I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but
it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question
is the kitchen ring)?

Andy Burns wrote:
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but
it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question
is the kitchen ring)?

Not an uncommon result if you are very close to the tripping point in
total leakage, or the trip is being caused by something intermittent.

Finding the cause will take some detective work:

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


--
Cheers,

John.

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

Andy Burns formulated on Sunday :
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on the
RCD protected half of the consumer unit, and tried to reset the MCB, the
first two made no difference (I switched them each back on before trying the
next) when I got to the third MCB, the RCD did reset, so I switched the third
MCB back on expecting it to trip the RCD again, but it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question is
the kitchen ring)?

RCD's can trip simply due to various filters on the input to equipment
each having their own contribution to the overall leakage. Microwaves
have them, as do PC's, some washing machines and etc.. Added together
these small leakages can be enough to cause the trip.

An alternative problem might be leakage from a cooker element or
similar.

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

Harry Bloomfield wrote:

RCD's can trip simply due to various filters on the input to equipment
each having their own contribution to the overall leakage. Microwaves
have them, as do PC's, some washing machines and etc.. Added together
these small leakages can be enough to cause the trip.

Thanks, I know about the filters each adding a small amount of leakage;
on the kitchen circuit are the washer/drier, the gas hob (for the
igniter and presumably the flame failure device) the microwave,
extractor hood, breadmaker and toaster, also an unused socket in the
garage.

Fridge/freezer, underfloor heating and boiler are on a different circuit.

An alternative problem might be leakage from a cooker element or similar.

Oven is on a different circuit with the kettle on the 13A socket of the
45A cooker switch.

But my question was really why turning the relevant MCB off/on allowed
the RCD to reset? I mean the circuit in question (along with all the
other RCB protected ones) was "off" due to the RCD tripping.

"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but it
didn't.

Can anyone suggest a reason for that behaviour (the circuit in question is
the kitchen ring)?

probably the toaster

Andy Burns has brought this to us :
But my question was really why turning the relevant MCB off/on allowed the
RCD to reset? I mean the circuit in question (along with all the other RCB
protected ones) was "off" due to the RCD tripping.

There could be many reasons...

Slight changes in temperature making the RCD less sensitive or the
leakage less as the filter cools slightly. Possible moisture which
evaporated as you were messing with the RCD / MCB's.

All you can really do is wait and see if it happens again and take note
of what fixes the problem, until you can pin it down to a single MCB
circuit - then progress it to an actual appliance causing the trip.

These intermitant things can be notoriously difficult to pin down and
the best person to pin it down is the one reseting it, by applying
logic to the problem.

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

Ian wrote:
"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but it
didn't.

Can anyone suggest a reason for that behaviour (the circuit in question is
the kitchen ring)?

probably the toaster

One of the less likely suspects I would have thought. Things with
mineral insulated immersed elements are more likely to cause problems.

--
Cheers,

John.

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

Andy Burns wrote:
Harry Bloomfield wrote:

RCD's can trip simply due to various filters on the input to equipment
each having their own contribution to the overall leakage. Microwaves
have them, as do PC's, some washing machines and etc.. Added together
these small leakages can be enough to cause the trip.

Thanks, I know about the filters each adding a small amount of leakage;
on the kitchen circuit are the washer/drier, the gas hob (for the
igniter and presumably the flame failure device) the microwave,
extractor hood, breadmaker and toaster, also an unused socket in the
garage.

Cooker, and socket in the garage would be worth checking.

Fridge/freezer, underfloor heating and boiler are on a different circuit.

Different or no RCD I take it?

An alternative problem might be leakage from a cooker element or similar.

Oven is on a different circuit with the kettle on the 13A socket of the
45A cooker switch.

But my question was really why turning the relevant MCB off/on allowed
the RCD to reset? I mean the circuit in question (along with all the
other RCB protected ones) was "off" due to the RCD tripping.

Well its not off once you reset (or try to) the RCD. Note also that
something like a neutral earth short can be enough to trip a RCD. You
will probably only then see the problem under high load conditions - and
the load in question can include that of your neighbours as well as yours.

