So,

There I was, minding my own business when 'click' all the power went
off in the room. I only had a side light on at the time so thought it
could have been a power cut. Went to the hall and found the light on
so realised it was just my power ccts. Reset the RCD and it tripped
again. Switched off all the MCB's, reset the RCD and turned the MCB's
back on one at a time .. upstairs ring .. 'click'. I then removed all
plugs from the ring and RCD reset ok. Plugging things back in found
the washing machine (there is a little utility area also on upstairs
ring) was the cause (but only when switched on at the socket to it's a
live earth fault?)

So, what is likely to be the cause of a washing machine tripping the
power when it's not actually turned on itself (ie what's between the
plug and the main switch in the machine .. It's an AEG OKO-Lavamat
6100 Digitronic if that makes any difference?)

I'll pull it out and have a look tomorrow but I fancied some pointers
meantime?

"Place yer bets please .. " ;-)

All the best ..

T i m

T i m wrote:

So, what is likely to be the cause of a washing machine tripping the
power when it's not actually turned on itself (ie what's between the
plug and the main switch in the machine .. It's an AEG OKO-Lavamat
6100 Digitronic if that makes any difference?)

Mains input filter?


Lee
--
Email address is valid, but is unlikely to be read.

Lee wrote:
T i m wrote:

So, what is likely to be the cause of a washing machine tripping the
power when it's not actually turned on itself (ie what's between the
plug and the main switch in the machine .. It's an AEG OKO-Lavamat
6100 Digitronic if that makes any difference?)


Mains input filter?


Lee
Sounds like it to me too. Large electrolytic capacitor gone all leaky.

Andrew

T i m wrote:


plugs from the ring and RCD reset ok. Plugging things back in found
the washing machine (there is a little utility area also on upstairs
ring) was the cause (but only when switched on at the socket to it's a
live earth fault?)

Depends on if your socket has a double pole switch or not... ;-)

I'll pull it out and have a look tomorrow but I fancied some pointers
meantime?

Input filter as others said, or possibly water seeping into some place
it should not (which would suggest an internal leak!). Any puddles under
the machine when you move it?

--
Cheers,

John.

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

On Fri, 14 Jan 2005 19:12:29 +0000, T i m wrote:

So,

There I was, minding my own business when 'click' all the power went
off in the room. I only had a side light on at the time so thought it
could have been a power cut. Went to the hall and found the light on
so realised it was just my power ccts. Reset the RCD and it tripped
again. Switched off all the MCB's, reset the RCD and turned the MCB's
back on one at a time .. upstairs ring .. 'click'. I then removed all
plugs from the ring and RCD reset ok. Plugging things back in found
the washing machine (there is a little utility area also on upstairs
ring) was the cause (but only when switched on at the socket to it's a
live earth fault?)

So, what is likely to be the cause of a washing machine tripping the
power

Water (where oughta not be).

In a place I run, the damp pump windings have gone short circuit (4.4
Ohms @ 3V DC) which in turn have taken out the control module (£80). 8-(((


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html

On Fri, 14 Jan 2005 21:03:08 +0000, John Rumm
wrote:

T i m wrote:


plugs from the ring and RCD reset ok. Plugging things back in found
the washing machine (there is a little utility area also on upstairs
ring) was the cause (but only when switched on at the socket to it's a
live earth fault?)

Depends on if your socket has a double pole switch or not... ;-)

Std MK double switched socket .. single pole? And if it is only single
pole that pins it down to a live earth fault though eh?).

I'll pull it out and have a look tomorrow but I fancied some pointers
meantime?

Input filter as others said,

Are you all talking one of the inline sealed jobbys here .. L & N in
and out, common Gnd (like you sometimes get built into IEC chassis
sockets?

or possibly water seeping into some place
it should not (which would suggest an internal leak!). Any puddles under
the machine when you move it?

I'll let you know tomorrow ;-) Theres no 'visible' water leak as yet
(it *has* leaked in the past .. I changed the drum bearings / seal and
the water pump, both a few years ago now ..) and the drain backed up
once (used the drain wand on my pressure washer). ;-)

It was working perfectly last night and wasn't on when it tripped the
cct?

We will see ...

All the best ..

T i m

On Fri, 14 Jan 2005 21:03:09 +0000, Ed Sirett
wrote:


So, what is likely to be the cause of a washing machine tripping the
power

Water (where oughta not be).

In a place I run, the damp pump windings have gone short circuit (4.4
Ohms @ 3V DC) which in turn have taken out the control module (£80). 8-(((

Ouch ... let's hope not, else it will be back down the brook with the
washing for her ... !

All the best ..

T i m

In article , T i m
writes
On Fri, 14 Jan 2005 21:03:08 +0000, John Rumm
wrote:

T i m wrote:


plugs from the ring and RCD reset ok. Plugging things back in found
the washing machine (there is a little utility area also on upstairs
ring) was the cause (but only when switched on at the socket to it's a
live earth fault?)

Depends on if your socket has a double pole switch or not... ;-)

Std MK double switched socket .. single pole? And if it is only single
pole that pins it down to a live earth fault though eh?).

I'll pull it out and have a look tomorrow but I fancied some pointers
meantime?

Input filter as others said,

Are you all talking one of the inline sealed jobbys here .. L & N in
and out, common Gnd (like you sometimes get built into IEC chassis
sockets?

or possibly water seeping into some place
it should not (which would suggest an internal leak!). Any puddles under
the machine when you move it?

I'll let you know tomorrow ;-) Theres no 'visible' water leak as yet
(it *has* leaked in the past .. I changed the drum bearings / seal and
the water pump, both a few years ago now ..) and the drain backed up
once (used the drain wand on my pressure washer). ;-)

It was working perfectly last night and wasn't on when it tripped the
cct?

We will see ...

All the best ..

T i m



Also don't forget to check and measure leakage from Neutral to Earth
this is also a cause for RCD tripping....
--
Tony Sayer

On Fri, 14 Jan 2005 22:45:56 +0000, tony sayer
wrote:


Also don't forget to check and measure leakage from Neutral to Earth
this is also a cause for RCD tripping....

Will I need some 'special' gear for that .. only got a DMM?

If it only trips when I turn the thing on at the socket (ie netural
and earth would be connected all the time) would that suggest the
neutral earth leakage was ok (or below the trip current at least?)

All the best ..

T i m

In article , T i m
writes
On Fri, 14 Jan 2005 22:45:56 +0000, tony sayer
wrote:


Also don't forget to check and measure leakage from Neutral to Earth
this is also a cause for RCD tripping....

Will I need some 'special' gear for that .. only got a DMM?

