave Osborne wrote:
An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.
I wouldn't have thought so. Surely if there is a leakage (sorry,
residual) current of 17mA then there is something seriously wrong
somewhere and it's worth finding it and fixing it.
It depends a bit on what you have on the circuit. Lots of IT kit and
other electronic items for example will typically "leak" a small amount
due to the mains input filtering. With enough items on the circuit the
cumulative "normal" leakage can add up and take a lump out of your
available trip threshold.

And more to the point, if there is any step surge or interference on
the mains input, can transfer quite large currents resulting from the
harmonics higher than 50Hz.


I don't find either of these two answers particularly illuminating.

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?

Doesn't in all circumstances "what goes in must come out" apply? In
other words why should the live current ever (at any instant) differ
from the neutral current (except in direction)?

Or is it that there is never a real leak, but the breaker is fooled into
thinking there is, in which case what is the mechanism for fooling it?


OK, so the RCD measures the current flowing in the live wire and
measures the current flowing in the neutral wire and compares the two.
Notionally, if the current in the neutral wire is less than the current
in the live wire, then some of the current from the live wire *must* be
flowing to earth, as the neutral is bonded to earth at the substation
and therefore earth provides an alternative return path to neutral for
the current from the live wire. If the neutral current is lower than the
live current by a threshold value, then the RCD trips out the supply.

Some types of equipment can inject some nasty signals into the mains
which cause interference with other equipment, so to comply with the EMC
directive, suppression components have to be installed to stop the
interference from getting out of the unit.

One of the techniques for limiting the interference is to connect small
capacitors between live and earth and between neutral and earth. These
are called Class Y capacitors[1].

Now, some small current flows in these capacitors. As the nominal
potential difference between live and earth is 240V and the nominal
potential difference between neutral and earth is 0V, then the current
flowing from live to earth is 240V/Xc and the current flowing from
neutral to earth is 0V/Xc (i.e. zero), where Xc is the impedance of the
capacitor.

So, with this type of interference suppressor, current (say 0.5mA)
legitimately flows from live to earth and this current is detected by
the RCD. These currents all add up, so, for example, thirty pieces of IT
equipment[2] may have a leakage of 15mA. If the nominal 30mA RCD
actually trips at 25mA then effectively, RCD protection is sensitised to
9mA of unacceptable fault current instead of 24mA of unacceptable fault
current.

Now, the impedance of the suppression capacitor (Xc) is a function of
the frequency of the signal passing through it. As you will see from
http://en.wikipedia.org/wiki/Capacitor the impedance is inversely
proportional to the the frequency, which is another way of saying that
when the frequency rises, the impedance falls.

So if the device being protected generates harmonic interference (i.e
interference at multiples of the mains frequency), then the harmonic
interference is going to cause an even higher current to flow between
live and earth than that just due to the 50Hz mains.

In summary, there are circumstances where a legitimate current flows
from live to earth; these circumstances are commonplace in a modern
house and they cumulatively serve to "sensitise" the tripping
characteristic of the RCD.



[1] http://my.execpc.com/~endlr/line-filter.html

[2] How many pieces if IT equipment in your home?

Computers?
Laptop power supplies?
TVs?
DECT phones?
DVD players?
Satellite boxes?
Printers?
Scanners?
Monitors?
ADSL modem?
Phone chargers?
Microwave oven?
etc.
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I am pretty sure I am right, that a vacuum cleaner, which presents
no path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


Luke


The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to
Earth that can trip it.


Yes, but the vacuum cleaner has no earth cable and in any case I wired
it directly to the CU. There was no earth wire involved at all.


Was this wired directly into the RCD with all the other connections to the
RCD removed (apart from the incoming L and N supply) or was it wired into
the RCD with the outgoing neutral still connected. It does make a
difference.

Adam


Thank you.

To answer your question, I left all the neutrals connected to the
neutral side of the RCD but only connected one live, which was to the
vacuum cleaner. I would take a photo so you could see exactly what I
did, but I have disconnected it now and don't want to cut the power at
this time of night to recreate it.

So, I decided to simplify matters yet further by wiring the RCD into a
socket:

(Health and safety fanatics look away now)
http://tinyurl.com/LukeRCD

And this time the RCD didn't trip, and the vacuum cleaner worked fine.
I must admit I am confused as to how leaving all those neutrals
connected could have been a contributory factor, as only 1 live was
connected. Is it therefore possible there is a leak between a Live on
the non-RCD side and Neutral on the RCD side then? If you like, I will
re-execute my original experiment tomorrow morning and post a
picture.

