2.5mm2 cable protected by 13A BS1362 fuse...

[MattChan]

Evening chaps, theoretical situation for my own interest / learning purposes and to check my assumptions / calcs.

One item on a ring final (protected by a B32 breaker) is an FCU with a 13A BS1362 fitted to it whos output is 2.5mm2 T+E radial that feeds 4 double sockets.

Fuse type correction factor:
Fusing factor for the BS1362 is 1.9 compared to 1.45 for a type B. Therefore tabulated cable rating needs to be:
13A x 1.9 / 1.45 = 17.03A

No grouping:
Therefore tabulated cable rating:
17.03 / 1 = 17.03A

On the wall in the garage:
Ambient temp assumed to be 30degC:
Therefore tabulated cable rating:
17.03 / 1 = 17.03A

2.5mm2 clipped direct has a rating of 27A so suitable.

In the loft:
Ambient temp assumed to be 50degC worst case:
Therefore tabulated cable rating:
17.03 / 0.71 = 23.99A (is 0.71 correct for the fuse type, I see some records point to 0.87 for non 1.45 fuse factor devices)

Cable laid over 100mm of rockwool with 200mm of rockwool on top of it for 3m length. Reference method 103 or clipped direct x 0.5 assumed.

2.5mm2 = 13.5A which is too low
4mm2 = 17.5A which is fine at a 30degC ambient
10mm2 = 32A is the smallest cable with a valid rating at 50degC

Have I screwed my calcs / made any bad assumptions?

This isn't my day job, I work on the electronics / product design end of the world and I'm just interested in how the above is calculated by the pros.

 

[JohnW2]

Evening chaps, theoretical situation for my own interest / learning purposes and to check my assumptions / calcs. .... Have I screwed my calcs / made any bad assumptions? ... This isn't my day job, I work on the electronics / product design end of the world and I'm just interested in how the above is calculated by the pros.

My first observation is that you are probably thinking about this more deeply (and quite possibly, strictly speaking, more correctly) than many/most electricians probably would. In particular ....

Fuse type correction factor: Fusing factor for the BS1362 is 1.9 compared to 1.45 for a type B. Therefore tabulated cable rating needs to be: 13A x 1.9 / 1.45 = 17.03A

You may be right about the fusing factor for a BS1362 fuse (I can't recall ever having seen a figure quoted). However, as far as BS7671 is concerned (433.1.1 & 433.1.202), it appears that there are only two fusing factors one has to consider - 2.0 (1.45 /0.725) for BS3036 fuses and 1.45 for 'everything else'.

In other words (and looking at it 'the other way around', as BS7671 does) in the absence of any of the de-rating factors (for installation method, grouping, ambient temp etc.), it appears that the limiting (max) In of the over-current device (fuse, MCB etc.) is 'the tabulated current' ('CCC') for everything other than BS3036 fuses, in which case it's 'tabulated current x 0.725'.

That's how I read it, anyway

Kind Regards, John

 

[flameport]

Fusing factor for the BS1362 is 1.9

That depends on where you look, various figures between 1.6 and 1.9 are quoted.

The rest is ok - and the conclusion from all of it is that installing cables in thermal insulation is best avoided for all types of circuits.

 

[JohnW2]

That depends on where you look, various figures between 1.6 and 1.9 are quoted.

Interesting. What do you think about what I've just written - i.e. that BS7671 seems to be concerned about only two fusing factors - 2.0 for 3036's and 1.45 for 'everything else'?

The rest is ok - and the conclusion from all of it is that installing cables in thermal insulation is best avoided for all types of circuits.

Very much so - any practice that leads to the conclusion that one needs 10mm² cable for a "13A circuit" is clearly a little on the crazy side!

Kind Regards, John

 

[MattChan]

Thanks chaps.

For the BS1362 fuse I was working on the basis that I2 has a tolerance at 10^4seconds for a 13A fuse of approx 21A to 25A.

Therefore fusing factor to 1dp is in the range:
21/13A = 1.6
25/13A = 1.9 must be assumed due to it being worst case.

Mind you 1.45 in a breaker must have a tolerance on it too.

Is there a reg that says the following must be met I2 <= 1.45Iz or additional factors must be used?

 

[MattChan]

I found this reference but don't have the regs to hand...
Regulation 433-02-01 can be summarised as follows:
(i) In > Ib
(ii) Iz > In
(iii)1.45 x Iz > I2

 

[JohnW2]

I found this reference but don't have the regs to hand...
Regulation 433-02-01 can be summarised as follows:
(i) In > Ib
(ii) Iz > In
(iii)1.45 x Iz > I2

Indeed (albeit that reg number you quote is from an ancient version of BS7671/Wiring Regs - but the content hasn't changed since) - but, as I just wrote, that is (now) modified by ...

433.1.202 Where the protective device is a semi-enclosed fuse to BS 3036 compliance with condition (iii) of
Regulation 433.1.1 is afforded if its rated current (In) does not exceed 0.725 times the current-carrying capacity (Iz)
of the lowest rated conductor in the circuit protected

So, as I said, that in effect means that one considers the fusing factor as 1.45 in all cases unless it's a BS3036 fuse, in which case it becomes 2.0 (1.45/.0.725).

Kind Regards, John

 

[MattChan]

Cheers John

Just for interest whats the wording of 433.1.1?

 

[EFLImpudence]

So, as I said, that in effect means that one considers the fusing factor as 1.45 in all cases unless it's a BS3036 fuse, in which case it becomes 2.0 (1.45/.0.725).

