Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......

Go on I'll bite at the risk of being buried by the group.
Answers,
Neither,both,either one or the 60a main incomer.
Answers based on experience of many cable shorts,cuts and snaps.
Bit like tossing a coin and it falling in the sand will it land heads ,tails
or end on
or if you live in some areas never hit the ground. :-)
To many variables.
CJ

"Stephen H" wrote in message ...

Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......

Stephen H wrote:

Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.
watts.

Discuss.......
Discuss.......

From experience, if the plug had a 5A fuse and a dead short, the 32A CU
device would *usually* not blow.

13A fuse in the plug has a much lower relative discrimination so it is quite
possible the 32A MCB (assuming a Type B) would trip.

We could get technical and look at the energy let-through of a plugtop 13A
fuse, but that would involve getting books out...

--
Tim Watts Personal Blog: http://www.dionic.net/tim/

"History will be kind to me for I intend to write it."

On 10/12/2012 21:57, Stephen H wrote:

Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Not enough information really (supply impedance, at CU and at socket saw
was connected to, plus fault current?)

In general, the fuse will go first. However not always. It can even come
down to individual fuses. Fuse ok, and MCB tripping can also happen, as
can both opening.


--
Cheers,

John.

/================================================== ===============\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article ,
Stephen H writes:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......

Depends on the short circuit current, which depends on supply
impedance at the point of the short circuit.
If you can get 120A or higher fault current (supply impedance
2ohms or less), then a 32A type B MCB will open faster than
some 13A fuses.

I've had a 15W pigmy oven light blow and take out the 32A MCB.

OTOH, I ran over the mower flex a few months ago, and in spite
of cutting right through, that didn't blow anything, leaving
a live end on the lawn. The blade probably made contact for
far too short a time.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Mon, 10 Dec 2012 23:12:29 +0000 (UTC),
(Andrew Gabriel) wrote:


OTOH, I ran over the mower flex a few months ago, and in spite
of cutting right through,

That's why I prefer a petrol mower, in spite of my fairly small lawns
:-)

I've hardly been able to use any sort of mower this summer since
breaking my left humerus at the start of August, so the gardens have
become wildernesses.
Next year...

--
Frank Erskine

On 11/12/2012 00:50, Frank Erskine wrote:
On Mon, 10 Dec 2012 23:12:29 +0000 (UTC),
(Andrew Gabriel) wrote:


OTOH, I ran over the mower flex a few months ago, and in spite
of cutting right through,

That's why I prefer a petrol mower, in spite of my fairly small lawns
:-)

I've hardly been able to use any sort of mower this summer since
breaking my left humerus at the start of August,

Was tempted to say that's not funny - but decided to give you a break as
it was a a bit close to the bone ;-)


--
Cheers,

John.

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

On Mon, 10 Dec 2012 21:57:22 +0000, Stephen H wrote:

Which pops first, the 13A fuse in the circular saw's plug or the 32A
MCB at the CU?

Do you have the answer or is this just cough hypothetical?

I'd guess the MCB as the magnetic rather thermal overload side of those
is very quick, probably quicker than the time required to heat up and
blow a 13A fuse.

--
Cheers
Dave.

Erm why? Surely it would be the fastes device in the circuit, which is
probably the electronic device but not always.

Brian

--
From the Sofa of Brian Gaff Reply address is active
"Stephen H" wrote in message
...
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a 13A
fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power tool's
mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......

It happens that Stephen H formulated :
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a 13A
fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power tool's
mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB at
the CU?

In case you need to know, the power rating of the circular saw is 1200 watts.

Discuss.......

In my experience, usually a 32amp MCB will trip before a 13amp plugtop
cartridge fuse. The cartridge fuse has some thermal lag, the MCB does
not.

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

Harry Bloomfield wrote:

In my experience, usually a 32amp MCB will trip before a 13amp plugtop
cartridge fuse. The cartridge fuse has some thermal lag, the MCB does
not.

That was always my understanding, but when I tripped a 16A type B CB
last week, it also blew the DNO fuse.
That was a 'surprise'.

Later revelations showed the fault current was massively high - 16kA,
with a 0.02 ohm earth loop - the transformer was 20 feet away, so that
could account for the fuse going, but it is still surprising.

--
To reply by e-mail, change the ' + ' to 'plus'.

In article ,
(A.Lee) writes:
Harry Bloomfield wrote:

In my experience, usually a 32amp MCB will trip before a 13amp plugtop
cartridge fuse. The cartridge fuse has some thermal lag, the MCB does
not.

That was always my understanding, but when I tripped a 16A type B CB
last week, it also blew the DNO fuse.
That was a 'surprise'.

Later revelations showed the fault current was massively high - 16kA,
with a 0.02 ohm earth loop - the transformer was 20 feet away, so that
could account for the fuse going, but it is still surprising.

Can happen in blocks of flats with the substation in the basement.
The first few flats near it often get a long supply cable snaked
back and fourth a bit to decrease the PSSC.

