On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:
On 16/12/2018 20:57, Roger Hayter wrote:
John Rumm wrote:
On 16/12/2018 15:36, Roger Hayter wrote:

This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

From the picture he has a type AC RCD, and they are insensitive to
unipolar currents. So switching to any of the other types would make
that kind of trip more likely.

Ok, sorry, I got type A and type AC the wrong way round. I'll go with
everyone else that it is a defective component, very probably a heater
element, especially given that it happens on switching off.

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.

Andy Burns wrote:

Turn off all the other MCBs on the left side of the CU

leaving the 'oven' MCB switched on

then see if
turning the oven on/off still reliably trips the left RCD

wrote:

On Sunday, 16 December 2018 20:14:00 UTC, Roger Hayter wrote:
tabbypurr wrote:
On Sunday, 16 December 2018 15:36:20 UTC, Roger Hayter wrote:

Older ovens used to have lazily designed electronics power supplies
using a dropper capacitor from the mains. If this is the case, on
switching off a transient from discharging the DC supply at the bottom
of the capacitor to earth could pass through the capacitor via the (now
disconnected) live circuitry of the oven which may be fairly low
resistance to neutral, and via the intact neutral oven connection
through the RCD to incoming neutral. This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

An annoying workaround would be to always turn the oven off at the
isolating switch on the wall.

Not true of course.

It does assume the oven has a mains switch in the live supply which,
come to think of it, my last two cookers didn't have. If it does, it
*could* be true.

It can't possibly be true. Capacitor power supplies don't run live to
earth, but live to neutral. And no new appliance passes enough i to E to
trip an RCD.

I find that hard to believe. That would be going back to the
arrangements for live chassis TVs of fifty years ago. I am suspect that
the ground reference for the electronics must be earth. I am prepared
to be proved wrong, but certainly the only oven I have ever worked on
the electronics was earth (and indeed chassis) referenced.

And the size of the dropper capacitor does not limit the peak current,
only the total charge.


And there's nothing lazy about capacitor PSUs.


NT


--

Roger Hayter

On 17/12/2018 13:00, Roger Hayter wrote:
wrote:

On Sunday, 16 December 2018 20:14:00 UTC, Roger Hayter wrote:
tabbypurr wrote:
On Sunday, 16 December 2018 15:36:20 UTC, Roger Hayter wrote:

Older ovens used to have lazily designed electronics power
supplies using a dropper capacitor from the mains. If this
is the case, on switching off a transient from discharging
the DC supply at the bottom of the capacitor to earth could
pass through the capacitor via the (now disconnected) live
circuitry of the oven which may be fairly low resistance to
neutral, and via the intact neutral oven connection through
the RCD to incoming neutral. This probably should not be
enough to trip if the RCD is not already carrying a leakage
current, but would be a design fault. It is true that an
RCD insensitive to unipolar currents (?? type A) would
prevent this tripping, but you shouldn't have to change it!

An annoying workaround would be to always turn the oven off
at the isolating switch on the wall.

Not true of course.

It does assume the oven has a mains switch in the live supply
which, come to think of it, my last two cookers didn't have. If
it does, it *could* be true.

It can't possibly be true. Capacitor power supplies don't run live
to earth, but live to neutral. And no new appliance passes enough i to E to trip an RCD.

I find that hard to believe. That would be going back to the
arrangements for live chassis TVs of fifty years ago. I am suspect
that the ground reference for the electronics must be earth. I am
prepared to be proved wrong, but certainly the only oven I have ever
worked on the electronics was earth (and indeed chassis) referenced.

On class 1 devices, mains RFI input filters are typically so called 5
terminal devices, so live and neutral in and out plus an earth
connection. The earth connection normally acting as a centre point
between a pair of caps[1].

So they leak a very small amount at 50Hz ( 1mA), but that can rise at
higher frequencies. So a switching transient which has more HF content
may cause a brief spike in earth leakage.