Luke
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In article
s.com, Luke scribeth thus
On Nov 7, 8:40*pm, tony sayer wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


Luke


No its not if its a Neutral Earth short, then that can cause that with
no problems at all as can a lamp or anything else that causes currents
to flow.

The only exception might be a faulty or leaky "suppression" capacitor
that is permitting a current to flow from the Live wire in it to Earth
that can trip it.

Way to find out is to tray another load such as an electric fire on the
same circuit, tripping then Earth Neutral short very likely, no tripping
then prolly faulty caps in the vacuum cleaner..

--
Tony Sayer


Yes, but the vacuum cleaner has no earth cable and in any case I wired
it directly to the CU. There was no earth wire involved at all.


Won't be a leak from Live to Earth then!, prolly you do have an Earth
Neutral short...
--
Tony Sayer



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Luke wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents
no path to earth at all, and hence no means of providing a leakage
current, should not under any circumstances trip the RCD. That's
correct isn't it?


Luke


The only exception might be a faulty or leaky "suppression"
capacitor that is permitting a current to flow from the Live wire
in it to Earth that can trip it.


Yes, but the vacuum cleaner has no earth cable and in any case I
wired it directly to the CU. There was no earth wire involved at
all.


Was this wired directly into the RCD with all the other connections
to the RCD removed (apart from the incoming L and N supply) or was
it wired into the RCD with the outgoing neutral still connected. It
does make a difference.

Adam


Thank you.

To answer your question, I left all the neutrals connected to the
neutral side of the RCD but only connected one live, which was to the
vacuum cleaner. I would take a photo so you could see exactly what I
did, but I have disconnected it now and don't want to cut the power at
this time of night to recreate it.

So, I decided to simplify matters yet further by wiring the RCD into a
socket:

(Health and safety fanatics look away now)
http://tinyurl.com/LukeRCD


OK. So if that is the RCD from your CU then what socket did you plug this
Darwin Medal into?


And this time the RCD didn't trip, and the vacuum cleaner worked fine.
I must admit I am confused as to how leaving all those neutrals
connected could have been a contributory factor, as only 1 live was
connected. Is it therefore possible there is a leak between a Live on
the non-RCD side and Neutral on the RCD side then? If you like, I will
re-execute my original experiment tomorrow morning and post a
picture.


I'd video it, the coroner may want a copy.

You have the symptoms of a classic neutral earth fault on the RCD side of
your house electrics.

The RCD needs the live and neutral that pass through it to have the same
current, any imbalance more than 30mA and the RCD will trip.

If the neutral and earth are shorted after the RCD (in the T&E house cable
not the vacuums flex) then some of the current that should flow down the
neutral cable back to the RCD is flowing to earth and the RCD will trip.

--
Adam




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On Nov 7, 7:45*pm, Ronald Raygun
wrote:

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? *Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". *


Older equipment, with transformers, doesn't usually do this. The real
problems began with the advent of switch mode power supplies (c.1980):
a way of building power supplies out of complex silicon (which is
cheap these days) and small high-frequency transformers, rather than
the old heavy 50Hz transformers. As these are high-frequency devices,
they make electrical interference that can be transmitted to, and
interfere with, other equipment.

The solution to this (for both interfererence generator and
susceptible equipment) is to fit input filters. These act as short
circuits to high-frequency noise, shorting it out and dissipating it
between live, neutral and earth. In practice they're usually five
components: a pair of inductors or chokes in the live & neutral lines
act as an open circuit to high frequencies whilst a triangular delta
network of capacitors between the three wires, including the earth,
acts as short circuits to high frequencies.

Capacitors are an open circuit to DC and nearly so to low frequency
50Hz AC, so they don't usually trigger RCDs, certainly not when it's
an older RCD of 100mA. However, as John Rumm points out, it's a
cumulative effect and 30mA isn't a huge leakage allowance to begin
with.

And why would interference harmonics cause a leak?


Harmonics isn't the right term. The thing is that if you switch an
inductive load (such as a transformer or motor) and you don't switch
exactly on the zero-crossing point (which only happens with
sophisticated transistor or thyristor switches) then you're attempting
to switch a sizable voltage instantly. With an inductive load, that
would require an infinite rate of change in the current, which an
inductor will resist. So the outcome instead is a "ringing" current,
where the current and voltage oscillate until damped out. The extra
current drawn during these oscillations is also out of phase between
live & neutral (as they're on opposite legs of the inductive load), so
it's seen by the RCD as if it was a leakage current (it isn't, it's
purely between live and neutral, but it's still picked up by the RCD's
differential transformer). This is a relatively slow oscillation,
compared to that of a switched-mode supply's oscillator, so RCDs are
still fairly sensitive to it.
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Andy Dingley wrote:
On Nov 7, 7:45 pm, Ronald Raygun
wrote:

Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins".