What was the breaking capacity of the MCB? 6kA is highest you
normally get on domestic boards, although you can get up to 10kA
for commercial boards. It probably needs replacing anyway.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Andrew Gabriel wrote:
In article ,
(A.Lee) writes:
Harry Bloomfield wrote:

In my experience, usually a 32amp MCB will trip before a 13amp
plugtop cartridge fuse. The cartridge fuse has some thermal lag,
the MCB does not.

That was always my understanding, but when I tripped a 16A type B CB
last week, it also blew the DNO fuse.
That was a 'surprise'.

Later revelations showed the fault current was massively high -
16kA, with a 0.02 ohm earth loop - the transformer was 20 feet
away, so that could account for the fuse going, but it is still
surprising.

Can happen in blocks of flats with the substation in the basement.
The first few flats near it often get a long supply cable snaked
back and fourth a bit to decrease the PSSC.

What was the breaking capacity of the MCB? 6kA is highest you
normally get on domestic boards, although you can get up to 10kA
for commercial boards. It probably needs replacing anyway.

Were you there when the NICEIC guy came to my house? It was wintertime 3
years ago and my earth loop reading is zero. He suggestested using the tails
from my CU as UFH on the path to the front door:-).

You may actually be relying on the 100A 1361 fuse to clear a fault with such
a low reading earth loop reading and that the MCB and some cabling will be
knackered.

--
Adam

On 10/12/12 21:57, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......
Id say the MCB

they are faster acting usually.


--
Ineptocracy

(in-ep-toc-ra-cy) €“ a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

On 11/12/2012 11:20, The Natural Philosopher wrote:
On 10/12/12 21:57, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......
Id say the MCB

they are faster acting usually.



It was the 32A MCB that popped.

When the circular saw's plug fuse was tested, it was still intact. So a
new flex and reset the 32A MCB.

Still an interesting one.... Wonder what would have happened if the 32A
MCB was a rewirable 30A piece of fusewire or a cartridge type fuse?

On Dec 10, 9:57*pm, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with a
13A fuse plugged into a house's ring main which has a 32A MCB at the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead with
spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the 32A MCB
at the CU?

In case you need to know, the power rating of the circular saw is 1200
watts.

Discuss.......

I did this in a much simpler way the other day and have all the
answers for you - not the why answers, but this is what happened
answers.

I'd been given a vacuum cleaner to repair - "why did you include your
dirty disposable bag" - I asked " because you need it in place to get
it to run" came the answer. You do normally believe people when they
tell you such !

As I took the motor assembly out, I noticed a 2 pin connector - " ahh
the bag microswitch lead " thought I and proceeded to put a 2.5mm^2
jumper across it. I sorted the broken mains lead, plugged-in the
still dismantled machine and switch on. Workshop - 6pm at this time
of year - can be very dark when all the lights go out.

So - once the torch and the exit door were located, I found that the
B16 mcb in the workshop CU for the ring had tripped, but then so had
the feed B32 one in the house CU, hence the being plunged into
darkness. The 13A fuse was OK.

Cause of all this was my believing the owner's assertion as the
connector in question was in fact a mains feed to an auxiliary mains
outlet and I had put a big short across it. The machine worked quite
happily without the damn bag!!

I have got a C32 now to fit on the main CU. Ohh - and yes I replaced
the 13A fuse as well.

Rob

robgraham wrote:
On Dec 10, 9:57 pm, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with
a 13A fuse plugged into a house's ring main which has a 32A MCB at
the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead
with spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the
32A MCB at the CU?

In case you need to know, the power rating of the circular saw is
1200 watts.

Discuss.......

I did this in a much simpler way the other day and have all the
answers for you - not the why answers, but this is what happened
answers.

I'd been given a vacuum cleaner to repair - "why did you include your
dirty disposable bag" - I asked " because you need it in place to get
it to run" came the answer. You do normally believe people when they
tell you such !

As I took the motor assembly out, I noticed a 2 pin connector - " ahh
the bag microswitch lead " thought I and proceeded to put a 2.5mm^2
jumper across it. I sorted the broken mains lead, plugged-in the
still dismantled machine and switch on. Workshop - 6pm at this time
of year - can be very dark when all the lights go out.

So - once the torch and the exit door were located, I found that the
B16 mcb in the workshop CU for the ring had tripped, but then so had
the feed B32 one in the house CU, hence the being plunged into
darkness. The 13A fuse was OK.

Cause of all this was my believing the owner's assertion as the
connector in question was in fact a mains feed to an auxiliary mains
outlet and I had put a big short across it. The machine worked quite
happily without the damn bag!!

I have got a C32 now to fit on the main CU. Ohh - and yes I replaced
the 13A fuse as well.

Unless you have done the proper calculations and tests then I would not fit
a C32 MCB to your CU.

--
Adam

On 12/12/2012 00:21, ARW wrote:
robgraham wrote:
On Dec 10, 9:57 pm, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with
a 13A fuse plugged into a house's ring main which has a 32A MCB at
the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead
with spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the
32A MCB at the CU?

In case you need to know, the power rating of the circular saw is
1200 watts.

Discuss.......

I did this in a much simpler way the other day and have all the
answers for you - not the why answers, but this is what happened
answers.