Also worth noting that the filter does not actually need to be present
in the device being switched to cause a RCD trip - elsewhere on the
circuit in another appliance, can be enough.


[1] See the circus diagram:

http://www.farnell.com/datasheets/2340758.pdf


--
Cheers,

John.

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

On Monday, 17 December 2018 13:00:15 UTC, Roger Hayter wrote:
tabbypurr wrote:
On Sunday, 16 December 2018 20:14:00 UTC, Roger Hayter wrote:
tabbypurr wrote:
On Sunday, 16 December 2018 15:36:20 UTC, Roger Hayter wrote:

Older ovens used to have lazily designed electronics power supplies
using a dropper capacitor from the mains. If this is the case, on
switching off a transient from discharging the DC supply at the bottom
of the capacitor to earth could pass through the capacitor via the (now
disconnected) live circuitry of the oven which may be fairly low
resistance to neutral, and via the intact neutral oven connection
through the RCD to incoming neutral. This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

An annoying workaround would be to always turn the oven off at the
isolating switch on the wall.

Not true of course.

It does assume the oven has a mains switch in the live supply which,
come to think of it, my last two cookers didn't have. If it does, it
*could* be true.

It can't possibly be true. Capacitor power supplies don't run live to
earth, but live to neutral. And no new appliance passes enough i to E to
trip an RCD.

I find that hard to believe. That would be going back to the
arrangements for live chassis TVs of fifty years ago. I am suspect that
the ground reference for the electronics must be earth. I am prepared
to be proved wrong, but certainly the only oven I have ever worked on
the electronics was earth (and indeed chassis) referenced.

And the size of the dropper capacitor does not limit the peak current,
only the total charge.

Most of that is as unambiguous as mud.


NT

On 17/12/2018 10:31, wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:
On 16/12/2018 20:57, Roger Hayter wrote:
John Rumm wrote:
On 16/12/2018 15:36, Roger Hayter wrote:

This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

From the picture he has a type AC RCD, and they are insensitive to
unipolar currents. So switching to any of the other types would make
that kind of trip more likely.

Ok, sorry, I got type A and type AC the wrong way round. I'll go with
everyone else that it is a defective component, very probably a heater
element, especially given that it happens on switching off.

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

--
Adam

On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:
On 16/12/2018 20:57, Roger Hayter wrote:
John Rumm wrote:
On 16/12/2018 15:36, Roger Hayter wrote:

This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

From the picture he has a type AC RCD, and they are insensitive to
unipolar currents. So switching to any of the other types would make
that kind of trip more likely.

Ok, sorry, I got type A and type AC the wrong way round. I'll go with
everyone else that it is a defective component, very probably a heater
element, especially given that it happens on switching off.

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.


NT

On 17/12/2018 09:14, Andy Burns wrote:
John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off.

Turn off all the other MCBs on the left side of the CU, then see if
turning the oven on/off still reliably trips the left RCD, will tell you
if some other circuit is taking the RCD to the brink, or whether the
oven alone can trip the RCD ... might save finger pointing between John
Lewis and the sparky

OP he Indeed that would be a good test. Turning off the oven does NOT
reliably trip the RCD. When the NEFF engineer was present he saw that it
tripped sometimes, same with the sparky. When I've had a go, it has not
tripped after several attempts.

We had a NEW Neff dishwasher.

Seiuosly Faulty within 3 months., repaired, then faulty again, then my daughter got a shock off it (with hindsight, this was probably just static (mind you, its not happened before or since). The supplier agreed a refund, and we stumped up the extra for a Miele with which we are very pleased (last one lasted 10 years, Miele Washer lasted 20 and never needed brushes or a bearing).

Grumps wrote:

On 17/12/2018 09:14, Andy Burns wrote:
John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off.