(any circumstance that tends to pull the neutral away from earth
potential, can allow a neutral to earth current flow. More likely to be
an issue on TN-S installs)


--
Cheers,

John.

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

John Rumm wrote:

Cooker, and socket in the garage would be worth checking.

My initial though was that it did rain overnight, not that the garage
leaks but just that it is more damp than it has been for a while, it has
only tripped once before like this, about 18 months ago, and I *think*
it was also damp then ...

Fridge/freezer, underfloor heating and boiler are on a different circuit.

Different or no RCD I take it?

At the moment they are on the same RCD. They were deliberately not put
onto the newer kitchen/garage ring circuit, the intention being to
eventually move the fridge/freezer and boiler from the main house ring
circuit to a non-RCD radial circuit.

Well its not off once you reset (or try to) the RCD. Note also that
something like a neutral earth short can be enough to trip a RCD. You
will probably only then see the problem under high load conditions - and
the load in question can include that of your neighbours as well as yours.

Certainly nothing connected to the kitchen ring (whose MCB seemed to
clear it) was taking any significant current when the RCD tripped, just
appliances left on standby.

(any circumstance that tends to pull the neutral away from earth
potential, can allow a neutral to earth current flow. More likely to be
an issue on TN-S installs)

TN-C-S installation.

John Rumm wrote:
Ian wrote:
"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under
the stairs, the RCD wouldn't reset, so one by one I switched off the
MCBs on the RCD protected half of the consumer unit, and tried to
reset the MCB, the first two made no difference (I switched them each
back on before trying the next) when I got to the third MCB, the RCD
did reset, so I switched the third MCB back on expecting it to trip
the RCD again, but it didn't.

Can anyone suggest a reason for that behaviour (the circuit in
question is the kitchen ring)?

probably the toaster

One of the less likely suspects I would have thought. Things with
mineral insulated immersed elements are more likely to cause problems.

My brother used to live in the middle of nowhere with overhead supply.
If there was a storm in the area it would trip their RCD out if the
kettle was plugged in - didn't have to be on. kids putting 'bent' bread
that touches the elements in your toaster will do it as well. Happy
hunting for the source.

Chewbacca wrote:
John Rumm wrote:
Ian wrote:
"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under
the stairs, the RCD wouldn't reset, so one by one I switched off the
MCBs on the RCD protected half of the consumer unit, and tried to
reset the MCB, the first two made no difference (I switched them each
back on before trying the next) when I got to the third MCB, the RCD
did reset, so I switched the third MCB back on expecting it to trip
the RCD again, but it didn't.

Can anyone suggest a reason for that behaviour (the circuit in
question is the kitchen ring)?
probably the toaster
One of the less likely suspects I would have thought. Things with
mineral insulated immersed elements are more likely to cause problems.

My brother used to live in the middle of nowhere with overhead supply.
If there was a storm in the area it would trip their RCD out if the
kettle was plugged in - didn't have to be on. kids putting 'bent' bread
that touches the elements in your toaster will do it as well. Happy
hunting for the source.
as will kids burning silver foil on the exposed elements of old
fashioned bar fires, I know I was that child :-)
never diiiiiiid mmmmme annny harrrmmmmmmmmmm
Might have stopped me growing old gracefully possibly yea ha

--
Kevin R
Reply address works

Andy Burns wrote:
John Rumm wrote:

Cooker, and socket in the garage would be worth checking.

My initial though was that it did rain overnight, not that the garage
leaks but just that it is more damp than it has been for a while, it has
only tripped once before like this, about 18 months ago, and I *think*
it was also damp then ...

Fridge/freezer, underfloor heating and boiler are on a different
circuit.

Different or no RCD I take it?

At the moment they are on the same RCD. They were deliberately not put
onto the newer kitchen/garage ring circuit, the intention being to
eventually move the fridge/freezer and boiler from the main house ring
circuit to a non-RCD radial circuit.

Boiler would be another place water an electricity are in close proximity.



--
Cheers,

John.

