Circuit breaker trip puzzle

[Arfa Daily]

This puzzle now has another twist. I decided to do some methodical
leakage tests. My intention was to introduce a leakage from the live
connection of the circuit under test to a convenient earth, and
observe what tripped. That would at least confirm that I understood
what behaviour I was *supposed* to get from my Crabtree StarBreaker
CU. I used a couple of robust 15k resistors in parallel, theoretically
giving me a leakage around 32 mA (in my UK 240 V circuits).

I decided to start with an unimportant circuit, which also had an
extra RCD unit over and above the main CU. This is a double socket RCD
in the garage, fed from #3 on the Main Switch (non-RCD) side of the
CU. As expected, momentarily connecting this leakage current
immediately tripped the garage RCD. However, to my surprise, maybe 1-2
seconds later (with the leakage removed), the *main* switch on the CU
was tripped. So my PC and various household devices and clocks went
down despite my cunning plan ;-)

I'd not have expected this to happen, so could someone offer a
possible explanation please? Does it offer more insight into the odd
behaviour already reported?

In case it helps, the electrician's scrawled installation notes record
trip times of 18/19 ms for all the RCD circuits, and 7 ms for the
external garage RCD unit. There are various other columns like 'Loop
Impedance', 'CPC-CPC ohms', 'Ph-Ph ohms', 'R1+R2 or R2 ohms'; they
mean nothing to me but please let me know if they could help the
diagnosis.

Well, that's certainly strange to me, and like you, not what I would have
expected. I suspect that you are now getting beyond my level of expertise
with this, so if I catch up with him in the next day or two, I will ask my
next door neighbour, who is a fully UK qualified electrician, and talks
about this sort of stuff all the time.

Arfa

 

[Arfa Daily]

Pondering this over a coffee shortly after hitting Send, I realised
that I was wrong to imply the devices on this circuit were
insignificant. I left all the devices in place when I applied the
leakage. Amongst other things, I have an ancient large DC power supply
in my shed/workshop (30A), permanently connected to provide automatic
garden lighting at dusk. I'm wondering if perhaps some sort of
inductive effect might be responsible? Could the sudden removal of
power from that and other lower-rated transformers have had the effect
observed? If so, my starting choice was poor, and I'll try another
circuit. Or maybe disconnect the two plugs from the garage RCD unit.

Ahhh ... Very possibly ... I'll still ask my neighbour, though ( see my
other reply )

Arfa

 

[Graham]

Pondering this over a coffee shortly after hitting Send, I realised
that I was wrong to imply the devices on this circuit were
insignificant. I left all the devices in place when I applied the
leakage. Amongst other things, I have an ancient large DC power supply
in my shed/workshop (30A), permanently connected to provide automatic
garden lighting at dusk. I'm wondering if perhaps some sort of
inductive effect might be responsible? Could the sudden removal of
power from that and other lower-rated transformers have had the effect
observed? If so, my starting choice was poor, and I'll try another
circuit. Or maybe disconnect the two plugs from the garage RCD unit.

Can you confirm that it was the main switch on the far left that
tripped or was it the RCD in the middle? I suspect that the socket
outlet only disconnects the live conductor and not the neutral, very
common, hence your leakage providing a link between N and E possibly
combined with the power supply tripping the RCD. The main switch on
the left is purely a switch and contains no over current or RCD
device, if that has turned itself off then someone's playing tricks or
it needs to be changed, however I have never come across that 15
years.

On you main board the switch on the far left controls everything up to
the RCD (RCD included), and the RCD everything after that. I my
experience it's usually caused by an appliance somewhere, washing
machine, cooker and anything plugged in outside seem to be the popular
ones. Other less likely causes are loose contacts in a socket, and
mice.

Wait and see if it happens again, and note down what was on when it
went. If it seems to be at random get a spark in and get him to do an
insulation test at 250v with live and neutral joined to earth with
everything connected. It's not accepted practice but it usually finds
the fault! In my mind nothing has happened that seems especially odd,
and other than doing a quick test of earth loop impedance and RCD trip
times for peace of mind, I wouldn't look any further and advise to
wait and see.