Turn off all the other MCBs on the left side of the CU, then see if
turning the oven on/off still reliably trips the left RCD, will tell you
if some other circuit is taking the RCD to the brink, or whether the
oven alone can trip the RCD ... might save finger pointing between John
Lewis and the sparky

OP he Indeed that would be a good test. Turning off the oven does NOT
reliably trip the RCD. When the NEFF engineer was present he saw that it
tripped sometimes, same with the sparky. When I've had a go, it has not
tripped after several attempts.

Is that just turning off the oven heating, or has it got a switch that
turns it entirely off including the display, or are you referring to a
separate (from the oven) isolating switch?

--

Roger Hayter

On 19/12/2018 12:08, Roger Hayter wrote:
Grumps wrote:

On 17/12/2018 09:14, Andy Burns wrote:
John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off.

Turn off all the other MCBs on the left side of the CU, then see if
turning the oven on/off still reliably trips the left RCD, will tell you
if some other circuit is taking the RCD to the brink, or whether the
oven alone can trip the RCD ... might save finger pointing between John
Lewis and the sparky

OP he Indeed that would be a good test. Turning off the oven does NOT
reliably trip the RCD. When the NEFF engineer was present he saw that it
tripped sometimes, same with the sparky. When I've had a go, it has not
tripped after several attempts.

Is that just turning off the oven heating, or has it got a switch that
turns it entirely off including the display, or are you referring to a
separate (from the oven) isolating switch?

Just turning off the oven.

Grumps wrote:

On 19/12/2018 12:08, Roger Hayter wrote:
Grumps wrote:

On 17/12/2018 09:14, Andy Burns wrote:
John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off.

Turn off all the other MCBs on the left side of the CU, then see if
turning the oven on/off still reliably trips the left RCD, will tell you
if some other circuit is taking the RCD to the brink, or whether the
oven alone can trip the RCD ... might save finger pointing between John
Lewis and the sparky

OP he Indeed that would be a good test. Turning off the oven does NOT
reliably trip the RCD. When the NEFF engineer was present he saw that it
tripped sometimes, same with the sparky. When I've had a go, it has not
tripped after several attempts.

Is that just turning off the oven heating, or has it got a switch that
turns it entirely off including the display, or are you referring to a
separate (from the oven) isolating switch?

Just turning off the oven.

I can say with certainty that a leaky element *can* do that, though
obviously I don't know if that applies to your oven. The element(s) can
probably easily be removed, and tested with an insulation tester. If
faulty if could easily be replaced without replacing the whole oven.
Assuming they have any good ones at all, anyway. No common insulation
tester (up to, say, 5Gigaohms) should show any leakage at all at 500V.

--

Roger Hayter

On 2018-12-15, John Rumm wrote:

On 15/12/2018 09:20, Grumps wrote:
....
So, can anyone decipher this and say what needs changing and why?
Can you even get a class/type A MCB? Why would changing the RCD type
from B to A make a difference?

There are different types of MCB; Type B, C, or D. They differ in how
tolerant they are to switch on surges. Typically you use type B devices
on most domestic circuits. Can't see that being an issue here -
especially on switch off.

OK, I'll bite --- why is type A missing?


There are also different types of RCD. The basic one we know and love is
a type AC. Other types include a type A, F, and B. Each of those adds
abilities to detect additional leakage scenarios that the lower level
device may not respond to. Again it seems unlikely that there is an
issue here.

And why this odd set of types?

On 20/12/2018 17:00, Adam Funk wrote:
On 2018-12-15, John Rumm wrote:

On 15/12/2018 09:20, Grumps wrote:
...
So, can anyone decipher this and say what needs changing and why?
Can you even get a class/type A MCB? Why would changing the RCD type
from B to A make a difference?

There are different types of MCB; Type B, C, or D. They differ in how
tolerant they are to switch on surges. Typically you use type B devices
on most domestic circuits. Can't see that being an issue here -
especially on switch off.

OK, I'll bite --- why is type A missing?

Presumably because they thought there was too much scope for confusion
with the current rating... e.g. a Type A 32A MCB.