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

Chewbacca wrote:
John Rumm wrote:
Ian wrote:
"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under
the stairs, the RCD wouldn't reset, so one by one I switched off the
MCBs on the RCD protected half of the consumer unit, and tried to
reset the MCB, the first two made no difference (I switched them each
back on before trying the next) when I got to the third MCB, the RCD
did reset, so I switched the third MCB back on expecting it to trip
the RCD again, but it didn't.

Can anyone suggest a reason for that behaviour (the circuit in
question is the kitchen ring)?
probably the toaster
One of the less likely suspects I would have thought. Things with
mineral insulated immersed elements are more likely to cause problems.

My brother used to live in the middle of nowhere with overhead supply.
If there was a storm in the area it would trip their RCD out if the
kettle was plugged in - didn't have to be on. kids putting 'bent' bread

Trips from power transients (lightening, bulbs blowing etc) tend to
indicate a sensitized RCD - i.e. one where the cumulative leakage is
getting close to the tripping point.

that touches the elements in your toaster will do it as well. Happy
hunting for the source.

Yup, the moisture in the bread may be enough to conduct a few mA.


--
Cheers,

John.

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

My initial though was that it did rain overnight... it has
only tripped once before like this, about 18 months ago, and I *think*
it was also damp then ...

Assuming you have cross bonding to water/gas/oil pipes, then saturated
ground might lower the earth impedance/potential slightly relative to
neutral.

Any appliance that does have some earth leakage, may pass slightly
more current to earth.

wrote:

Assuming you have cross bonding to water/gas/oil pipes,

I do now. Having lived here for 18 years, when doing up the kitchen I
discovered someone had chopped out the bonding to the water pipe while
using a disc-cutter to make the opening in the wall!

wrote:
My initial though was that it did rain overnight... it has
only tripped once before like this, about 18 months ago, and I *think*
it was also damp then ...

Assuming you have cross bonding to water/gas/oil pipes, then saturated
ground might lower the earth impedance/potential slightly relative to
neutral.

Should not make much difference on a TN-C-S setup.

--
Cheers,

John.

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

In article , Andy Burns
scribeth thus
Harry Bloomfield wrote:

RCD's can trip simply due to various filters on the input to equipment
each having their own contribution to the overall leakage. Microwaves
have them, as do PC's, some washing machines and etc.. Added together
these small leakages can be enough to cause the trip.

Thanks, I know about the filters each adding a small amount of leakage;
on the kitchen circuit are the washer/drier, the gas hob (for the
igniter and presumably the flame failure device) the microwave,
extractor hood, breadmaker and toaster, also an unused socket in the
garage.

Fridge/freezer, underfloor heating and boiler are on a different circuit.

An alternative problem might be leakage from a cooker element or similar.

Oven is on a different circuit with the kettle on the 13A socket of the
45A cooker switch.

But my question was really why turning the relevant MCB off/on allowed
the RCD to reset? I mean the circuit in question (along with all the
other RCB protected ones) was "off" due to the RCD tripping.



What can cause a lot of head and arse scratching is when there is a live
to neutral short which only shows up when there is sufficient power
flowing to raise the neutral above the trip threshold whereas you would
normally be looking for a Live to Earth fault;!...
--
Tony Sayer

Andy Burns wrote:
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but
it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question
is the kitchen ring)?


I get episodes of this.

I have never tracked it down. It seems I have a high static leakage due
to probably a load of electronic filters everywhere. Once the trip has
gone, its remarkably reluctant to be reset under load.

I THINK, but cannot be sure, that the mechanism is more or less that the
total wiring and RFI filters in house represents a fair bit of
capacitance between live and earth. This is not in itself a problem, BUT
if there is any voltage transient on the house - or indeed incoming -
mains., its enough to push the setup over the edge. When I lost mains
supply for a week and EDF put a generator outside, it was markedly
worse, with the most apparently offensive ring being the one that runs
all the computer equipment.

So anything hat represents a bloody great spike on the mains, will trip
it. Yea even unto simply switching the house on.

Its bloody irritating, and one day I will get a clamp meter and measure
all the earth currents and see if there is a prime offender. Or fit
RCBO's everywhere.and get rid of the whole house trip altogether.
..
To fully load a 30mA trip at 250v is about 8K ohms. I make that about
380nF at 50hz.