If it only trips when I turn the thing on at the socket (ie netural
and earth would be connected all the time) would that suggest the
neutral earth leakage was ok (or below the trip current at least?)

All the best ..

T i m




That leakage is quite awkward to measure, its that sometimes if theres a
short between neutral and earth some current needs to flow before it
will show up. Bit late to explain all the niceties about RCD's.

Bet summats got damp somewhere
--
Tony Sayer

"T i m" wrote in message
...
On Fri, 14 Jan 2005 22:45:56 +0000, tony sayer
wrote:


Also don't forget to check and measure leakage from Neutral to Earth
this is also a cause for RCD tripping....

Will I need some 'special' gear for that .. only got a DMM?

If it only trips when I turn the thing on at the socket (ie netural
and earth would be connected all the time) would that suggest the
neutral earth leakage was ok (or below the trip current at least?)

All the best ..

T i m

And it actually trips the RCD, or just the MCB for that circuit. If it's
the whole house RCD then it's a fault from live to earth through a cable or
loose connection somewhere. Or the mains RFI filter on the appliance as
others have already said. A fault to earth through the negative conductor
will only show when the supply to the appliance is actually being used.
That's the nature of un-balanced current flow in both conductors. Earth and
negative are usually at the same, or bloody close to the same, potential
while the appliance is lying dormant. But this being a washing machine
supply, then look for water as well as burn marks on the wires inside. Mice
can also chew through the flex in the most inconceivable places.

On Sat, 15 Jan 2005 00:10:15 GMT, "BigWallop"
strung together this:

And it actually trips the RCD, or just the MCB for that circuit. If it's
the whole house RCD then it's a fault from live to earth through a cable or
loose connection somewhere.

Nope.

Or the mains RFI filter on the appliance as
others have already said.

Possibly.

A fault to earth through the negative conductor
will only show when the supply to the appliance is actually being used.

Nope.

That's the nature of un-balanced current flow in both conductors. Earth and
negative are usually at the same, or bloody close to the same, potential
while the appliance is lying dormant.

Not always. It doesn't take much to knock the RCD out while other
circuits are on load if there is a slight difference between N and E
and there is a bit of a N\E fault somewhere..

But this being a washing machine
supply, then look for water as well as burn marks on the wires inside. Mice
can also chew through the flex in the most inconceivable places.

*ignore all this and carry on following the advice from elsewhere in
the thread. *

BW, you can have absolutely nothing pulling a load through an RCD and
a NEUTRAL (not negative, that's DC), to earth fault *can* trip it.

Sorry.
--

SJW
Please reply to group or use 'usenet' in email subject

"Lurch" wrote in message
...
On Sat, 15 Jan 2005 00:10:15 GMT, "BigWallop"
strung together this:

snipped
*ignore all this and carry on following the advice from elsewhere in
the thread. *

BW, you can have absolutely nothing pulling a load through an RCD and
a NEUTRAL (not negative, that's DC), to earth fault *can* trip it.

Sorry.


Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth. If the appliance is open circuit, all
switches open, then the phase should not be connected to neutral through any
load, and so this can't cause an imbalance in the phase to neutral loop
which the RCD is there to detect. So, for arguments sake, we'll say that
neutral and earth are in the same potential state. The phase is at higher
potential than both neutral and earth. Phase and neutral are "not"
connected through any load (open circuit). A straight short occurs between
a neutral and an earth conductor. What happens at the RCD?

Therefore, the fault must lie between phase and earth for the RCD to trip
the supply, because neutral is already at, or so close to, earth potential
that the RCD will not detect any difference on this side of the supply.
This would only happen if there was a closed circuit between phase and
neutral, and a neutral conductor passed a fault current to earth at such a
low impedance, which should be highly unlikely because neutral is already,
or very nearly, at earth potential.

How I was told and read how it all worked was like this. The conductors and
loads in a circuit act as resistance to the passage of electrical current.
Placing two resistances in series only increases the total resistance in the
circuit. But, if two resistors are placed in parallel to each other, the
total resistance is actually reduced by a tangible amount in the whole
circuit because the electrical current has two paths to flow through on that
part of the circuit rather than just one. This reduction in total
resistance in the circuit by placing a parallel resistor in it is what
creates the imbalance for the RCD to detect. The RCD should only have one
conductor with current going out and one conductor with current coming in.
By making two conductors send current out, and still have only one conductor
sending current in, the circuit becomes unbalanced.

That's why I always thought an RCD was placed in line to prevent shock or
damage from an imbalance on the phase to earth side of the supply, not on
the neutral to earth side of the supply as well. An RCD should also give no
detection of a fault across phase and neutral because such a fault would
also be detected as a balanced load across them. Reading the IEE and RCD
manufacturers literature would also point me to believe this is true.

Yes you can have a slight increase in potential through heavy loading on
other parts of the same neutral path, but these should never be enough to
create a dangerous situation for the RCD to detect and trip open because
earth should also be induced by these same small increases in potential that
are effecting the neutral paths. So what you are saying is that the neutral
or earth of the OP's supply are not operating properly to remove the
unlikely event of a low resistance current flow between neutral and earth,
while the neutral conductor is being induced by some other loadings on the
same part of the circuit. This would cause the neutral to have a much
higher potential than earth, and so would cause an imbalance for the RCD to
detect. So do you think the OP should also have the supply and wiring
tested to be sure that the safety devices fitted are going to properly
protect from the potential harm of an electric shock?

(BTW, I use the word "negative" for circuit descriptions nearly every day
and I was wrong to use it here as we're talking AC not DC, but old habits
die hard. My apologies)

http://www.safety.ed.ac.uk/policy/part3/52.html
http://www.tlc-direct.co.uk/Technica...CD%20Works.htm
http://www.reo.co.uk/files/kbase/How...CD%20works.pdf
http://www.collinseducation.com/reso...20Breakers.pdf
http://www.tlc-direct.co.uk/Book/5.9.2.htm

On Sat, 15 Jan 2005 04:55:43 GMT, "BigWallop"
wrote:


Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

Not sure this helps anything but when I was rewiring Dads garage a
while back I fitted one of those little RCD + 2 x MCB CU's.

I had powered it all up and was using the 'lighting' CCT and had the
live switched out on the MCB. I cut through the T&E on the 'Power' cct
and the RCD tripped (because the RCD had detected earth current
derived from the neutral potential as they were shorted by my
cutters?).

All the best ..

T i m

On Sat, 15 Jan 2005 00:10:15 GMT, "BigWallop"
wrote:

A fault to earth through the negative conductor
will only show when the supply to the appliance is actually being used.
That's the nature of un-balanced current flow in both conductors.

