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Topic Title: Oven Lamp.
Topic Summary: Big Bang.
Created On: 02 January 2018 03:02 PM
Status: Read Only
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 02 January 2018 03:02 PM
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Zoomup

Posts: 3478
Joined: 20 February 2014

I was called to a total blackout at a domestic property.

The main 63 Amp. submain M.C.B. had tripped off in the flat.

The reason was a blown oven lamp.

The flat's 32 Amp. consumer unit cooker circuit M.C.B. has tripped off as well as the flat's 63 Amp. M.C.B.

Some fault I thought, to be caused by a small oven lamp. The remains of the lamp showed internal lamp arcing.

The lamp had only just been replaced. A new one came on a card marked oven lamps. Were they really heat resistant oven lamps, or China's best I wondered?

Z.
 02 January 2018 03:09 PM
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Nedryerson

Posts: 126
Joined: 12 December 2009

Z - The fault may have occurred at the top of the half cycle taking out both breakers.
 02 January 2018 05:13 PM
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potential

Posts: 1646
Joined: 01 February 2007

All quite likely on a low impedance circuit.
A failing incandescent lamp is capable of passing a high arc current for several cycles.
 02 January 2018 05:16 PM
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Weirdbeard2

Posts: 126
Joined: 29 November 2017

It's not a common occurrence, but is why generally mcbs of the same type aren't great at discriminating fault current, but on the bright side on the rare occasion something like this happens it is easy for the consumer to switch it back on
 03 January 2018 08:16 AM
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ectophile

Posts: 758
Joined: 17 September 2001

But if it had been a 63A fuse, would it ever have blown?

-------------------------
S P Barker BSc PhD IEng MIET
 03 January 2018 09:33 AM
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potential

Posts: 1646
Joined: 01 February 2007

Originally posted by: ectophile

But if it had been a 63A fuse, would it ever have blown?
Had it been a 60A wired fuse, almost certainly not.
Had it been a 60A cartridge fuse protecting a 30A wired fuse possibly yes.
 03 January 2018 06:43 PM
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AJJewsbury

Posts: 16183
Joined: 13 August 2003

But if it had been a 63A fuse, would it ever have blown?

If there was a fault (or overload) on the submain, I'd hope so.
Had it been a 60A cartridge fuse protecting a 30A wired fuse possibly yes.

I though the general rule of thumb was that fuses even of mixed types generally discriminate OK when their ratings are 2:1 - if both are of the same type (at least modern cartridge ones) then 1.6 : 1 is usually OK.

- Andy.
 03 January 2018 07:30 PM
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mapj1

Posts: 9789
Joined: 22 July 2004

The honest answer is a resounding 'Jein' as my co-workers in Germany used to say when the answer was unclear.
It will for example, depend on what other loads are on - the 63A fuse may already be preheated, and the 30A stone cold. Re-wireables are far more variable than the sand filled cartridges, as how much cooling the wire gets where it touches the terminals and the insulation blocks, and how tight the wire is pulled, all contribute to this uncertainty , and that is before we get to the effects of grouping and air circulation that affect all fuse types to a degree.
Equally a nice tight fusewire not rubbing the block, will probably beat a cold 63a cartridge. Or maybe both would fail , as one may keep an arc between recently opened and separating ends for long enough for the other to fail.
It gets far more interesting with circuit breakers, as unlike fuses they don't speed up with really high fault currents, so at many kA, the 63A fuse may beat even a 6A MCB, or again, as the contacts may have started moving but not yet separated when the fuse opens, you may find again, both operate.

-------------------------
regards Mike
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