I have not been able to identify what most phase-to-earth capacitors in
RFI filters applied to domestic equipment is, but 10nF would seem to be
what I would expect. That puts 40 such units as well able to trip a 30mA
trip. I am certain I have more. Which is why I am on a 100mA trip and it
still goes sometimes on surges.

In addition, inter-conductor capacitance on T & E is about 100pF/meter
allegedly, so a couple of hundred neters will get a further 20nF on that
load.

Add in CFL bulbs with RFI filters..and the whole way in which houses are
protected starts to look incredibly shaky.

I suspect that in due course the regulations will catch up, and RCBO's
bet the way to go on a per circuit basis.

John Rumm wrote:
Chewbacca wrote:
John Rumm wrote:
Ian wrote:
"Andy Burns" wrote in message
et...
I had the RCD trip out this afternoon, so went to investigate under
the stairs, the RCD wouldn't reset, so one by one I switched off the
MCBs on the RCD protected half of the consumer unit, and tried to
reset the MCB, the first two made no difference (I switched them each
back on before trying the next) when I got to the third MCB, the RCD
did reset, so I switched the third MCB back on expecting it to trip
the RCD again, but it didn't.

Can anyone suggest a reason for that behaviour (the circuit in
question is the kitchen ring)?
probably the toaster
One of the less likely suspects I would have thought. Things with
mineral insulated immersed elements are more likely to cause problems.

My brother used to live in the middle of nowhere with overhead supply.
If there was a storm in the area it would trip their RCD out if the
kettle was plugged in - didn't have to be on. kids putting 'bent' bread

Trips from power transients (lightening, bulbs blowing etc) tend to
indicate a sensitized RCD - i.e. one where the cumulative leakage is
getting close to the tripping point.

that touches the elements in your toaster will do it as well. Happy
hunting for the source.

Yup, the moisture in the bread may be enough to conduct a few mA.


FWIW I calculate 8.3Kohms or 380 NF between live and earth to be enough
to trip a 30mA trip.

The former is unlikely in the absence of some pretty drastic insulation
breakdown. The latter is all too easy to arrange, and as I said in the
previous post, thats at 50Hz. Switching a switch ON at anything other
than a zero crossing point of the mains can be infinitely worse.

When my board trips, I nearly always have to flip every circuit off and
bring them up one by one after resetting the RCD.: A pretty clear
indication that my problem is capacitative and not resistive.


I was thinking of gong masiveley RCBO with a 180mA main RCD..but how do
you isolate a board to refit a main switch?

Should not make much difference on a TN-C-S setup.

I'm not an expert in this - and I'd agree if all cross bonding was
brought back to a mecca close to the earth/neutral bond at the cut-out
- it would lower the impedance of both neutral an earth near equally.

However if a wiring earth is bonded close to a fixed appliance (e.g.
from an appliance chassis to a water pipe) and away from the mecca -
wouldn't that lower the impedance of earth to possibly a significantly
greater extent than neutral?

In article , tony sayer
scribeth thus
In article , Andy Burns
scribeth thus
Harry Bloomfield wrote:

RCD's can trip simply due to various filters on the input to equipment
each having their own contribution to the overall leakage. Microwaves
have them, as do PC's, some washing machines and etc.. Added together
these small leakages can be enough to cause the trip.

Thanks, I know about the filters each adding a small amount of leakage;
on the kitchen circuit are the washer/drier, the gas hob (for the
igniter and presumably the flame failure device) the microwave,
extractor hood, breadmaker and toaster, also an unused socket in the
garage.

Fridge/freezer, underfloor heating and boiler are on a different circuit.

An alternative problem might be leakage from a cooker element or similar.

Oven is on a different circuit with the kettle on the 13A socket of the
45A cooker switch.

But my question was really why turning the relevant MCB off/on allowed
the RCD to reset? I mean the circuit in question (along with all the
other RCB protected ones) was "off" due to the RCD tripping.



What can cause a lot of head and arse scratching is when there is a live
to neutral short

Correction!!!