I wonder if the different devices work differently? I'm aware of
ELCB's and RCD's (the RCD.s with MCB's built in are the same thing)
and I think there was something before ELCB?


Earth and
negative are usually at the same, or bloody close to the same, potential
while the appliance is lying dormant.

Wouldn't that also depend of / if other devices are also running on
that (or other rings) from the same RCD? That device may be off but
beside it is say the tumble dryer pulling a few amps ...? With the
washer plugged in (but turned off at the wall) and assuming the
sockets only switch the live then the neutral / earth will still be
continued into the washer and a 'fault' between them still detected?

But this being a washing machine
supply, then look for water as well as burn marks on the wires inside.

I'll look for that later today .. ;-(

Mice
can also chew through the flex in the most inconceivable places.

Well, I'm not aware we have any mickys in here at the moment (we have
had a few years ago .. came in from under the communial suspended
ground floor .. holes now all filled .. ), I'll look for PVC chippings
as well then ;-)

All the best ..

T i m

In article ,
BigWallop wrote:
[big snip]
This would only happen if there was a closed circuit between
phase and neutral, and a neutral conductor passed a fault current
to earth at such a low impedance, which should be highly unlikely
because neutral is already, or very nearly, at earth potential.

I don't think that is correct and in fact we have had
an RCD trip caused by an electric kettle. Both the
wall and kettle switches were Off and it required the
kettle to be unplugged to stop the tripping.

An RCD trip from an N-E short is quite possible in this
house when you look at the sums. We are on a two-wire
supply from a transformer that is a few hundred yards
away. Neutral is connected to Earth at that transformer.
The N-E resistance measured at our house is 33 ohms, and
is a measure of the resistance of that few hundred yards
of clay soil, between our Earth rod and the Earth rod at
the transformer.

Our N-E voltage, measured at the house, averages 0.7Vrms.
This is probably due to the loadings of the five other
houses upstream of us.

I= V/R, so an N-E short would result in 0.7/33 (21mArms)
of unbalance current through the RCD.... even when there
is nothing connected to the Line wire through the RCD.

If the N-E voltage or the ground resistance varies (long
hot summers?), then the RCD unbalance current (and the
chance of an RCD trip) would also vary.

--
Tony Williams.

The message
from T i m contains these words:

A fault to earth through the negative conductor
will only show when the supply to the appliance is actually being used.
That's the nature of un-balanced current flow in both conductors.

Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

--
Roger

BigWallop wrote:

Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

Well, you are completely and utterly wrong.

A common question here goes along the lines of "I isolated the circuit
at the fuse/MCB to do some work on it and when I cut a cable the RCD
tripped. Why?"
Answer: because you created a neutral-earth (strictly neutral-CPC) short
with your wire cutters.

If the appliance is open circuit, all switches open, then the phase
should not be connected to neutral through any load, and so this
can't cause an imbalance in the phase to neutral loop which the RCD
is there to detect.

But there are always other loads to think about. These are passing
current through neutral conductors somewhere. A N-E short creates an
opportunity for the neutral current to take two parallel paths, one of
which is through your RCD. The RCD then sees current in the neutral but
not the phase(s); if this exceeds the threshold the RCD will oblige by
tripping.

So, for arguments sake, we'll say that neutral and earth are in the
same potential state. The phase is at higher potential than both
neutral and earth. Phase and neutral are "not" connected through any
load (open circuit). A straight short occurs between a neutral and
an earth conductor. What happens at the RCD?

It probably trips. As soon as you short the neutral to earth the
neutral is no longer isolated -- unless its isolated upstream at an
all-pole isolator (not usually the case unless you've opened the main
switch) -- current can now flow in the neutral and trip the RCD.

Therefore, the fault must lie between phase and earth for the RCD to trip
the supply, because neutral is already at, or so close to, earth potential
that the RCD will not detect any difference on this side of the supply.

"At or so close to earth potential" is missing the point. It's easier
to think about where current is flowing, and how it will be shared if
there are parallel paths. If you want to think in terms of voltage
though, consider the fact that you've got low impedances (copper wires)
and that 30mA (or whatever) to trip an RCD is not much current. If the
neutral is, say, only 0.1 volt 'above' earth then a N-E fault loop
impedance of 3 ohms will cause over 30mA of residual current and trip
the RCD. Typical Z-E shorts in domestic wiring will have a loop
impedance of well under that. Even with the fault on the end of 50
metres of 1 mm^2 T&E cable you'd see under 2 ohms.

This would only happen if there was a closed circuit between phase and
neutral, and a neutral conductor passed a fault current to earth at such a
low impedance, which should be highly unlikely because neutral is already,
or very nearly, at earth potential.

I hope you can now see the flaw in that argument. It's not highly
unlikely at all; it's highly likely.

The type of earthing of the installation makes a difference. Let's work
through the 3 usual cases:

1. TN-S (earth comes via supplier's cable sheath)
-------------------------------------------------
Here there is always a voltage difference between N and E -- even if
*everything* in your house is switched off. This is because other
consumers' loads are causing a voltage drop in the supply cable neutral
between the point where your service cable is connected and the earthed
point at the transformer. A N-E short in the house diverts a little bit
of their neutral current through your neutral, your RCD and back to the
transformer via the cable sheath. The RCD will almost always trip in
these circumstances.

2. TN-C-S (PME)
---------------
Here your earth comes from the supply neutral entering the premises. If
there are no loads on in your house there is no neutral current to
divert and the RCD will not trip on a N-E short.

However if you are working on one RCD protected circuit (with its phase
isolated) and there's load current flowing in another circuit protected
by the same RCD, a N-E short in the circuit you're working on will
divert some of the other circuit's load current through the earth,
causing an imbalance and tripping the RCD.

3. TT (no supplier earth, own earth rod)
----------------------------------------
This is essentially the same as the TN-S case, except that the impedance
of the parallel earth path is much higher and the risk of tripping
somewhat lower. Nevertheless if your earth electrode is good and the
incoming neutral is a volt or two above the local ground (again because
of other people's loads) then a N-E short will trip the RCD -- e.g.
neutral 1 volt above earth, earth electrode resistance 30 ohms (neglect
other impedances): residual current = 33 mA, so a 30mA RCD will trip but
a 100mA one won't trip.


BTW-1: RCD tripping on a N-E short is a Good Thing because it alerts you
to the presence of a potentially dangerous problem. Neutrals and earths
are not fused. If there's a parallel path and the main desired neutral
path happens to go o/c then lots of load current could end up going
through a bit of 1 mm^2 wire in your house, which might respond by
catching fire...