Just to be picky -

The fusing factor is not 2 because 1.45/0.725 = 2
rather the derating factor is 0.725 because 1.45/2 = 0.725.

 

[JohnW2]

Cheers John ... Just for interest whats the wording of 433.1.1?

I imagine probably essentially identical to the 'ancient' version you quoted ....

433.1.1 The operating characteristics of a device protecting a conductor against overload shall satisfy the
following conditions:
(i) The rated current or current setting of the protective device (In) is not less than the design current (Ib) of the
circuit, and
(ii) the rated current or current setting of the protective device (In) does not exceed the lowest of the currentcarrying
capacities (Iz) of any of the conductors of the circuit, and
(iii) 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 any of the conductors of the circuit.
For adjustable protective devices, the rated current (In) is the current setting selected.
The current (I2) causing effective operation of the protective device is given in the product standard or may be
provided by the manufacturer.
NOTE 1: Where overload protection is provided by BS 3036 fuses, refer to Regulation 433.1.202.
NOTE 2: Protection in accordance with this regulation may not provide protection in all cases, for example, where sustained
overcurrents less than I2 occur.

Kind Regards, John

 

[JohnW2]

Just to be picky - The fusing factor is not 2 because 1.45/0.725 = 2 ... rather the derating factor is 0.725 because 1.45/2 = 0.725.

I have no argument with either that statement nor the fact that it is '"picky" .

I did, after all, write "... that in effect means ...", and there was no "because" (or any other implication of causality) in sight in my sentence!

Kind Regards, John

 

[MattChan]

John, I have to admit my experience is in complying with EN61010, EN60950, RED, EN61000 etc and having them evaluated at a lab rather than BS7671.

With that in mind my reading of the snippet of standard you posted is that 433.1.202 is only applicable for BS3036 devices so must be disregarded in this case. It doesn't mean that the requirement of 433.1.1(iii) does not need to be met.

Therefore we must comply with 433.1.1(iii) and the only way I can see to do this is to apply the correction I did above!

Happy to be wrong but as I say that is my reading. Maybe I should get some use out of my IET membership and seek clarification, I have given them enough cash over the years.

 

[JohnW2]

... With that in mind my reading of the snippet of standard you posted is that 433.1.202 is only applicable for BS3036 devices so must be disregarded in this case. It doesn't mean that the requirement of 433.1.1(iii) does not need to be met. ... Therefore we must comply with 433.1.1(iii) ...

Agreed ....

... and the only way I can see to do this is to apply the correction I did above!

I must say that I don't see that. As I've said, 433.1.1(iii) effectively means that (to comply) one takes the fusing factor of the device (any device) as 1.45, the only exception (per 433.1.202) being if it is a BS3036 fuse, in which case one is implicitly assuming the fusing factor to be 2.0.

I would therefore to say is that one simply looks at the tabulated Iz (which tabulation already assumes a 1.45 'fusing factor') and compares that with the In of the device, and hence does not ever need to consider any other 'fusing factor' (e.g. your 1.9 for a BS1362 fuse), unless its a BS3036 fuse.

Do you see something wrong with that reasoning?

However, as flameport has said, you really ought to find a way of not having the cable buried in thermal insulation, since that results in the need for a ridiculously large cable for a "13A circuit"!

Kind Regards, John

 

[MattChan]

John

I would read 433.1.1(iii) that:
I2 / 1.45 < Iz

The important bit being that it is I2 and not In.

For a breaker:
I2 = In x 1.45 giving
In x 1.45 / 1.45 < Iz
simplified
In < Iz

For our BS1362:
I2 = In x 1.9 giving
In x 1.9 / 1.45 < Iz
simplified
In x 1.31 < Iz
Or
In < Iz x 0.763

Would you agree?

Agreed on avoiding thermal insulation being the best option.

 

[JohnW2]

John ... I would read it that: I2 / 1.45 < Iz ... The important bit being that it is I2 and not In.

Indeed - and, as you go on to say/illustrate, for an MCB (or anything else with a 'fusing factor' of 1.45), that simplifies to In ≤ Iz.

For our BS1362: I2 = In x 1.9 giving .... In x 1.9 / 1.45 < Iz ... simplified .... In x 1.31 < Iz ... Or
In < Iz x 0.763 .... Would you agree?

Yes, I have no choice but to agree with that! .... which I suppose means that I have always 'thought' that 433.1.1(iii) talked about In and not I2 !

This confusion (at least, in my mind!) seems to arise at least partially because 433.1.202 has been included completely unnecessarily, tempting one to think that BS3036 fuses are the only devices that require 'special attention/thought'. In reality, it is but just one example of a device with a 'fusing factor' which is not 1.45 (hence I2 is not 1.45In) , such that they could also have included (as another 'special case'), say, your BS1362 with a fusing factor of 1.9 (or whatever) ... or, better, not have this reg at all, and just let people work out I2 for themselves..

However, given this realisation that 433.1.1(iii) is written in terms of I2 (not In) (which makes sense), I'm now a bit more confused ... what is the point of 433.1.1(ii)? Is it simply there in case one has a device with a fusing factor less than 1.45 (if they exist) - since, if it is 1.45 or more, it's surely inevitable that anything which satisfies (iii) will also satisfy (ii)?

Agreed on avoiding thermal insulation being the best option.

Yes, I really think you should try hard to think of a way of avoiding it - since the consequences are so 'ridiculous'!

Kind Regards, John