Older equipment, with transformers, doesn't usually do this. The real
problems began with the advent of switch mode power supplies (c.1980):
a way of building power supplies out of complex silicon (which is
cheap these days) and small high-frequency transformers, rather than
the old heavy 50Hz transformers. As these are high-frequency devices,
they make electrical interference that can be transmitted to, and
interfere with, other equipment.

The solution to this (for both interfererence generator and
susceptible equipment) is to fit input filters. These act as short
circuits to high-frequency noise, shorting it out and dissipating it
between live, neutral and earth. In practice they're usually five
components: a pair of inductors or chokes in the live & neutral lines
act as an open circuit to high frequencies whilst a triangular delta
network of capacitors between the three wires, including the earth,
acts as short circuits to high frequencies.

Capacitors are an open circuit to DC and nearly so to low frequency
50Hz AC, so they don't usually trigger RCDs, certainly not when it's
an older RCD of 100mA. However, as John Rumm points out, it's a
cumulative effect and 30mA isn't a huge leakage allowance to begin
with.

And why would interference harmonics cause a leak?


Harmonics isn't the right term. The thing is that if you switch an
inductive load (such as a transformer or motor) and you don't switch
exactly on the zero-crossing point (which only happens with
sophisticated transistor or thyristor switches) then you're attempting
to switch a sizable voltage instantly. With an inductive load, that
would require an infinite rate of change in the current, which an
inductor will resist. So the outcome instead is a "ringing" current,
where the current and voltage oscillate until damped out. The extra
current drawn during these oscillations is also out of phase between
live & neutral (as they're on opposite legs of the inductive load), so
it's seen by the RCD as if it was a leakage current (it isn't, it's
purely between live and neutral, but it's still picked up by the RCD's
differential transformer). This is a relatively slow oscillation,
compared to that of a switched-mode supply's oscillator, so RCDs are
still fairly sensitive to it.


Er no. Thats bull****.

That's not what happens. Current cant be 'lost' in an inductor or a
capacitor. What goes in comes out.


Its purely down to the capacitative coupling between L&N, and E.

There is a current path via any interference suppression capacitors
between those and earth. Now at 50Hz sine, that's rated as a very low
current BUT if you slam a big load on the mains, you get a sharp step in
voltage (equivalent to a much higher frequency than 50Hz being applied)
on the mains, and that translates to a big surge current through those
capacitors.


In short, if you have maybe 20 electronic bits plugged into the mains,
and you connect a powerful motor and switch it on, you are almost
guaranteed to flip a 30mA RCD.


That, and neutral earth shorts are the most common causes of phantom
tripping.

When I had a lorry sized generator instead of a proper mains supply, I
could reliably flip the RCD any time switching on a hoover or kettle.

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On Nov 7, 10:13*pm, The Natural Philosopher
wrote:

Its purely down to the capacitative coupling between L&N, and E.


Cable capacitance (which is pretty damned small), or (as I described)
the filter capacitors?


BUT if you slam a big load on the mains, you get a sharp step in voltage


We're talking about inductive loads here - so you _don't_ get this
sharp step (which would indeed require those harmonics), you get a
slower rise instead, with overshoot and ringing.
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ARWadsworth wrote:
Luke wrote:
I am pretty sure I am right, that a vacuum cleaner, which
presents no path to earth at all, and hence no means of
providing a leakage current, should not under any circumstances
trip the RCD. That's correct isn't it?

Luke

The only exception might be a faulty or leaky "suppression"
capacitor that is permitting a current to flow from the Live wire
in it to Earth that can trip it.

Yes, but the vacuum cleaner has no earth cable and in any case I
wired it directly to the CU. There was no earth wire involved at
all.

Was this wired directly into the RCD with all the other connections
to the RCD removed (apart from the incoming L and N supply) or was
it wired into the RCD with the outgoing neutral still connected. It
does make a difference.

Adam


Thank you.

To answer your question, I left all the neutrals connected to the
neutral side of the RCD but only connected one live, which was to the
vacuum cleaner. I would take a photo so you could see exactly what I
did, but I have disconnected it now and don't want to cut the power
at this time of night to recreate it.

So, I decided to simplify matters yet further by wiring the RCD into
a socket:

(Health and safety fanatics look away now)
http://tinyurl.com/LukeRCD


OK. So if that is the RCD from your CU then what socket did you plug
this Darwin Medal into?