BTW-2: None of this is relevant to the original subject of this thread.
From the original article we know that the washing m/c does not trip
the RCD when plugged in with the circuit MCB open. Therefore a N-E
short can be ruled out and phase-earth leakage is the problem. Probably
the EMI filter, or water leakage, as someone said a while ago.

--
Andy

On Sat, 15 Jan 2005 04:55:43 GMT, "BigWallop"
strung together this:

Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

An RCD doesn't trip on an earth fault as such, it measures the current
in the phase and neutral conductors and if they are not equal, (to
within the rating of the RCDs trip current), then it will trip.

If the appliance is open circuit, all
switches open, then the phase should not be connected to neutral through any
load, and so this can't cause an imbalance in the phase to neutral loop
which the RCD is there to detect. So, for arguments sake, we'll say that
neutral and earth are in the same potential state.

But they might not be, let's say for arguments sake there isn't a
fault, what does that prove?

The phase is at higher
potential than both neutral and earth. Phase and neutral are "not"
connected through any load (open circuit). A straight short occurs between
a neutral and an earth conductor. What happens at the RCD?

Nothing, there has to be potential between the N and E for the RCD to
trip, like I've said.

I scanned through this bit then snipped it

It appears that you haven't understood the workings of an RCD
glasshopper!

An RCD detects an imbalance between the current leaving on the phase
conductors and the current returning on the neutral conductor. They
don't measure earth to detect a fault.

As you are basing your whole argument on your incorrect understanding
of the function of RCDs then I'll let you try again, but with the
correct answer!
--

SJW
Please reply to group or use 'usenet' in email subject

"T i m" wrote in message
...
On Sat, 15 Jan 2005 04:55:43 GMT, "BigWallop"
wrote:


Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply
unless
there is a fault from Phase to Earth.

Not sure this helps anything but when I was rewiring Dads garage a
while back I fitted one of those little RCD + 2 x MCB CU's.

I had powered it all up and was using the 'lighting' CCT and had the
live switched out on the MCB. I cut through the T&E on the 'Power' cct
and the RCD tripped (because the RCD had detected earth current
derived from the neutral potential as they were shorted by my
cutters?).

All the best ..

T i m


No it didn't, it detected induced current through what should have been a
neutral conductor, but which was then a phase conductor because the lighting
was still in use when you cut the cable to the socket circuit. I can bet
that the lighting and power cables run along side each other on some stretch
of the wiring plan, and it was either the induced current, even though very
small, or a larger impedance on the earth conductors that caused the RCD to
trip open. It could have either been the earth was not properly conducting
the residual current away from the induced leakage through the cable
insulation, and this created a higher potential in the neutral conductor, or
what should have been a neutral conductor had risen more toward phase
potential from some other means as yet undiscovered. This would cause a
Residual Current Devices to detect an imbalance between phase and earth and
activate the breaker circuitry.

If you have a look at the inner workings of an RCD you see there is no way
that a neutral to earth leak will cause the device to trip open. That's
unless there is an earth loop impedance fault or, to put it another way, a
higher resistance path on the earth conductors than the neutral conductors,
which will then causes the neutral path to become higher in potential energy
than earth and so cause an imbalance between a positive potential and
negative potential at earth (where common current flow specific is used).
This type of fault causes what should be a neutral potential to literally
become phase potential and so again cause an imbalance between phase
potential and earth potential which an RCD is designed to detect. He
http://www.memonline.com/rcd1.html for instance, shows how the device works,
with a diagram of the inner parts.

The whole point of equipotential bonding using larger cross sectional area
(csa') conductors than those used in the supply, is to greatly reduce the
impedance along these conductors. The reduction in impedance should be to
such a level that will not cause what should be a neutral potential to
obtain phase potential. This use of large csa' cables for earth bonding
should be capable of removing all residual current obtained by the neutral
paths, either through inductance or contact with a phase potential
conductor, and should also be capable of preventing the neutral path from
creating an imbalanced current flow between it and earth. Neutral and earth
should always remain at the same potential, or at a very negligible measure
from it, no matter where it is on the whole installation plan. This is the
requirement, stated in all the good guideline hand books and other
literature, for the earth bonding to have proper residual current removal
properties.

An example. Phase and neutral conductors can be created using, lets say,
2.5 mm csa' conductors but, to remove the residual current created by close
proximity between the two supply conductors and any other means of induced
current flow, the earth bonding conductor needs to be of a larger csa', of
more commonly 4 mm to 6 mm, to reduce its impedance further than the supply
conductors and so be capable of reducing induced current flow and prevent
nuisance trips on an RCD. So, larger cross sectional area cables are used
for equipotential bonding to produce a much lower impedance path to ensure
proper earth leakage current detection from the phase supply. This, in
turn, creates a much safer reaction time if such a leak to earth does occur.
It isn't just a small safety thing to use equipotential bonding, it is also
there to keep both phase and neutral paths in balance so that a higher
residual current, commonly 30mA, can be detected quickly.

It is natural to have a current flow of anything up to 12 mA, or even 15 mA
in some installations, between phase and earth/neutral conductors, even when
they are not under load. This is due mainly to induction through their
close proximity with each other inside the insulated outer sleeve of the
cable. The totally bare earth protective conductor is placed in the cable
to try to reduce this natural induced current flow phenomenon as much as
possible. The bare conductor should also help bring the two supply
conductors back in to a balanced state again. It isn't because the cable
makers are stingy with the insulation materials that the earth protective
conductor is left totally bare naked, it's because an insulated coating
would create a much higher resistance and reduce the bare conductors
capabilities in removing the residual current and so be unable to creating
some kind of harmony inside the cable again. Electricity suppliers also use
tests to make sure that the earth bonding at a supply is within certain
limits. This is to create a proper equipotential bond between all the
conductors. To high an impedance on any one of them, phase, neutral or
earth, will put the whole power balance out of sync and cause problems.

So, to sum up, I still say an RCD will not trip open unless the fault
current it detects is between a phase potential and earth mass.

Boy, can I go on a bit when I get started. :-) LOL !!!

"Tony Williams" wrote in message
...
In article ,
BigWallop wrote:
[big snip]
This would only happen if there was a closed circuit between
phase and neutral, and a neutral conductor passed a fault current
to earth at such a low impedance, which should be highly unlikely
because neutral is already, or very nearly, at earth potential.

I don't think that is correct and in fact we have had
an RCD trip caused by an electric kettle. Both the
wall and kettle switches were Off and it required the
kettle to be unplugged to stop the tripping.