With B32, C6 etc , there is no scope for misunderstandings.

There are also different types of RCD. The basic one we know and love is
a type AC. Other types include a type A, F, and B. Each of those adds
abilities to detect additional leakage scenarios that the lower level
device may not respond to. Again it seems unlikely that there is an
issue here.

And why this odd set of types?

I suppose I can see some logic for the F since that is the one sensitive
to high frequency waveform leakage (as you may get on a VFD drive
feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for rectified
DC, and S for smoothed DC would have made more sense?)


--
Cheers,

John.

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

On 20/12/2018 18:46, John Rumm wrote:
On 20/12/2018 17:00, Adam Funk wrote:
On 2018-12-15, John Rumm wrote:

On 15/12/2018 09:20, Grumps wrote:
...
So, can anyone decipher this and say what needs changing and why?
Can you even get a class/type A MCB? Why would changing the RCD type
from B to A make a difference?

There are different types of MCB; Type B, C, or D. They differ in how
tolerant they are to switch on surges. Typically you use type B devices
on most domestic circuits. Can't see that being an issue here -
especially on switch off.

OK, I'll bite --- why is type A missing?

Presumably because they thought there was too much scope for confusion
with the current rating... e.g. a Type A 32A MCB.

With B32, C6 etc , there is no scope for misunderstandings.

There are also different types of RCD. The basic one we know and love is
a type AC. Other types include a type A, F, and B. Each of those adds
abilities to detect additional leakage scenarios that the lower level
device may not respond to.Â* Again it seems unlikely that there is an
issue here.

And why this odd set of types?

I suppose I can see some logic for the F since that is the one sensitive
to high frequency waveform leakage (as you may get on a VFD drive
feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for rectified
DC, and S for smoothed DC would have made more sense?)



I agree with the MCBs, but there is already a S type RCD (the time
delayed one).

So S was already used. S meaning selective.

as for A and B maybe it was just a lack of imagination? Why not start at
Z and work down the alphabet?



--
Adam

On 20/12/2018 19:50, ARW wrote:
On 20/12/2018 18:46, John Rumm wrote:
On 20/12/2018 17:00, Adam Funk wrote:
On 2018-12-15, John Rumm wrote:

On 15/12/2018 09:20, Grumps wrote:
...
So, can anyone decipher this and say what needs changing and why?
Can you even get a class/type A MCB? Why would changing the RCD type
from B to A make a difference?

There are different types of MCB; Type B, C, or D. They differ in how
tolerant they are to switch on surges. Typically you use type B devices
on most domestic circuits. Can't see that being an issue here -
especially on switch off.

OK, I'll bite --- why is type A missing?

Presumably because they thought there was too much scope for confusion
with the current rating... e.g. a Type A 32A MCB.

With B32, C6 etc , there is no scope for misunderstandings.

There are also different types of RCD. The basic one we know and
love is
a type AC. Other types include a type A, F, and B. Each of those adds
abilities to detect additional leakage scenarios that the lower level
device may not respond to.Â* Again it seems unlikely that there is an
issue here.

And why this odd set of types?

I suppose I can see some logic for the F since that is the one
sensitive to high frequency waveform leakage (as you may get on a VFD
drive feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for
rectified DC, and S for smoothed DC would have made more sense?)



I agree with the MCBs, but there is already a S type RCD (the time
delayed one).

So S was already used. S meaning selective.

Indeed... good point.

(Although I note that MK quite often like to stick a "s" suffix on all
their RCDs just to confuse you!)

as for A and B maybe it was just a lack of imagination? Why not start at
Z and work down the alphabet?



--
Cheers,

John.

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

On 21/12/2018 18:48, John Rumm wrote:
On 20/12/2018 19:50, ARW wrote:

I suppose I can see some logic for the F since that is the one
sensitive to high frequency waveform leakage (as you may get on a VFD
drive feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for
rectified DC, and S for smoothed DC would have made more sense?)