It's a shame you don't live in Herefordshire or I'd offer to pop round
and do a quick check for a cup of coffee!

Graham

 

[Terry Pinnell]

Can you confirm that it was the main switch on the far left that
tripped or was it the RCD in the middle?

Thanks Graham, appreciate your reply. Yes, it was the RCD switch that
had opened.

I suspect that the socket
outlet only disconnects the live conductor and not the neutral, very
common, hence your leakage providing a link between N and E possibly
combined with the power supply tripping the RCD. The main switch on
the left is purely a switch and contains no over current or RCD
device, if that has turned itself off then someone's playing tricks or
it needs to be changed, however I have never come across that 15
years.

On you main board the switch on the far left controls everything up to
the RCD (RCD included), and the RCD everything after that. I my
experience it's usually caused by an appliance somewhere, washing
machine, cooker and anything plugged in outside seem to be the popular
ones. Other less likely causes are loose contacts in a socket, and
mice.

Wait and see if it happens again, and note down what was on when it
went. If it seems to be at random get a spark in and get him to do an
insulation test at 250v with live and neutral joined to earth with
everything connected. It's not accepted practice but it usually finds
the fault! In my mind nothing has happened that seems especially odd,
and other than doing a quick test of earth loop impedance and RCD trip
times for peace of mind, I wouldn't look any further and advise to
wait and see.

As mentioned in my original post, it was the thought of it happening
during my imminent holiday and leaving the fridge/freezer unpowered
that troubled me most. But as you saw up-thread, I can at least avoid
that risk by powering it from the cooker socket. Nevertheless, that's
hardly satisfactory so I won't be comfortable until I've diagnosed the
problem properly.

I've arranged for the original installing electrician to visit this
evening. Apart from anything else he wants to do, I propose to repeat
my earlier experiment. Both with the various devices connected and
also disconnected.

It's a shame you don't live in Herefordshire or I'd offer to pop round
and do a quick check for a cup of coffee!

Indeed! But a very welcome second-best would be for you to hang on in
here with me in the thread until either I get a resolution or terminal
boredom sets in ;-)

 

[Graham]

Thanks Graham, appreciate your reply. Yes, it was the RCD switch that
had opened.


As mentioned in my original post, it was the thought of it happening
during my imminent holiday and leaving the fridge/freezer unpowered
that troubled me most. But as you saw up-thread, I can at least avoid
that risk by powering it from the cooker socket. Nevertheless, that's
hardly satisfactory so I won't be comfortable until I've diagnosed the
problem properly.

I've arranged for the original installing electrician to visit this
evening. Apart from anything else he wants to do, I propose to repeat
my earlier experiment. Both with the various devices connected and
also disconnected.


Indeed! But a very welcome second-best would be for you to hang on in
here with me in the thread until either I get a resolution or terminal
boredom sets in ;-)

If it only happened the once I would be inclined to leave it. There
are a whole heap of things that can cause it trip, most of which are
one time only events that dissappear as soon as it happens, such as
power spikes. I would only investgate it heavily if it was tripping
regually ( > once a month) and causing a nusance, otherwise your not
likely to find a fault. As for plugging the freezer into the cooker,
go ahead, it is common (and considered good practice) to provide a
single socket that is not RCD protected and on its own breaker for it,
Although that won't be allowed under the next regs, you will however
be able to plug it in in the bathroom......

Graham

 

[Arfa Daily]

If it only happened the once I would be inclined to leave it. There
are a whole heap of things that can cause it trip, most of which are
one time only events that dissappear as soon as it happens, such as
power spikes. I would only investgate it heavily if it was tripping
regually ( > once a month) and causing a nusance, otherwise your not
likely to find a fault. As for plugging the freezer into the cooker,
go ahead, it is common (and considered good practice) to provide a
single socket that is not RCD protected and on its own breaker for it,
Although that won't be allowed under the next regs, you will however
be able to plug it in in the bathroom......

Graham

Is that a joke, or a serious comment for your neck of the woods ?