An RCD trip from an N-E short is quite possible in this
house when you look at the sums. We are on a two-wire
supply from a transformer that is a few hundred yards
away. Neutral is connected to Earth at that transformer.
The N-E resistance measured at our house is 33 ohms, and
is a measure of the resistance of that few hundred yards
of clay soil, between our Earth rod and the Earth rod at
the transformer.

Our N-E voltage, measured at the house, averages 0.7Vrms.
This is probably due to the loadings of the five other
houses upstream of us.

I= V/R, so an N-E short would result in 0.7/33 (21mArms)
of unbalance current through the RCD.... even when there
is nothing connected to the Line wire through the RCD.

If the N-E voltage or the ground resistance varies (long
hot summers?), then the RCD unbalance current (and the
chance of an RCD trip) would also vary.

Tony Williams.


So, I'll start again. A neutral / earth short circuit will not cause an RCD
to trip. If a fault or high impedance earth loop is used, then the neutral
becomes higher in potential than it should be, so it isn't at neutral
potential any more because neutral potential is the same as earth potential.
What has happened is, the conductor that is used as the neutral bond has now
risen to phase potential because earth hasn't removed the residual current
from it. The neutral is now a phase. If the phase then leaks to proper
earth potential through a lower impedance path, then an RCD is designed to
detect this leak and trip open. Again. Neutral and earth are at the same
potential, so a straight short across them will do nothing. If an induce
current of any kind lifts the neutral conductor to phase potential, then it
no longer is at neutral potential. This can be caused, as you have
demonstrated yourself above, when the earth bonding does not remove the
residual current properly and causes what should be the neutral conductor to
rise in potential energy, and so it becomes more to phase potential. Now
create a short circuit across the neutral conductor which is now sitting
well above earth potential and you cause an imbalance between phase
potential and earth.

An RCD will not trip open with a fault between neutral potential and earth
potential, because both neutral potential and earth potential have exactly
the same potential. An RCD will only trip open circuit if it detects an
imbalance between phase potential and earth potential.

Roger wrote:


Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

Exactly so. Congratulations: you've managed to say in 5 lines what
usually takes me several screenfuls - and just took Andy W nearly as
long, but with more cases more thoroughly considered - on the frequent
occasions that this FAQ comes up!

Stefek

Stefek Zaba wrote:

Exactly so. Congratulations: you've managed to say in 5 lines what
usually takes me several screenfuls - and just took Andy W nearly as
long, but with more cases more thoroughly considered - on the frequent
occasions that this FAQ comes up!

None of it seems to have stopped Mr. Wallop continuing to post complete
******** though. I'll be sorely tempted to adopt the IMM approach next
time and resort to snip_drivel.

--
Andy

BigWallop wrote:


So, I'll start again. A neutral / earth short circuit will not cause an RCD
to trip.

It does, demonstrably, and without any further faults.

Your mental model of what an RCD does is all at sea, I'm afraid. Since
it's a CURRENT BALANCE device, it's easiest to see what happens in an
N-to-E fault in terms of CURRENT.

By "it's a CURRENT BALANCE" device, what I mean is this: it compares how
much current goes 'out' from its L or 'phase' terminal, to how much
comes 'back' to its other, N terminal (both these on its load side).
While wot-comes-back is within 30mA of wot-goes-out, it's happy; if the
imbalance exceeds 30mA, it trips. This contributes to safety, since that
30mA of current is going somewhere it oughtn't to - just maybe through a
person; 30mA is chosen as it's around the current needed, given typical
body resistance, to be approaching Serious Consequences.

When there's an N-to-E short, there's a second current return path
created: the nature of this return path and the direction of the
imbalance vary according to the source of the 'outgoing' current and the
earthing arrangements.

In the case most relevant to the OP: the RCD is supplying the load (here
a washing machine) with the 'outbound' current off its L terminal, and
wants to see it all come back to its N terminal. With no N-E fault, this
is just what it gets. (Indeed, if the E for the circuit/socket/plug
supplying the washing machine becomes disconnected, the RCD is *still*
in balance, and doesn't trip.) Now, if an N-to-E short happens on the
circuit supplying the washing machine - or inside the washing machine
itself - all the current which went 'out' to the washing machine now has
a choice of routes 'back': the 'original', intended route, back through
the RCD's N terminal (and back along the relevant CU's N busbar, the
black meter tail, and back to the substation/transformer where the
supply's L side comes from too), but also a new route through the 'E'
conductors (the circuit-protective-conductor in the T&E cable supplying
the washing m/c), crucially BYPASSING the RCD's N terminal, back through
the mess of E wiring around the CU, and finding its way back to the
substation and points closer to the installation through the many paths
between the installation's main E terminal and the supply's N. (In the
case of a PME (TN-C-S) installation, there's a local low-resistance path
right at the supply entry point; in the case of a TN-S installation the
supply E and supply N may not be in good metallic contact until we're
back at the substation; in the case of a TT installation there's only
the 'mass of earth' path back to the originating N.)

So, how much of the load current coming out of the RCD's L terminal
'chooses' the 'balancing' path back to the RCD's N terminal, and how
much goes the 'wrong', 'imbalancing' way? It depends completely on the
relative resistance (impedance, if you want to get picky) of those two
paths. For a PME installation, as outlined above, the 'imbalance' path
is all within the house, and is almost as low as the 'balancing' path -
a little higher, since CPCs are usually a bit thinner than N conductors;
so for a 10A load you might find 7A going back the 'right' way and 3A
going the 'wrong' way. The RCD will certainly trip in this case! The
TN-S case isn't that different, except that the nearest link between the
installation's E and the supplier's N isn't within the house, but might
be as far back as the substation/transformer: but both paths are
low-resistance and metallic, so again you can expect a substantial
proportion of the load current to 'choose' the 'imbalance' path, and
again the RCD will trip. At the other extreme, for a TT installation,
with a relatively high earth electrode resistance, the 'imbalance' path
could be at least 100 times higher resistance than the 'balancing' one,
so an on-load N-to-cpc fault isn't nearly so likely to cause a trip.

The analysis is similar for the related FAQ case of 'I was working on a
ring circuit I'd turned off the MCB for, touched its N to its E and all
the RCD-protected circuits in my house went dead'. Here the just-created
N-to-E short provides an alternative path bypassing the RCD's N terminal
for all the loads in the *OTHER* circuits the RCD is supplying, since
the N in the 'isolated' ring isn't isolated, but connected to all the
other N's through the consumer-unit busbar. Andy W's essay covers those
cases in detail, so I won't go on any more.

