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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

    Well a 13 A fuse will carry 13A all day, and may not blow in any sensible time even at 20-25A, depending on manufacturing tolerances and environmental temperatures.

     

    Look at these curves and look at the ‘all blow’ right hand curve, vs the left hand ‘never blow’ limit for 13 and 3A fuses. 

    Now cable ratings are also a function of manufacturing variations, and most importantly cooling.  An extension lead knotted in a bundle or wrapped in pillows will overheat quickly even at the nominal rating, while a cable  in even a modest breeze will be fine at considerably more than the nominal ‘free air’  rating.

    Now the ratings tend to assume that the cable fails the moment the copper cores reach 70C but anyone who has dropped an off cut of cable into their coffee will know it does not melt immediately nor spontaneously combust. so the cable can be over run quite a lot before they actually fail. (power is proportional to current squared so if we neglect convection air movement increasing as things get hot, we might expect double the temperature rise from ambient from root 2 ~ 1.4 times the load current. so if for example 16A gets us from 30 degrees to 70, then 24 A may get us from 30 degrees to 110 degrees - still not cable failure but not good for you if you touch it..)

    But would you really want a cable at even  90 degrees in  a place you could touch it ?

    probably not..

    Mike.

Reply
  • 0

    Well a 13 A fuse will carry 13A all day, and may not blow in any sensible time even at 20-25A, depending on manufacturing tolerances and environmental temperatures.

     

    Look at these curves and look at the ‘all blow’ right hand curve, vs the left hand ‘never blow’ limit for 13 and 3A fuses. 

    Now cable ratings are also a function of manufacturing variations, and most importantly cooling.  An extension lead knotted in a bundle or wrapped in pillows will overheat quickly even at the nominal rating, while a cable  in even a modest breeze will be fine at considerably more than the nominal ‘free air’  rating.

    Now the ratings tend to assume that the cable fails the moment the copper cores reach 70C but anyone who has dropped an off cut of cable into their coffee will know it does not melt immediately nor spontaneously combust. so the cable can be over run quite a lot before they actually fail. (power is proportional to current squared so if we neglect convection air movement increasing as things get hot, we might expect double the temperature rise from ambient from root 2 ~ 1.4 times the load current. so if for example 16A gets us from 30 degrees to 70, then 24 A may get us from 30 degrees to 110 degrees - still not cable failure but not good for you if you touch it..)

    But would you really want a cable at even  90 degrees in  a place you could touch it ?

    probably not..

    Mike.

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