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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.

  • mapj1: 
     

    I assume your vacuum vessel is mirror lined so no radiation loss ?.

     

     

    And do we ignore conduction heat losses to the outside world via the supply cables?

    Z.

  • Very well said Mike and particularly Chris, I think that you are about right. This reinforces the long piece I wrote above, the time component of overload is very important, and usually not even realised to be there. 

  • Thanks for all your contributions chaps. I'm going to have to re-read some of these a few times. I'm particularly loving the disparity between what I would consider a fag packet computation and yours though! ? But then I'm one of those who, at least historically, has followed the numbers because I do err in favour of an easy life. Food for though though and clearly some new sums to learn… 

     

  • And do we ignore conduction heat losses to the outside world via the supply cables?

    Well the electricity has to join or leave at the ends of the test section of cable, depending how resistively that is done heat may flow in or out. But let us concern ourselves with the cable in the middle, and note that in these Gedankenversuchen  (*) we can simply imagine making  the experiment longer & longer until the end effects are far from the section of interest and so become safe to ignore.

    In a real experiment, necessarily short, we would instead externally heat or cool the wire ends near the terminations to set a temperature at the ends close to the measured centre temperature, so there was no great lengthways thermal gradient to drive heat either way in that direction. In the language of the maths dept. we would impose a boundary condition of no significant longitudinal  heat flow. Its a lot cheaper than asking site facilities to provide an infinitely long thin lab bench and vacuum vessel ?

    Mike

     * something like an experiment in the mind only, literally "thought-experiement"

  • Have a look at Section 10.14 and Table 10.6 of the IET Code of Practice for In-Service Inspection and Testing of Electrical Equipment.

    A 13 A or 10 A fuse may be fitted for appliances rated at 10 A, with 0.75 sq mm flex up to 2 m in length. Someone else has already done the sums to demonstrate this is probably going to be OK, including for fault current protection. BS 7671 permits the non-adiabatic approach in BS 7454 for fault current protection as well (but don't forget there may well be a requirement for protection against overload current).

  • I would be much more worried about installation conditions than exact fuse ratings

    I recently saw an extension reel in a very sorry condition because someone had run it fairly near full load without unreeling it first. A couple of hours and the whole thing had sagged with the cables in the middle fused into each other