Whether this is enough to convince you that your 'neutral potential'
model is misleading, I can't tell. I assert again that a 'pure'
current-balance RCD doesn't give a monkey's about the relative potential
of L, N, and the installation's E (to which it's not directly
connected), though obviously you need to have an idea of their
potentials to see whether any current will flow among them. (Old-style
voltage-operated earth-leakage breakers are different, and some 'modern'
electronic RCDs have further refinements beyond the pure 'current
balance' idea; but that's another discussion). I hope it at least gives
you some pause for thought, and might even inspire you to do a (safe ;-)
bench experiment with a small load, a current-balance RCD, and
selected-resistance N-to-E faults ;-)

Cheers, Stefek

"Stefek Zaba" wrote in message
...
BigWallop wrote:


So, I'll start again. A neutral / earth short circuit will not cause an
RCD
to trip.

It does, demonstrably, and without any further faults.

No it doesn't. Neutral and earth points are taken from the place at earth
mass, so how can a short across neutral and earth cause an imbalance between
them, when they are already short circuited together at the point where
they're both connected to earth mass. It can only happen when one of the
conductors of either earth or neutral has risen or dropped below the
potential at which they where both connected to earth.


snipped
balance' idea; but that's another discussion). I hope it at least gives
you some pause for thought, and might even inspire you to do a (safe ;-)
bench experiment with a small load, a current-balance RCD, and
selected-resistance N-to-E faults ;-)

Cheers, Stefek

In article , Stefek Zaba s-nilnews-
writes
Roger wrote:


Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

Exactly so. Congratulations: you've managed to say in 5 lines what
usually takes me several screenfuls - and just took Andy W nearly as
long, but with more cases more thoroughly considered - on the frequent
occasions that this FAQ comes up!

Stefek

Just to chuck my 2 penny worth in Stefek has this right and Roger has
put the same thing the same way.

All you need to remember is the FUNDAMENTAL thing about a RCD is that it
compares the current IN to the current OUT if they are unbalanced by
more that the rated trip current nominally 30 or 100 ma then the device
will trip.

Remember the word !!CURRENT!! flow thats the important part.

It matters not whether the RCD is supplying a single table light or a
shower and cooker on the go together. As long as the CURRENT IN matches
the current OUT then all will be well. Offer the electricity an
alternative path and this balance is upset and off the trip will go.

That is all there is to it.

Let me try to put something else in perspective.

Imagine an RCD connected to the incoming supply. Nothing is connected to
the output of it at all.

Now take a wire thats connected to a good earth like a stake in the
ground outside in very damp soil and after its been raining. Touch this
to the live and the trip will immediately fire off.

Now do the same with the neutral output side of the trip and it won't
trip. So why does this happen?. Remember CURRENT flow. In the first
instance from the live to your earthstick current is flowing from the
incoming live line, through the RCD and then via your stick and then
through the earth back to the substation where one side of the supply
the neutral is connected to earth so you can see that current flows
through the live side of the RCD back through the earth connection to
the supplying transformer. It is NOT going back through the RCD so a
current imbalance has taken place.

The important thing is that there was an !!imbalance!! because more was
going OUT of the RCD than what was coming BACK through it!.

However when you touched the earth stick to the neutral NO CURRENT was
flowing through the live side, so no current was or could have been
flowing back!.

Now suppose you have a one bar electric fire connected to the output
terminals of the RCD. Normally what is going out is going back through
the RCD, so all is well.

Now connect that earth stick to the neutral again and it will trip.
Reason is that the CURRENT has an alternative way back to the earthed
side at the substation, some current is flowing through the RCD back to
the substation , but some is now taking the alternative route home via
the earth stick.

So an imbalance is caused, their being current flowing OUT through the
RCD but it has an alternative route back and if its going back and its
NOT through the RCD so its not the same as what's coming IN, then off
will go the trip.

Simple as that.

So thats why sometimes when you have an Earth to Neutral fault, it
doesn't show up until some "current" is flowing through the RCD through
the alternative path back to the substation, as until their is some
current flowing "through" the RCD, then it won't trip. Course that only
needs to be more then the rated tripping current but as long as thats
exceeded then off it will go, but it CAN'T trip unless there is some
current actually flowing through it in order to exceed this rated trip
imbalance level...
--
Tony Sayer

"Stefek Zaba" wrote in message
...
BigWallop wrote:


So, I'll start again. A neutral / earth short circuit will not cause an
RCD
to trip.

It does, demonstrably, and without any further faults.

Your mental model of what an RCD does is all at sea, I'm afraid. Since
it's a CURRENT BALANCE device, it's easiest to see what happens in an
N-to-E fault in terms of CURRENT.

Amongst your lengthy explanation which is a fair approximation to real life
you missed the point that in many installations a N-E voltage exists. (I'm
not entirely happy about describing an AC current as flowing "from" the live
but lets avoid pedantry.) O.K. its often "small" but present nevertheless. A
short between N E wiries will thus have a resultant current which is not
being cancelled out in the current balance detector and thus the device will
trip. Depending on the N E voltage and the rating of the RCD this can
require a full short or it can sometimes be sufficient to get your body
parts (fingers etc) across the N E wires.

BigWallop wrote:

No it doesn't. Neutral and earth points are taken from the place at earth
mass, so how can a short across neutral and earth cause an imbalance between
them, when they are already short circuited together at the point where
they're both connected to earth mass. It can only happen when one of the
conductors of either earth or neutral has risen or dropped below the
potential at which they where both connected to earth.


Sigh. My last word follows, you're welcome to have the real last word.

If you persist in the mistaken belief that 'neutral and earth points'
are in all non-fault situations already 'shorted together' at zero
resistance, I'm sure there's nothing I can say which will shake your
belief. The fact that the belief is wrong, will mislead you in doing
fault-finding on RCD trips (and propagating the belief would mislead
others), doesn't take into account the finite resistance of copper
conductors, and that in the case of current-balance RCDs the *location*
of any such 'final' bonding of N to E is crucial in understanding how
the current flow divides up between a through-the-RCD path and an
around-the-RCD path - all of that is is just sad; at least until such
time as parts of that belief affect the safety of others.

I don't doubt that the bulk, and quite possibly all, of the work you do
both personally and professionally in the electrical sphere is both safe
and competent. If part of your mental model about how it all works is
interestingly non-standard, it doesn't matter that much. Hell, one could
go through an entire career in electrical installation and design work
believing that current was carried by little green worms (which don't
like wriggling through narrow wires), or indeed that it was the mass
movement of electrons themselves, without doing anything dangerous...

Stefek

"Stefek Zaba" wrote in message
...
BigWallop wrote:

No it doesn't. Neutral and earth points are taken from the place at
earth
mass, so how can a short across neutral and earth cause an imbalance
between
them, when they are already short circuited together at the point where
they're both connected to earth mass. It can only happen when one of
the
conductors of either earth or neutral has risen or dropped below the
potential at which they where both connected to earth.