Earth to Neutral short!!!!


which only shows up when there is sufficient power
flowing to raise the neutral above the trip threshold whereas you would
normally be looking for a Live to Earth fault;!...



--
Tony Sayer

In article , The Natural
Philosopher scribeth thus
Andy Burns wrote:
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but
it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question
is the kitchen ring)?


I get episodes of this.

I have never tracked it down. It seems I have a high static leakage due
to probably a load of electronic filters everywhere. Once the trip has
gone, its remarkably reluctant to be reset under load.

I THINK, but cannot be sure, that the mechanism is more or less that the
total wiring and RFI filters in house represents a fair bit of
capacitance between live and earth. This is not in itself a problem, BUT
if there is any voltage transient on the house - or indeed incoming -
mains., its enough to push the setup over the edge. When I lost mains
supply for a week and EDF put a generator outside, it was markedly
worse, with the most apparently offensive ring being the one that runs
all the computer equipment.

So anything hat represents a bloody great spike on the mains, will trip
it. Yea even unto simply switching the house on.

Its bloody irritating, and one day I will get a clamp meter and measure
all the earth currents and see if there is a prime offender. Or fit
RCBO's everywhere.and get rid of the whole house trip altogether.
.
To fully load a 30mA trip at 250v is about 8K ohms. I make that about
380nF at 50hz.

I have not been able to identify what most phase-to-earth capacitors in
RFI filters applied to domestic equipment is, but 10nF would seem to be
what I would expect. That puts 40 such units as well able to trip a 30mA
trip. I am certain I have more. Which is why I am on a 100mA trip and it
still goes sometimes on surges.

In addition, inter-conductor capacitance on T & E is about 100pF/meter
allegedly, so a couple of hundred neters will get a further 20nF on that
load.

Add in CFL bulbs with RFI filters..

Yep but their phase to neutral not Earth?..

and the whole way in which houses are
protected starts to look incredibly shaky.

I suspect that in due course the regulations will catch up, and RCBO's
bet the way to go on a per circuit basis.



Just how much electronic stuff are you running on the one 30 ma trip
NP?...
--
Tony Sayer

tony sayer wrote:
In article , The Natural
Philosopher scribeth thus
Andy Burns wrote:
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on
the RCD protected half of the consumer unit, and tried to reset the MCB,
the first two made no difference (I switched them each back on before
trying the next) when I got to the third MCB, the RCD did reset, so I
switched the third MCB back on expecting it to trip the RCD again, but
it didn't.

Can anyone suggest a reason for that behaviour (the circuit in question
is the kitchen ring)?

I get episodes of this.

I have never tracked it down. It seems I have a high static leakage due
to probably a load of electronic filters everywhere. Once the trip has
gone, its remarkably reluctant to be reset under load.

I THINK, but cannot be sure, that the mechanism is more or less that the
total wiring and RFI filters in house represents a fair bit of
capacitance between live and earth. This is not in itself a problem, BUT
if there is any voltage transient on the house - or indeed incoming -
mains., its enough to push the setup over the edge. When I lost mains
supply for a week and EDF put a generator outside, it was markedly
worse, with the most apparently offensive ring being the one that runs
all the computer equipment.

So anything hat represents a bloody great spike on the mains, will trip
it. Yea even unto simply switching the house on.

Its bloody irritating, and one day I will get a clamp meter and measure
all the earth currents and see if there is a prime offender. Or fit
RCBO's everywhere.and get rid of the whole house trip altogether.
.
To fully load a 30mA trip at 250v is about 8K ohms. I make that about
380nF at 50hz.

I have not been able to identify what most phase-to-earth capacitors in
RFI filters applied to domestic equipment is, but 10nF would seem to be
what I would expect. That puts 40 such units as well able to trip a 30mA
trip. I am certain I have more. Which is why I am on a 100mA trip and it
still goes sometimes on surges.

In addition, inter-conductor capacitance on T & E is about 100pF/meter
allegedly, so a couple of hundred neters will get a further 20nF on that
load.

Add in CFL bulbs with RFI filters..

Yep but their phase to neutral not Earth?..

and the whole way in which houses are
protected starts to look incredibly shaky.