And this time the RCD didn't trip, and the vacuum cleaner worked
fine. I must admit I am confused as to how leaving all those neutrals
connected could have been a contributory factor, as only 1 live was
connected. Is it therefore possible there is a leak between a Live on
the non-RCD side and Neutral on the RCD side then? If you like, I
will re-execute my original experiment tomorrow morning and post a
picture.


I'd video it, the coroner may want a copy.

You have the symptoms of a classic neutral earth fault on the RCD
side of your house electrics.

The RCD needs the live and neutral that pass through it to have the
same current, any imbalance more than 30mA and the RCD will trip.

If the neutral and earth are shorted after the RCD (in the T&E house
cable not the vacuums flex) then some of the current that should flow
down the neutral cable back to the RCD is flowing to earth and the
RCD will trip.


Here is a crappy picture for you that might help.

http://s428.photobucket.com/albums/q...urrent=rcd.jpg

Note that any current flowing through earth does not go through the RCD and
so causes an imbalance in the RCD that causes it to trip.

--
Adam


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On 07/11/2010 14:23, Dave Osborne wrote:

Given that Hager (for example) sell RCDs that consistently trip at
27-28mA, I can (and I do) consider one that trips at 17mA as over
sensitive.

To be clear, though I meant over sensitive in the sense of "10mA more
sensitive than it needs to be" rather than "failing the 1/2 Idn test".


OK, understood. I've no experience of Hager products, so thanks for the
info. Do they guarantee a minimum tripping current though, other than
that implied by compliance with EN 60898?

Actually, the standard says that they must not trip at 15mA and must
trip at 30mA (within 200ms). It is entirely with the manufacturer to
decide what actual trip level they set their product at.


Indeed, but I still think that the prudent fixed installation designer
should assume a possible trip current of 15 mA and divide the
installation such that no RCD should see more than (say) 10 mA of Idn
leakage under normal load conditions (insofar as 'normal' can be
predicted). Bear in mind BS 7671's requirements for maintainability -
your Hager '27 mA' RCD could get replaced by a different make that trips
at 18 mA, say.

Your own interest is more specialised and under 'skilled person'
supervision, and I can see your need to avoid spurious tripping at all cost.

[snip]


To answer your question, which I take to mean "how many times have I
found an RCD that tripped at the required fault current, but *not*
within the prescribed time" then I would say at least a couple each season.

Sometimes you get a duff batch and (usually after a season or two of
being fine) most or all of them fail to survive over-wintering.


Interesting. I take it you've seen the ESC's reliability study:
http://short.zen.co.uk/?id=faa

--
Andy


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Andy Wade wrote:
On 07/11/2010 14:23, Dave Osborne wrote:



[snip]


To answer your question, which I take to mean "how many times have I
found an RCD that tripped at the required fault current, but *not*
within the prescribed time" then I would say at least a couple each
season.

Sometimes you get a duff batch and (usually after a season or two of
being fine) most or all of them fail to survive over-wintering.


Interesting. I take it you've seen the ESC's reliability study:
http://short.zen.co.uk/?id=faa


No haven't seen that before; thanks for the heads-up. It makes an
interesting read and correlates closely with my observed experience.

I had assumed that problems with my RCDs were due to the fact that they
were un-powered (and therefore cold and damp) over the winter months,
but it seems that there is a general issue with detrimental ageing.

I quite often find that if you test an (in-service rather than new) RCD
without first exercising it on the trip button two or three times, it
fails on trip time, but it's thereafter OK. I have got into the habit of
exercising the mechanism a few times on the trip button before testing
on the RCD tester, so I don't have to record a test fail.
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Default Another RCD puzzler

On 8 Nov,
Dave Osborne wrote:

I quite often find that if you test an (in-service rather than new) RCD
without first exercising it on the trip button two or three times, it
fails on trip time, but it's thereafter OK. I have got into the habit of
exercising the mechanism a few times on the trip button before testing
on the RCD tester, so I don't have to record a test fail.


I've got into the habit of routinely testing mine when the clocks change. It
saves having to reset all the clocks an extra time. I know they /should/ be
tested twice as often but at least it's done twice a year.

--
B Thumbs
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Default Another RCD puzzler


SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT

Here is a crappy picture for you that might help.

http://s428.photobucket.com/albums/q.../?action=view¤...

Note that any current flowing through earth does not go through the RCD and
so causes an imbalance in the RCD that causes it to trip.