Arfa

 

[Graham]

Is that a joke, or a serious comment for your neck of the woods ?

Arfa

Unfortunately not, under the next revision of the regs (17th edition)
(proposed) socket outlets are permitted providing they are 3 m from a
bath or shower and RCD protected. Currently no sockets can be
installed in a room containing a bath or shower no matter how big the
room is. It's mainly for B&B's and hotels that have a shower in the
corner of the room. The rest of Europe has allowed them for many
years. Local bonding in a bathroom has also been axed if all circuits
in the bathroom are RCD protected.

I see a few RCD's fail every year, either taking too long trip or not
tripping at all. I will not be installing sockets in bathrooms (except
hotel rooms) and I will continue to do bonding regardless of whether
or not the circuits have a RCD protection!

Please note that the 17th edition is still in draft stage and may be
subject to change on final release. For anyone interested the current
draft copy can be found here:

http://www.iee.org/Publish/WireRegs/DPC/index.cfm


Graham

 

[Ron(UK)]

Unfortunately not, under the next revision of the regs (17th edition)
(proposed) socket outlets are permitted providing they are 3 m from a
bath or shower and RCD protected. Currently no sockets can be
installed in a room containing a bath or shower no matter how big the
room is. It's mainly for B&B's and hotels that have a shower in the
corner of the room. The rest of Europe has allowed them for many
years. Local bonding in a bathroom has also been axed if all circuits
in the bathroom are RCD protected.

One of the entertainment venues I work at has no sockets in the dressing
rooms because there are showers. When I enquired if we could fit RCD
protected sockets, I was told in no uncertain terms No.

So what happens now is that artists plug their own mains extension cable
into a socket in the corridor outside the dressing room and under the door!

Ron(UK)

 

[Arfa Daily]

Unfortunately not, under the next revision of the regs (17th edition)
(proposed) socket outlets are permitted providing they are 3 m from a
bath or shower and RCD protected. Currently no sockets can be
installed in a room containing a bath or shower no matter how big the
room is. It's mainly for B&B's and hotels that have a shower in the
corner of the room. The rest of Europe has allowed them for many
years. Local bonding in a bathroom has also been axed if all circuits
in the bathroom are RCD protected.

I see a few RCD's fail every year, either taking too long trip or not
tripping at all. I will not be installing sockets in bathrooms (except
hotel rooms) and I will continue to do bonding regardless of whether
or not the circuits have a RCD protection!

Please note that the 17th edition is still in draft stage and may be
subject to change on final release. For anyone interested the current
draft copy can be found here:

http://www.iee.org/Publish/WireRegs/DPC/index.cfm


Graham

Boy oh boy. Considering all the crap that they recently subjected us all to
with regard to not being allowed to wire up your own garden shed, or add new
circuits, or even alter light fittings without getting Building Inspector
approval and an electrician who's signed off to certify the job, that
beggars belief ...

Arfa

 

[Dave Plowman (News)]

One of the entertainment venues I work at has no sockets in the dressing
rooms because there are showers. When I enquired if we could fit RCD
protected sockets, I was told in no uncertain terms No.

So what happens now is that artists plug their own mains extension cable
into a socket in the corridor outside the dressing room and under the
door!

Yup - this is exactly what some do for drying/clipping hair in their
bathroom - so they can use the mirror.

I've oft wondered if a transformer isolated 13 amp socket - 3kW isolating
transformer - would conform to the current regs?

 

[Gary Tait]

Yup - this is exactly what some do for drying/clipping hair in their
bathroom - so they can use the mirror.

I've oft wondered if a transformer isolated 13 amp socket - 3kW
isolating transformer - would conform to the current regs?

I doubt it. I belive current regs only permit an isolated transformer
receptical for small motorised appliances or chargers, such as razors or
toohtbrushes.

 

[hr(bob) [email protected]]

I doubt it. I belive current regs only permit an isolated transformer
receptical for small motorised appliances or chargers, such as razors or
toohtbrushes.- Hide quoted text -

- Show quoted text -

And I thought some of the USA regulations went too far.

H. R. (Bob) Hofmann