I suspect that in due course the regulations will catch up, and RCBO's
bet the way to go on a per circuit basis.



Just how much electronic stuff are you running on the one 30 ma trip
NP?...
Its 100mA now.

2 Mac G4's
2 PCs.
2 powered speakers for 2 machines.
PABX
laser printer
DSL router
Ethernet switch
TV distribution amp.
3 set top boxes.
3 TVS on standby. Possibly 2 more not.
One Hifi setup often on standby.
Scanner
Print server box.
two home chargers and camera battery charger.
three radios.
Often a 12v bench PSU. nd two trickle chargers.
Radio thermostat.
Oil level monitor.
The odd other wall wart for something obscure like recharageable torches..


Then add in a fridge, two fridge freezers and a chest freezer..
plus an oil boiler, autocycling water softener, and dishwasher and
washing machine..

Nightmare innit?

Andy Burns formulated on Sunday :
I had the RCD trip out this afternoon, so went to investigate under the
stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on the
RCD protected half of the consumer unit, and tried to reset the MCB, the
first two made no difference (I switched them each back on before trying the
next) when I got to the third MCB, the RCD did reset, so I switched the third
MCB back on expecting it to trip the RCD again, but it didn't.

I don't think anyone has given Andy an explanation of just how RCD's
work....

Basically there are two current sensing coils, one sensing the current
passing through the neutral and the other the current passing through
the live conductor. The two are compared for any difference (high or
low) in the two current levels and any difference indicates a leakage.
If the leakage is assumed to be more than a preset level, then it trips
out. Any small amount of leakage (30mA?) from live to earth or from
neutral to earth, is enough to cause the trip.

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

Harry Bloomfield wrote:
Andy Burns formulated on Sunday :
I had the RCD trip out this afternoon, so went to investigate under
the stairs, the RCD wouldn't reset, so one by one I switched off the
MCBs on the RCD protected half of the consumer unit, and tried to
reset the MCB, the first two made no difference (I switched them
each back on before trying the next) when I got to the third MCB, the
RCD did reset, so I switched the third MCB back on expecting it to
trip the RCD again, but it didn't.

I don't think anyone has given Andy an explanation of just how RCD's
work....

Basically there are two current sensing coils, one sensing the current
passing through the neutral and the other the current passing through
the live conductor. The two are compared for any difference (high or
low) in the two current levels and any difference indicates a leakage.
If the leakage is assumed to be more than a preset level, then it trips
out. Any small amount of leakage (30mA?) from live to earth or from
neutral to earth, is enough to cause the trip.

nearly right there are two coils in most units but they are wound in
opposition through a transformer and the current flowing through one
cancels out the other, no need for fancy comparison circuits
see
http://www.powercords.co.uk/howrcd.htm

--
Kevin R
Reply address works

Kevin has brought this to us :
nearly right there are two coils in most units but they are wound in
opposition through a transformer and the current flowing through one cancels
out the other, no need for fancy comparison circuits
see
http://www.powercords.co.uk/howrcd.htm

I was trying explain the principle as simply as possible.

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

wrote:

if a wiring earth is bonded close to a fixed appliance (e.g.
from an appliance chassis to a water pipe) and away from the mecca -
wouldn't that lower the impedance of earth to possibly a significantly
greater extent than neutral?

The bonding to gas is close to the CU, both of which are just through
the outside wall to the electricity meter and main earth point. When I
discovered that the bonding to the water had been severed, I had to take
a longer route to reinstate it (about 10m of 16mm^2) and the "far" end
of that is close to the kitchen ring which is the suspect.

Harry Bloomfield wrote:

I don't think anyone has given Andy an explanation of just how RCD's
work....

Thanks, I know more or less how they work; I'm not puzzled at how/why it
tripped, if the fault persisted I'd expect the RCD to refuse to reset,
but I am puzzled at why turning off/on the particular MCB then allowed
the RCD to reset?

Add in CFL bulbs with RFI filters..

Yep but their phase to neutral not Earth?..

and the whole way in which houses are
protected starts to look incredibly shaky.

I suspect that in due course the regulations will catch up, and RCBO's
bet the way to go on a per circuit basis.



