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13A 1362 fuses and flex

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 mapj1 over 3 years ago

Well, it is a bit like peeping behind the curtain at the Wizard of Oz, to realise that the whole edifice of current ratings and so on is not perhaps as precise as the significant figures in the appendix suggest.
Belief in the numbers is for those who like a simple life. Life is not simple, as above, 13A fuses do not blow at 14A, and 27 amp cables may already overheat at 25 or be fine at 35, in very similar looking arrangements, and in many ways it is “measure with micrometer, mark in chalk and cut with axe” in that the apparent accuracy is a bit of a myth - well, if you re-create the exact test conditions you will probably re-produce the readings, more or less, but cables, especially flex, are never installed exactly like the test rig really. Given that convection is gravity dependant, you may ask why readings are not separated in to vertical and horizontal, with a few amps between them, or why the rating is not less near the ceiling. Luckily, there is usually a fair amount of slack in real systems relative to the book value, and most over loads are not on long enough to matter. (The warm up time is also very variable - the substation at the end of your street may be loaded to 200% for an hour a day without suffering any immediate issues, cables in plaster warm up over tens of minutes, and wires in free air are very affected by drafts, and the air in a house is almost never totally still)
There is a reason that double sockets on a 25mm spur from a 32A ring seldom suffer from cable failure.
Mike
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0 mapj1 over 3 years ago

Well, it is a bit like peeping behind the curtain at the Wizard of Oz, to realise that the whole edifice of current ratings and so on is not perhaps as precise as the significant figures in the appendix suggest.
Belief in the numbers is for those who like a simple life. Life is not simple, as above, 13A fuses do not blow at 14A, and 27 amp cables may already overheat at 25 or be fine at 35, in very similar looking arrangements, and in many ways it is “measure with micrometer, mark in chalk and cut with axe” in that the apparent accuracy is a bit of a myth - well, if you re-create the exact test conditions you will probably re-produce the readings, more or less, but cables, especially flex, are never installed exactly like the test rig really. Given that convection is gravity dependant, you may ask why readings are not separated in to vertical and horizontal, with a few amps between them, or why the rating is not less near the ceiling. Luckily, there is usually a fair amount of slack in real systems relative to the book value, and most over loads are not on long enough to matter. (The warm up time is also very variable - the substation at the end of your street may be loaded to 200% for an hour a day without suffering any immediate issues, cables in plaster warm up over tens of minutes, and wires in free air are very affected by drafts, and the air in a house is almost never totally still)
There is a reason that double sockets on a 25mm spur from a 32A ring seldom suffer from cable failure.
Mike
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13A 1362 fuses and flex

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