I agree with the MCBs, but there is already a S type RCD (the time
delayed one).

So S was already used. S meaning selective.

Indeed... good point.



I would have used T for time delayed:-)


--
Adam

On 17/12/2018 20:48, wrote:
On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:
On 16/12/2018 20:57, Roger Hayter wrote:
John Rumm wrote:
On 16/12/2018 15:36, Roger Hayter wrote:

This probably should not be
enough to trip if the RCD is not already carrying a leakage current, but
would be a design fault. It is true that an RCD insensitive to
unipolar currents (?? type A) would prevent this tripping, but you
shouldn't have to change it!

From the picture he has a type AC RCD, and they are insensitive to
unipolar currents. So switching to any of the other types would make
that kind of trip more likely.

Ok, sorry, I got type A and type AC the wrong way round. I'll go with
everyone else that it is a defective component, very probably a heater
element, especially given that it happens on switching off.

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.




Are you considering the NE fault as high resistance ?


--
Adam

ARW wrote:

On 17/12/2018 20:48, wrote:
On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:
On 16/12/2018 20:57, Roger Hayter wrote:
John Rumm wrote:
On 16/12/2018 15:36, Roger Hayter wrote:

This probably should not be enough to trip if the RCD is not
already carrying a leakage current, but would be a design fault.
It is true that an RCD insensitive to unipolar currents (?? type
A) would prevent this tripping, but you shouldn't have to change
it!

From the picture he has a type AC RCD, and they are insensitive to
unipolar currents. So switching to any of the other types would make
that kind of trip more likely.

Ok, sorry, I got type A and type AC the wrong way round. I'll go with
everyone else that it is a defective component, very probably a heater
element, especially given that it happens on switching off.

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on
there's very little leakage because there's very little voltage
difference between leaky spot & earth. If the switch is 2 pole,
sometimes the neutral opens before the live. Then the neutral end of the
element sees 240v briefly, and enough current flows to earth to trip the
RCD.




Are you considering the NE fault as high resistance ?

In case you're interested I'll describe my trip-on-turning-off fault to
you. It was high resistance, but the mechanism was not the double pole
switching one that tabbypurr postulates.

There were two grill elements I'll call A and B. Only the live was
switched and the switch positions were off, A on and A+B on. The fault
first manifested as intermitten trips on turning the switch from A+B to
A. It then became reproducible trips every time on switching from A+B
to A. Later, (when I had to fix it!) it became trips every time A was
furned on from the off position. I never tested turning the isolating
switch on with the switch in the A+B position, that might have been
instructive. Or not.

I didn't work out what was wrong and initially changed element A. This
had no effect and the removed element A seemed intact (2Gohm at 100V, I
somehow acquried a comms type Megger that provides 10-100V, a rather
specialised instrument!).

When I changed element B it completely cured the fault. Element B had
a resisance to earth of about 40kohms at 100V as I recollect.


My best theory is that the leak was very near the neutral end, and with
ilive switched on B the leackage from neutral and live was about equal,
but from neutral only when B live was switched off. And as it
progressed the leakage got greater even when B was not hot and the rise
in neutral voltage on turning element A on was sufficient to trip.
Had it progressed further, it would presumably have tripped even with
element A off, but I didn't leave it to happen. I could have checked
the element potentiometrically to see where the low insulation
resistance was along the element if I had a very high impedance
voltmeter, but I'm afraid I didn't.


I am quite prepared to be contradicted, if you have a better theory to
explain the fault.

--

Roger Hayter

On Friday, 21 December 2018 20:17:48 UTC, ARW wrote:
On 17/12/2018 20:48, tabbypurr wrote:
On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.




Are you considering the NE fault as high resistance ?

What the N-E resistance needs to be for this scenario to play out depends on where it is on the element, ie what the voltage difference is between fault & earth. If there were eg 10v between the faulty patch & earth, to get under 20mA rms flowing while powered would require at least 12kΩ of resistance.