--
Adam


Right! Thank you to you everyone for your very interesting and
humourous discourse into RCDs and their operation. My wife and I had a
good laugh about the Darwin Award reference, and the suggestion of a
vacuum cleaner in a bucket of salt water!

Just to clear a couple of things up:
1) I was able to test the Consumer Unit's RCD in a socket by
temporarily re-jigging the CU so that none of the circuits were RCD
protected. Good job I didn't post a picture of that!
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.

After examining Adam's picture, a light bulb went on, somewhat
belatedly I must admit. It finally dawned on me that because all the
neutrals are commoned together, then a path to earth can therefore
exist on a _DIFFERENT_ circuit to the one where a load is applied,
even if that _DIFFERENT_ circuit's MCB is actually switched off.

So, filled with enthusiasm, I isolated the neutrals one by one, until
finally the RCD no longer tripped, which was when I disconnected a
defunct cable to an old bathroom fan heater. I am pretty sure the
cable is just lying under the floorboards, no doubt with all its
conductors haphazardly twisted together - although the Live had long
since been disconnected, the neutral was still commoned together with
the other RCD protected circuits.

BINGO! There was my problem and now I have completely disconnected
this circuit the system works perfectly.

Thanks again

Luke

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Default Another RCD puzzler

In article
s.com, Luke scribeth thus

SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT

Here is a crappy picture for you that might help.

http://s428.photobucket.com/albums/q.../?action=view¤...

Note that any current flowing through earth does not go through the RCD and
so causes an imbalance in the RCD that causes it to trip.

--
Adam


Right! Thank you to you everyone for your very interesting and
humourous discourse into RCDs and their operation. My wife and I had a
good laugh about the Darwin Award reference, and the suggestion of a
vacuum cleaner in a bucket of salt water!

Just to clear a couple of things up:
1) I was able to test the Consumer Unit's RCD in a socket by
temporarily re-jigging the CU so that none of the circuits were RCD
protected. Good job I didn't post a picture of that!
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.

After examining Adam's picture, a light bulb went on, somewhat
belatedly I must admit. It finally dawned on me that because all the
neutrals are commoned together, then a path to earth can therefore
exist on a _DIFFERENT_ circuit to the one where a load is applied,
even if that _DIFFERENT_ circuit's MCB is actually switched off.

So, filled with enthusiasm, I isolated the neutrals one by one, until
finally the RCD no longer tripped, which was when I disconnected a
defunct cable to an old bathroom fan heater. I am pretty sure the
cable is just lying under the floorboards, no doubt with all its
conductors haphazardly twisted together - although the Live had long
since been disconnected, the neutral was still commoned together with
the other RCD protected circuits.

BINGO! There was my problem and now I have completely disconnected
this circuit the system works perfectly.

Thanks again

Luke

Neutral Earth short then;!...
--
Tony Sayer



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Default Another RCD puzzler

Luke wrote:
SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT

Here is a crappy picture for you that might help.

http://s428.photobucket.com/albums/q.../?action=view¤...

Note that any current flowing through earth does not go through the
RCD and so causes an imbalance in the RCD that causes it to trip.

--
Adam


Right! Thank you to you everyone for your very interesting and
humourous discourse into RCDs and their operation. My wife and I had a
good laugh about the Darwin Award reference, and the suggestion of a
vacuum cleaner in a bucket of salt water!

Just to clear a couple of things up:
1) I was able to test the Consumer Unit's RCD in a socket by
temporarily re-jigging the CU so that none of the circuits were RCD
protected. Good job I didn't post a picture of that!
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.

After examining Adam's picture, a light bulb went on, somewhat
belatedly I must admit. It finally dawned on me that because all the
neutrals are commoned together, then a path to earth can therefore
exist on a _DIFFERENT_ circuit to the one where a load is applied,
even if that _DIFFERENT_ circuit's MCB is actually switched off.

So, filled with enthusiasm, I isolated the neutrals one by one, until
finally the RCD no longer tripped, which was when I disconnected a
defunct cable to an old bathroom fan heater. I am pretty sure the
cable is just lying under the floorboards, no doubt with all its
conductors haphazardly twisted together - although the Live had long
since been disconnected, the neutral was still commoned together with
the other RCD protected circuits.

BINGO! There was my problem and now I have completely disconnected
this circuit the system works perfectly.


I am delighted that the RCD trips have stopped, but I am afraid that your
new post now gives me even greater concern about your electrical
installation than a tripping RCD.

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 see
http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing and
http://wiki.diyfaq.org.uk/index.php?...Earthing_Types)


--
Adam


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