I'd been given a vacuum cleaner to repair - "why did you include your
dirty disposable bag" - I asked " because you need it in place to get
it to run" came the answer. You do normally believe people when they
tell you such !

As I took the motor assembly out, I noticed a 2 pin connector - " ahh
the bag microswitch lead " thought I and proceeded to put a 2.5mm^2
jumper across it. I sorted the broken mains lead, plugged-in the
still dismantled machine and switch on. Workshop - 6pm at this time
of year - can be very dark when all the lights go out.

So - once the torch and the exit door were located, I found that the
B16 mcb in the workshop CU for the ring had tripped, but then so had
the feed B32 one in the house CU, hence the being plunged into
darkness. The 13A fuse was OK.

Cause of all this was my believing the owner's assertion as the
connector in question was in fact a mains feed to an auxiliary mains
outlet and I had put a big short across it. The machine worked quite
happily without the damn bag!!

I have got a C32 now to fit on the main CU. Ohh - and yes I replaced
the 13A fuse as well.

Unless you have done the proper calculations and tests then I would not fit
a C32 MCB to your CU.

The way I got round[1] that one was to fit a 32A Cartridge fuse in a MCB
form factor fuse carrier at the head end CU. (The workshop CU is also
split load to keep the lights on)

[1] In theory - never actually tripped the workshop MCB to try it!


--
Cheers,

John.

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

In article ,
John Rumm writes:

The way I got round[1] that one was to fit a 32A Cartridge fuse in a MCB
form factor fuse carrier at the head end CU. (The workshop CU is also
split load to keep the lights on)

BS1361 was withdrawn 2 or 3 years ago, and hence BS1361
cartridge fuse holders no longer seem to be available
(unless you can find old stock somewhere), although you
can still buy spare fuses.

This caught me out as I wanted a BS1361 cartridge fuse
for a sub-main some months back, to give some
discrimination over the downstream MCBs, but you can't
do that anymore. I don't think you could ever get BS88
ones to fit in CUs.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 12/12/2012 11:46, Andrew Gabriel wrote:
In article ,
John Rumm writes:

The way I got round[1] that one was to fit a 32A Cartridge fuse in a MCB
form factor fuse carrier at the head end CU. (The workshop CU is also
split load to keep the lights on)

BS1361 was withdrawn 2 or 3 years ago, and hence BS1361
cartridge fuse holders no longer seem to be available

Yes I had noticed they were harder to find... had not realised they had
been withdrawn.

(unless you can find old stock somewhere), although you
can still buy spare fuses.

This caught me out as I wanted a BS1361 cartridge fuse
for a sub-main some months back, to give some
discrimination over the downstream MCBs, but you can't
do that anymore. I don't think you could ever get BS88
ones to fit in CUs.

Probably too fat for a single module wide enclosure.


--
Cheers,

John.

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

On Dec 12, 12:21*am, "ARW" wrote:
robgraham wrote:
On Dec 10, 9:57 pm, Stephen H wrote:
Picture the scenario...

A double insulated Circular saw with twin core flex and a plug with
a 13A fuse plugged into a house's ring main which has a 32A MCB at
the CU.

No other appliances are plugged in at the same ring main.

Hapless DIYer accidentally slices through circular saw's mains lead
with spinning blade, creating a dead L - N short circuit.

RCD at CU does not pop due to no leakage current to earth as power
tool's mains lead does not have an earth conductor.

Now the conundrum:

Which pops first, the 13A fuse in the circular saw's plug or the
32A MCB at the CU?

In case you need to know, the power rating of the circular saw is
1200 watts.

Discuss.......

I did this in a much simpler way the other day and have all the
answers for you - not the why answers, but this is what happened
answers.

I'd been given a vacuum cleaner to repair - "why did you include your
dirty disposable bag" - I asked " because you need it in place to get
it to run" came the answer. * You do normally believe people when they
tell you such !

As I took the motor assembly out, I noticed a 2 pin connector - " ahh
the bag microswitch lead " thought I and proceeded to put a 2.5mm^2
jumper across it. *I sorted the broken mains lead, plugged-in the
still dismantled machine and switch on. *Workshop - 6pm at this time
of year - can be very dark when all the lights go out.

So - once the torch and the exit door were located, I found that the
B16 mcb in the workshop CU for the ring had tripped, but then so had
the feed B32 one in the house CU, hence the being plunged into
darkness. *The 13A fuse was OK.

Cause of all this was my believing the owner's assertion as the
connector in question was in fact a mains feed to an auxiliary mains
outlet and I had put a big short across it. *The machine worked quite
happily without the damn bag!!

I have got a C32 now to fit on the main CU. *Ohh - and yes I replaced
the 13A fuse as well.

Unless you have done the proper calculations and tests then I would not fit
a C32 MCB to your CU.

--
Adam

The 'garage' circuit feeds two sub-CU's both with 16A type B MCB's. I
cannot have the situation where there is a ring fault in the workshop
and all the lights go out too - unless I end up having to go the
emergency lighting root as suggested in another post today.

Rob