When the neutral feed is disconnected, but live still connected, that 12k would then pass... 480mA rms.


NT

On 22/12/2018 09:42, wrote:
On Friday, 21 December 2018 20:17:48 UTC, ARW wrote:
On 17/12/2018 20:48, tabbypurr wrote:
On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.




Are you considering the NE fault as high resistance ?

What the N-E resistance needs to be for this scenario to play out depends on where it is on the element, ie what the voltage difference is between fault & earth. If there were eg 10v between the faulty patch & earth, to get under 20mA rms flowing while powered would require at least 12kΩ of resistance.

When the neutral feed is disconnected, but live still connected, that 12k would then pass... 480mA rms.



None of which makes sense when it is only the live that is been switched.


--
Adam

On Saturday, 22 December 2018 15:39:13 UTC, ARW wrote:
On 22/12/2018 09:42, tabbypurr wrote:
On Friday, 21 December 2018 20:17:48 UTC, ARW wrote:
On 17/12/2018 20:48, tabbypurr wrote:
On Monday, 17 December 2018 19:47:52 UTC, ARW wrote:
On 17/12/2018 10:31, tabbypurr wrote:
On Monday, 17 December 2018 02:26:02 UTC, John Rumm wrote:

Dodgy heater elements are well known for RCD trips, however I would
expect it to do it either at switch on, or while running. Not
necessarily at switch off. So while a dodgy heater element is possible,
there are other possible causes.

I'm curious to see whether Adam understands how/why it can happen.



If I knew why a dodgy element would only cause a problem when switching
off the oven I would say so.

So why don't you tell us why it does?

If the leakage is at or near the neutral end of the element, when on there's very little leakage because there's very little voltage difference between leaky spot & earth. If the switch is 2 pole, sometimes the neutral opens before the live. Then the neutral end of the element sees 240v briefly, and enough current flows to earth to trip the RCD.




Are you considering the NE fault as high resistance ?

What the N-E resistance needs to be for this scenario to play out depends on where it is on the element, ie what the voltage difference is between fault & earth. If there were eg 10v between the faulty patch & earth, to get under 20mA rms flowing while powered would require at least 12kΩ of resistance.

When the neutral feed is disconnected, but live still connected, that 12k would then pass... 480mA rms.



None of which makes sense when it is only the live that is been switched.

It only applies to 2 pole switching, or 1 pole if some genius put the switch in the neutral. I guess in principle it might also happen with a dodgy neutral connection. And also for elements - there must be a formal name for them - where a switch puts sections of them in series & parallel to get different powers.


NT

On 2018-12-20, John Rumm wrote:

On 20/12/2018 17:00, Adam Funk wrote:
On 2018-12-15, John Rumm wrote:

On 15/12/2018 09:20, Grumps wrote:
...
So, can anyone decipher this and say what needs changing and why?
Can you even get a class/type A MCB? Why would changing the RCD type
from B to A make a difference?

There are different types of MCB; Type B, C, or D. They differ in how
tolerant they are to switch on surges. Typically you use type B devices
on most domestic circuits. Can't see that being an issue here -
especially on switch off.

OK, I'll bite --- why is type A missing?

Presumably because they thought there was too much scope for confusion
with the current rating... e.g. a Type A 32A MCB.

With B32, C6 etc , there is no scope for misunderstandings.

There are also different types of RCD. The basic one we know and love is
a type AC. Other types include a type A, F, and B. Each of those adds
abilities to detect additional leakage scenarios that the lower level
device may not respond to. Again it seems unlikely that there is an
issue here.

And why this odd set of types?

I suppose I can see some logic for the F since that is the one sensitive
to high frequency waveform leakage (as you may get on a VFD drive
feeding a machine tool).

Not sure what the logic was for A and B though. (perhaps R for rectified
DC, and S for smoothed DC would have made more sense?)

That all sounds plausible; thanks.