Sigh. My last word follows, you're welcome to have the real last word.

If you persist in the mistaken belief that 'neutral and earth points'
are in all non-fault situations already 'shorted together' at zero
resistance, I'm sure there's nothing I can say which will shake your
belief. The fact that the belief is wrong, will mislead you in doing
fault-finding on RCD trips (and propagating the belief would mislead
others), doesn't take into account the finite resistance of copper
conductors, and that in the case of current-balance RCDs the *location*
of any such 'final' bonding of N to E is crucial in understanding how
the current flow divides up between a through-the-RCD path and an
around-the-RCD path - all of that is is just sad; at least until such
time as parts of that belief affect the safety of others.

I don't doubt that the bulk, and quite possibly all, of the work you do
both personally and professionally in the electrical sphere is both safe
and competent. If part of your mental model about how it all works is
interestingly non-standard, it doesn't matter that much. Hell, one could
go through an entire career in electrical installation and design work
believing that current was carried by little green worms (which don't
like wriggling through narrow wires), or indeed that it was the mass
movement of electrons themselves, without doing anything dangerous...

Stefek


Please begin reading he http://www.tlc-direct.co.uk/Book/5.1.1.htm and
continue through the menu list until you reach the end. Maybe then some of
what I've been trying to explain will become clearer.

Thanks for the discussion on this though, it has made me mentally revise
many of my very early lessons from many, many years ago. :-)

"tony sayer" wrote in message
news
In article , Stefek Zaba s-nilnews-
writes
Roger wrote:


Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

Exactly so. Congratulations: you've managed to say in 5 lines what
usually takes me several screenfuls - and just took Andy W nearly as
long, but with more cases more thoroughly considered - on the frequent
occasions that this FAQ comes up!

Stefek

Just to chuck my 2 penny worth in Stefek has this right and Roger has
put the same thing the same way.

All you need to remember is the FUNDAMENTAL thing about a RCD is that it
compares the current IN to the current OUT if they are unbalanced by
more that the rated trip current nominally 30 or 100 ma then the device
will trip.

Remember the word !!CURRENT!! flow thats the important part.

It matters not whether the RCD is supplying a single table light or a
shower and cooker on the go together. As long as the CURRENT IN matches
the current OUT then all will be well. Offer the electricity an
alternative path and this balance is upset and off the trip will go.

That is all there is to it.

Let me try to put something else in perspective.

Imagine an RCD connected to the incoming supply. Nothing is connected to
the output of it at all.

Now take a wire thats connected to a good earth like a stake in the
ground outside in very damp soil and after its been raining. Touch this
to the live and the trip will immediately fire off.

Now do the same with the neutral output side of the trip and it won't
trip. So why does this happen?. Remember CURRENT flow. In the first
instance from the live to your earthstick current is flowing from the
incoming live line, through the RCD and then via your stick and then
through the earth back to the substation where one side of the supply
the neutral is connected to earth so you can see that current flows
through the live side of the RCD back through the earth connection to
the supplying transformer. It is NOT going back through the RCD so a
current imbalance has taken place.

The important thing is that there was an !!imbalance!! because more was
going OUT of the RCD than what was coming BACK through it!.

However when you touched the earth stick to the neutral NO CURRENT was
flowing through the live side, so no current was or could have been
flowing back!.

Now suppose you have a one bar electric fire connected to the output
terminals of the RCD. Normally what is going out is going back through
the RCD, so all is well.

Now connect that earth stick to the neutral again and it will trip.
Reason is that the CURRENT has an alternative way back to the earthed
side at the substation, some current is flowing through the RCD back to
the substation , but some is now taking the alternative route home via
the earth stick.

So an imbalance is caused, their being current flowing OUT through the
RCD but it has an alternative route back and if its going back and its
NOT through the RCD so its not the same as what's coming IN, then off
will go the trip.

Simple as that.

So thats why sometimes when you have an Earth to Neutral fault, it
doesn't show up until some "current" is flowing through the RCD through
the alternative path back to the substation, as until their is some
current flowing "through" the RCD, then it won't trip. Course that only
needs to be more then the rated tripping current but as long as thats
exceeded then off it will go, but it CAN'T trip unless there is some
current actually flowing through it in order to exceed this rated trip
imbalance level...

Tony Sayer


Correct !!! But can I ask what makes the current flow through these
conductors back to where ever?

Why didn't the RCD trip when the neutral was touched with the earth stick
whilst no load was connected across it?

Then why did it trip when the earth stick was touched to the phase terminal,
even though there was still no load connected across it?

What makes this all happen?

Is there something somewhere that creates these flowing currents?

If so, what is it?

What are the actual mechanics behind the generation and supply of this
electrickery stuff?

I think I'm meeting my full potential on this one guys. :-) LOL !!!

On Sat, 15 Jan 2005 11:22:16 GMT, "BigWallop"
strung together this:

This would cause a
Residual Current Devices to detect an imbalance between phase and earth and
activate the breaker circuitry.

RCD's don't detect anything between phase and earth. Most don't know
what earth is so can't detect it.

I'll not bother reading the rest as it's probably all based on the
incorrect fundementals again.

snip it all
--

SJW
Please reply to group or use 'usenet' in email subject

On Sat, 15 Jan 2005 11:47:21 GMT, "BigWallop"
strung together this:

So, I'll start again. A neutral / earth short circuit will not cause an RCD
to trip.

It can actually.

An RCD will not trip open with a fault between neutral potential and earth
potential, because both neutral potential and earth potential have exactly
the same potential. An RCD will only trip open circuit if it detects an
imbalance between phase potential and earth potential.

Incorrect.
--

SJW
Please reply to group or use 'usenet' in email subject

On Sat, 15 Jan 2005 14:10:11 GMT, "BigWallop"
strung together this:

So, I'll start again. A neutral / earth short circuit will not cause an
RCD
to trip.

It does, demonstrably, and without any further faults.

No it doesn't. Neutral and earth points are taken from the place at earth
mass,

No, not always.
--

SJW
Please reply to group or use 'usenet' in email subject

So thats why sometimes when you have an Earth to Neutral fault, it
doesn't show up until some "current" is flowing through the RCD through
the alternative path back to the substation, as until their is some
current flowing "through" the RCD, then it won't trip. Course that only
needs to be more then the rated tripping current but as long as thats
exceeded then off it will go, but it CAN'T trip unless there is some
current actually flowing through it in order to exceed this rated trip
imbalance level...

