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13A 1362 fuses and flex

Scrambled

Good evening everybody. 

I've been cogitating on the fusing factor of 1362 fuses (specifically 13A fuses)  and how this correlates with the protection of a 1.5mm2 flexible cable. As ever I am hoping you can shine a light!

The code of practice for the in service inspection and testing of equipment  Table 15.6 states that for flexes to be protected by the fuse in a BS1363 plug there is no limit to their length providing that their csa's are as in table 15.6 which states a minimum flex size of 1.25mm when using a 13A fuse. I am minded that it is quite common site to see a multi-gang extension lead on sale using 1.5mm2 flex where there is obviously potential for overload given the unknown nature of what would be plugged into them (even though there will be a warning not to intentionally do so).

From the Beama guide:

2.5 The BS1362 Fuse
The UK uses a fused plug which must be fitted with a BS 1362 fuse. For domestic
installations the use of the BS 1363 plug and socket system and the fitting of a BS 1362 fuse
into a plug is a legal requirement under the UK Plug and Socket Safety Regulations, 1995.
With a correctly fused BS 1363 plug, the flexible cable connected to equipment is always fully
protected against the effects of overload or small overcurrents as follows:
3A fuse protects 0.5mm² cords
5A (6A) fuse protects 0.75mm² cords
13A fuse protects 1.25mm2 cords
Protection against excessive damage by a short circuit is still achieved even if the smaller
cord sizes are inadvertently protected by a 13A fuse. In addition, it has been accepted in the
UK that some marginal damage to small flexible cords is tolerable under short circuit
conditions, for example where a 0.22mm² cord is used with a 13A BS 1362 fuse.

As far as I understand it the fusing factor of a 1362 fuse BS 1362 fuse is 1.9 (0.763) although in fairness I have seen lower fusing factors quoted (1.66?? which removes the particular problem I am wrestling with.)

Reference 4F3A a 1.5mm2 single phase AC flexible cable has a tabulated current carrying capacity of 16amps. 16x0.763 = 12.208 amps which is obviously less than the 13 amp rating of the fuse. 

I find it quite common to see 13A 1362 fuses inline on 32A cooker circuits protecting 1.5mm2 flexes to ovens. Is this deemed acceptable even though the oven isn't strictly speaking a fixed load (fan motor etc.)?

Is there another factor at play here which I am missing? Or do I just have the wrong fusing factor!

Thanks for your help in advance.

Parents
  • 0

    A real world example. 

    A few months ago I found myself with mapj1 junior in the car at a remote place with a defective battery. The jump leads were in Mrs mapj1s car at the time. Then it began to rain.  That sort of day.

    I was able to rustle up a short length of 2.5mm cable off the end of a reel, and the use of another car battery from an alarm engineer.

    Thought process ran like this..

    The car draws 250 to 300A to spin the starter.  How long can I crank the engine while my son and the nervous alarm man hold the 2.5mm onto the 2 batteries without burning either of them ?

    Now I did not quite use Chris's figure of 3 mins at 20 A, but if I had it would have scaled to 3mins/100 at 200A, or about 2 seconds for a 40 degree rise.  I had a more optimistic figure, allowing a bit for the PVC, and as I did not have the regs with me, so used a pidooma figure from memory.

    But I know the cable starts cold, I'm a bit more gung ho than the book ratings, as I do not care if the cable is ‘lifed’ afterwards  and so I can allow myself  a larger change in temp, perhaps 50 to 70C. 

    That “sum” suggested 

    “a-one and a-two and a-three.”  for cranking time, And relax and cool for 15 -20 seconds before retry  so average current is perhaps 20-30A.

    As it happened, the engine started midway through the  2nd attempt, and the cable was blood warm.

    When I described the thought process, the wide-eyed alarm installer said 

    “ I'd never have dared to do anything like that -  2.5mm is for 16 amps innit ?” well maybe under some condition, not sure what alarm chaps get taught about installation methods and de-rating..

    The only place that came slightly unstuck was one of the ends where the contact area to the battery was much less than the copper cross-section, and it spot welded. Well, sometimes wire cutters are a man's best friend, just keep the revs up a bit mapj1 junior while I find them.. 

    I now have a shiny new car battery without a tag of 2,5mm wire stuck to the terminal. 

    Mike.

    PS and the jump leads are back in the boot. all is well with the world.

    edited for a couple of nasty typos and readability. 

Reply
  • 0

    A real world example. 

    A few months ago I found myself with mapj1 junior in the car at a remote place with a defective battery. The jump leads were in Mrs mapj1s car at the time. Then it began to rain.  That sort of day.

    I was able to rustle up a short length of 2.5mm cable off the end of a reel, and the use of another car battery from an alarm engineer.

    Thought process ran like this..

    The car draws 250 to 300A to spin the starter.  How long can I crank the engine while my son and the nervous alarm man hold the 2.5mm onto the 2 batteries without burning either of them ?

    Now I did not quite use Chris's figure of 3 mins at 20 A, but if I had it would have scaled to 3mins/100 at 200A, or about 2 seconds for a 40 degree rise.  I had a more optimistic figure, allowing a bit for the PVC, and as I did not have the regs with me, so used a pidooma figure from memory.

    But I know the cable starts cold, I'm a bit more gung ho than the book ratings, as I do not care if the cable is ‘lifed’ afterwards  and so I can allow myself  a larger change in temp, perhaps 50 to 70C. 

    That “sum” suggested 

    “a-one and a-two and a-three.”  for cranking time, And relax and cool for 15 -20 seconds before retry  so average current is perhaps 20-30A.

    As it happened, the engine started midway through the  2nd attempt, and the cable was blood warm.

    When I described the thought process, the wide-eyed alarm installer said 

    “ I'd never have dared to do anything like that -  2.5mm is for 16 amps innit ?” well maybe under some condition, not sure what alarm chaps get taught about installation methods and de-rating..

    The only place that came slightly unstuck was one of the ends where the contact area to the battery was much less than the copper cross-section, and it spot welded. Well, sometimes wire cutters are a man's best friend, just keep the revs up a bit mapj1 junior while I find them.. 

    I now have a shiny new car battery without a tag of 2,5mm wire stuck to the terminal. 

    Mike.

    PS and the jump leads are back in the boot. all is well with the world.

    edited for a couple of nasty typos and readability. 

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