Conundrum.... which opens first, a 13A plug fuse or a 32A MCB during a L-N short circuit?

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.......

Reply to
Stephen H
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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

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.......

Reply to
CJ

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...

Reply to
Tim Watts

Do you have the answer or is this just 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.

Reply to
Dave Liquorice

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.

Reply to
John Rumm

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.

Reply to
Andrew Gabriel

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...

Reply to
Frank Erskine

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 ;-)

Reply to
John Rumm

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

Brian

Reply to
Brian Gaff

It happens that Stephen H formulated :

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.

Reply to
Harry Bloomfield

Id say the MCB

they are faster acting usually.

Reply to
The Natural Philosopher

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?

Reply to
Stephen H

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.

Reply to
A.Lee

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.

Reply to
Andrew Gabriel

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

Reply to
robgraham

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.

Reply to
ARW

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

Reply to
ARW

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!
Reply to
John Rumm

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.

Reply to
Andrew Gabriel

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 had not realised they had been withdrawn.

Probably too fat for a single module wide enclosure.

Reply to
John Rumm

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