Tony Sayer


Correct !!! But can I ask what makes the current flow through these
conductors back to where ever?

Why didn't the RCD trip when the neutral was touched with the earth stick
whilst no load was connected across it?

Then why did it trip when the earth stick was touched to the phase terminal,
even though there was still no load connected across it?

What makes this all happen?

Is there something somewhere that creates these flowing currents?

If so, what is it?


Its because the supply transformer has one side connected to earth which
forms the neutral line..

What are the actual mechanics behind the generation and supply of this
electrickery stuff?

I think I'm meeting my full potential on this one guys. :-) LOL !!!


What you need to consider is that the mains is tied to EARTH when it
comes out of the substation transformer. There is a drawing of this on
the TLC website.

Consider the circuit as follows. Say we have a mains transformer up a
power pole say 200 metres away from the subscribers premises. Now trace
the circuit from lets say the live side of that supplier transformer.
It then goes along the live line to the supply fuse through the meter
then through the RCD then through the fuse board isolator switches etc
then if the "earth stick" was connected to the live output side of the
RCD the circuit would continue back through earth to the earth plate at
the substation so there would be a "load" placed across the supplying
transformer, and hence current would be flowing through the live
conductor of the RCD but not back through the neutral side of the RCD so
the RCD will now be unbalanced and thus trip.

Now consider the same circuit from the earthed side of the supply
transformer trace it along the subs premises through the isolator switch
and then out through the neutral side of the RCD. Now consider the same
earth stick connected this time to the Neutral conductor this will
provide a path through earth back to the supplying transformer.

No current will flow and you should be able to see that no current will
flow 'cos we've "connected" one side of the supply transformer back to
itself so no current is flowing save a "very" small amount of induced
leakage!..

The problem arises with the neutral short to earth when the current that
comes in on the live line now has another path back to the earth plate
at the supplying transformer as well as the one, and only one, it should
have through the neutral side of the RCD.

So suppose we have 10 amps flowing through the live side of the RCD we
don't have the full 10 amps flowing back through the neutral as some of
its being diverted back through the neutral-to-earth short of the
consumer side, or protected side of the RCD, so as long as that current
is greater than the 30 milliamps difference that the RCD is set to, then
off goes the RCD.

--
Tony Sayer

tony sayer wrote:

Now do the same with the neutral output side of the trip and it won't
trip. So why does this happen?. Remember CURRENT flow. In the first

Errr, in fact it might. There is a very good chance that a potential
difference will exist between your earth stake and the neutral connected
to the incoming side of the RCD. Hence current can flow.

instance from the live to your earthstick current is flowing from the
incoming live line, through the RCD and then via your stick and then
through the earth back to the substation where one side of the supply
the neutral is connected to earth so you can see that current flows
through the live side of the RCD back through the earth connection to
the supplying transformer. It is NOT going back through the RCD so a
current imbalance has taken place.

This is true.

The important thing is that there was an !!imbalance!! because more was
going OUT of the RCD than what was coming BACK through it!.

However when you touched the earth stick to the neutral NO CURRENT was
flowing through the live side, so no current was or could have been
flowing back!.

No, there could be current flowing back - being sourced from your earth
stake.

Now suppose you have a one bar electric fire connected to the output
terminals of the RCD. Normally what is going out is going back through
the RCD, so all is well.

Now connect that earth stick to the neutral again and it will trip.
Reason is that the CURRENT has an alternative way back to the earthed
side at the substation, some current is flowing through the RCD back to
the substation , but some is now taking the alternative route home via
the earth stick.

true...

So an imbalance is caused, their being current flowing OUT through the
RCD but it has an alternative route back and if its going back and its
NOT through the RCD so its not the same as what's coming IN, then off
will go the trip.

Simple as that.

Yup, so long as you don't fall for the BigWallop assumption that E and N
are going to be at the same potential. The only setup where that is
likely is on a PME system, and then only where there are no circuits
turned on at all.


--
Cheers,

John.

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| John Rumm - john(at)internode(dot)co(dot)uk |
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In article , BigWallop
writes

Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

You don't have to. Wire a plug with two wires, one to neutral and the
other to earth. Plug it into a socket and touch the two wires together.
Your RCD will trip; if it doesn't, it's faulty.

--
..sigmonster on vacation

In article , BigWallop
writes

So, I'll start again.

Don't bother. *plonk*

--
..sigmonster on vacation

In article , Mike Tomlinson
writes
In article , BigWallop
writes

Well, I'm sorry too, because I can't find anything, anywhere, that says a
standard RCD in a normal domestic installation will trip the supply unless
there is a fault from Phase to Earth.

You don't have to. Wire a plug with two wires, one to neutral and the
other to earth. Plug it into a socket and touch the two wires together.
Your RCD will trip; if it doesn't, it's faulty.

Not necessarily.

Only if theres enough current flowing through the RCD.

Refer to the other post I made explaining this please.


--
Tony Sayer

O
So, to sum up, I still say an RCD will not trip open unless the fault
current it detects is between a phase potential and earth mass.

Boy, can I go on a bit when I get started. :-) LOL !!!

I must have a faulty one on my bench at work then, always trips on neutral
to eath fault.

Dave

--
For what we are about to balls up may common sense prevent us doing it
again
in the future!!

In article , John
Rumm writes
tony sayer wrote:

Now do the same with the neutral output side of the trip and it won't
trip. So why does this happen?. Remember CURRENT flow. In the first

Errr, in fact it might. There is a very good chance that a potential
difference will exist between your earth stake and the neutral connected
to the incoming side of the RCD. Hence current can flow.

Yes you could argue this, but I didn't want to cause any more confusion
more than what's perhaps been generated already

In fact if there was a heavy earth current into the earth locally than
that could be sufficient to cause upset, but its rather unlikely.

In fact I have to alter a circuit thats fed by around 900 metres of
overhead low voltage sometime this week I'll see what's induced there.



Snipppppppppppp

Yup, so long as you don't fall for the BigWallop assumption that E and N
are going to be at the same potential. The only setup where that is
likely is on a PME system, and then only where there are no circuits
turned on at all.

No of course if your e and n are "connected" on your premises this loop
is very short so no real chance of anything there at all!.

In fact years ago we used to use the earth to carry audio over from mine
to a friends house with two stakes set someway apart. Its just a
collection of an infinite number of resistor's really)

Surprising what audio you could get through, but there wasn't that much
hum!...

--
Tony Sayer

Bancom Communications Ltd U.K. Tel+44 1223 566577 Fax+44 1223 566588

P.O. Box 280, Cambridge, England, CB2 2DY E-Mail