Just how much electronic stuff are you running on the one 30 ma trip
NP?...
Its 100mA now.

2 Mac G4's
2 PCs.
2 powered speakers for 2 machines.
PABX
laser printer
DSL router
Ethernet switch
TV distribution amp.
3 set top boxes.
3 TVS on standby. Possibly 2 more not.
One Hifi setup often on standby.
Scanner
Print server box.
two home chargers and camera battery charger.
three radios.
Often a 12v bench PSU. nd two trickle chargers.
Radio thermostat.
Oil level monitor.
The odd other wall wart for something obscure like recharageable torches..


Some of that won't have caps to earth, across the line but not earth..


Then add in a fridge, two fridge freezers and a chest freezer..
plus an oil boiler, autocycling water softener, and dishwasher and
washing machine..

Nightmare innit?





Not much different to what we run here and trips are very, very, rare
perhaps once every two years or so and thats on a 30 ma...

We've got some comms sites with a lorra filtering all on 30 ma trips
perhaps now and again in a direct or near miss lightning strike;!..








--
Tony Sayer

Andy Burns wrote:
Harry Bloomfield wrote:

I don't think anyone has given Andy an explanation of just how RCD's
work....

Thanks, I know more or less how they work; I'm not puzzled at how/why it
tripped, if the fault persisted I'd expect the RCD to refuse to reset,
but I am puzzled at why turning off/on the particular MCB then allowed
the RCD to reset?



Cos you prolly had a BIG capacitative live-to-earth load on that one.

The Natural Philosopher wrote:

FWIW I calculate 8.3Kohms or 380 NF between live and earth to be enough
to trip a 30mA trip.

The former is unlikely in the absence of some pretty drastic insulation
breakdown. The latter is all too easy to arrange, and as I said in the
previous post, thats at 50Hz. Switching a switch ON at anything other
than a zero crossing point of the mains can be infinitely worse.

The RCDs (these days anyway) usually have a certain amount of filtering
to reduce the effect of capacitive coupling of transient related noise
and harmonics. IME you don't usually get trips on transients unless the
device is already sensitised to some extent.

I was thinking of gong masiveley RCBO with a 180mA main RCD..but how do
you isolate a board to refit a main switch?

Pull the main fuse...

FWIW, when I moved in here at the start of the year, there were 10
circuits all sharing a (oldish) 30mA RCD (including feeds to outside
sockets and outbuildings etc). We had a couple of trips out of the blue,
and then quite a number on transients (every thunderstorm, bulbs blowing
on so on) would also cause trips. So I ripped the lot and did a 16th
edition style TT setup, with 100mA time delayed RCDs in two split load
CUs (one for indoor and one for exterior / outbuilding circuits[1]).
With the 30mA RCD only covering the 3 main power circuits. Not had a
single RCD related problem since.


[1] That's the theory, however I still have to disentangle the outside
sockets and fish pond pump from the upstairs ring final circuit!

--

Cheers,

John.

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

wrote:
Should not make much difference on a TN-C-S setup.

I'm not an expert in this - and I'd agree if all cross bonding was
brought back to a mecca close to the earth/neutral bond at the cut-out
- it would lower the impedance of both neutral an earth near equally.

However if a wiring earth is bonded close to a fixed appliance (e.g.
from an appliance chassis to a water pipe) and away from the mecca -
wouldn't that lower the impedance of earth to possibly a significantly
greater extent than neutral?

Impedance to earth from where exactly? Not sure I follow.

The thing to bear in mind here is that the potential of the neutral wrt
to earth is only of relevance in the presence of a wiring fault (i.e.
neutral to earth short). The reason these are more "interesting" than
line to earth shorts is that they can go unnoticed some of the time.


--
Cheers,

John.

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

Harry Bloomfield wrote:
Kevin has brought this to us :
nearly right there are two coils in most units but they are wound in
opposition through a transformer and the current flowing through one
cancels out the other, no need for fancy comparison circuits
see
http://www.powercords.co.uk/howrcd.htm

I was trying explain the principle as simply as possible.

simplifying by inventing something thats not in it ????

--
Kevin R
Reply address works