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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

    Thanks all for responses and apologies for my tardy reply…. Bit hectic out there at the moment!

    Simon's response correlates with what my understanding is telling me - which seems at odds with the pretty confident guidance/statements in the Beama guide.

    In the spirit of learning from my betters I will lay myself open to ridicule! I'm faintly terrified at having my life changed though! ?

     

    So what is an overcurrent and what are we concerned about?

    131.4 Persons and livestock shall be protected against injury, and property shall be protected against damage due to excessive temperatures or electromechanical stresses caused by overcurrents likely to arise in live conductors.

    Note. Protection can be achieved by limiting the overcurrent to a safe value and/or duration.

    Part 2: Overcurrent: A current exceeding the rated value. For conductors the rated value is the current carrying capacity.

    We're concerned thermal effects of the cable overheating possibly causing fires or injury.

    433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

    1- The rated current or current setting of the protective device (In) is not less than the design current of the circuit and

    2- The rated current or current setting of the protective device (In) does not exceed the lowest of the current carrying (Iz) of any of the conductors of the circuit, and

    3- Where I'm getting turned around. The current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz) of the conductors of the circuit.

    Leading on to… 433.20/202 where an allowance is made for the fusing factor of a 3036 fuse. If the fusing factor of a 1362 is at 1.9 it is fairly comparable so surely we must apply a comparable derating factor?  This is where the wheels come off my bus if the answer is more than just… It doesnt protect against overload.

    I appreciate that there will be manufacturers tolerances both for the device and the cable (which could go both ways) and that there is undoubtedly going to be some headroom in the cable. I've seen a plumber leave a 9.0kW electric boiler on a bit of 1.5 flex before and yeah (prior to me fixing it) it was pretty hot after a few hours but it hadn't melted! Is the answer just that we're only talking about an extra amp in the example and whilst it may get a bit hotter than would be ideal this isn't the end of the world or are we into the realms of trying to work out how much heat is being dissipated during the fault by the flex as it is going to be happening over quite a prolonged timeframe?

    I'm braced. Be gentle and again - thanks for your time.

     

     

     

Reply
  • 0

    Thanks all for responses and apologies for my tardy reply…. Bit hectic out there at the moment!

    Simon's response correlates with what my understanding is telling me - which seems at odds with the pretty confident guidance/statements in the Beama guide.

    In the spirit of learning from my betters I will lay myself open to ridicule! I'm faintly terrified at having my life changed though! ?

     

    So what is an overcurrent and what are we concerned about?

    131.4 Persons and livestock shall be protected against injury, and property shall be protected against damage due to excessive temperatures or electromechanical stresses caused by overcurrents likely to arise in live conductors.

    Note. Protection can be achieved by limiting the overcurrent to a safe value and/or duration.

    Part 2: Overcurrent: A current exceeding the rated value. For conductors the rated value is the current carrying capacity.

    We're concerned thermal effects of the cable overheating possibly causing fires or injury.

    433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the following conditions:

    1- The rated current or current setting of the protective device (In) is not less than the design current of the circuit and

    2- The rated current or current setting of the protective device (In) does not exceed the lowest of the current carrying (Iz) of any of the conductors of the circuit, and

    3- Where I'm getting turned around. The current (I2) causing effective operation of the protective device does not exceed 1.45 times the lowest of the current carrying capacities (Iz) of the conductors of the circuit.

    Leading on to… 433.20/202 where an allowance is made for the fusing factor of a 3036 fuse. If the fusing factor of a 1362 is at 1.9 it is fairly comparable so surely we must apply a comparable derating factor?  This is where the wheels come off my bus if the answer is more than just… It doesnt protect against overload.

    I appreciate that there will be manufacturers tolerances both for the device and the cable (which could go both ways) and that there is undoubtedly going to be some headroom in the cable. I've seen a plumber leave a 9.0kW electric boiler on a bit of 1.5 flex before and yeah (prior to me fixing it) it was pretty hot after a few hours but it hadn't melted! Is the answer just that we're only talking about an extra amp in the example and whilst it may get a bit hotter than would be ideal this isn't the end of the world or are we into the realms of trying to work out how much heat is being dissipated during the fault by the flex as it is going to be happening over quite a prolonged timeframe?

    I'm braced. Be gentle and again - thanks for your time.

     

     

     

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