fcu spur off lighting circuit

[EFLImpudence]

The OP was about an extractor fan and I have introduced kettles - has anything else been mentioned?

What did you have in mind?

There are some 10A circuits here for "lighting" (I don't have any; all 16A) so I would presume (I haven't looked) that any appliance that did need protection would be supplied with the relevant sized flex - this in the case of a fan being a lot simpler than installing fusing and isolator switches - or, of couse, internal fusing when actually required would negate this.

 

[JohnW2]

The OP was about an extractor fan and I have introduced kettles - has anything else been mentioned? ... What did you have in mind?

Is that directed at me? If so, I have nothing particularly in mind - my recent posts have been follow-ons from jg321's trying to explore winston's views about fusing.

Winston is always talking about fuses existing only to protect cables. You have (I think) introduced the concept that cables may not actually need any overload protection - which, although true per BS7671 (I don't know about other country's regs), is something which a fair number of people seem to be a bit uncomfortable about. However, I've been attempting to ascertain whether winston is invoking that same concept, or whether he feels that protection of the cable is required, but finds it acceptable to protect a cable with an OPD whose In is greater than the CCC of the cable. .

There are some 10A circuits here for "lighting" (I don't have any; all 16A) so I would presume (I haven't looked) that any appliance that did need protection would be supplied with the relevant sized flex - this in the case of a fan being a lot simpler than installing fusing and isolator switches ...

As above, I thought we were talking about the protection of cables, not appliances (I certainly was). However, I don't fully understand what you're saying here. Extractor fans are not usually "supplied with flex" but, even if they (or any other appliance) were, I'm not sure what you would regard as a "relevant sized" flex (for connection to, say, a 16A supply) - unless you're saying that it should have a CCC of at least 16A (which I don't think you believe).

Kind Regards, John

 

[EFLImpudence]

As above, I thought we were talking about the protection of cables,

We are.

not appliances (I certainly was). However, I don't fully understand what you're saying here. Extractor fans are not usually "supplied with flex"

They are not.
Hence my saying that if you want to have a cable/flex which can carry the current of the OPD rating then it is easier to fit a larger cable/flex than to down-fuse somewhere.

but, even if they (or any other appliance) were, I'm not sure what you would regard as a "relevant sized" flex (for connection to, say, a 16A supply) - unless you're saying that it should have a CCC of at least 16A (which I don't think you believe).

That is what I meant if someone is "a bit uncomfortable about" it not being.

E.g. the kettle flex. If uncomfortable about the supplied flex then fit a larger one.
Of course, no one is uncomfortable about a kettle flex but installing a similar circuit themselves, they would/could be.

 

[JohnW2]

Hence my saying that if you want to have a cable/flex which can carry the current of the OPD rating then it is easier to fit a larger cable/flex than to down-fuse somewhere.

Indeed - but that, of course, would presumably not satisfy manufacturers who (I think we all assume) believe that a fuse is necessary to protect something other than the cable.

That is what I meant if someone is "a bit uncomfortable about" it not being. E.g. the kettle flex. If uncomfortable about the supplied flex then fit a larger one. ... Of course, no one is uncomfortable about a kettle flex but installing a similar circuit themselves, they would/could be.

All true - although I think that a major part of the reason people are not uncomfortable about 'under-protected' (overload-wise) kettle flexes is that the way in which kettles are used is such that a prolonged overload is virtually never going to happen. A similarly 'under-protected' cable supplying a cooker would probably be much more likely to make them 'a bit uncomfortable'.

Kind Regards, John

 

[EFLImpudence]

Indeed - but that, of course, would presumably not satisfy manufacturers who (I think we all assume) believe that a fuse is necessary to protect something other than the cable.

...but that is not the case, is it? They just demand a fuse because they can.

It is often said here that a 3A fuse will not discriminate with a 6A MCB so they must be selling dangerous products.

Is it really believed that they make products for Europe but for Britain they have a different design and production line using inferior innards?

 

[JohnW2]

...but that is not the case, is it? They just demand a fuse because they can.

There has to be some reason other than "because they can". There's an essentially unlimited of things (nearly all of which would be silly) which they could demand "because they can", but they don't. You really would have to ask them 'the reason' - since, as I said, it can't really be for protection of cable (which might well have a much higher CCC than 3A, or even 6A). It could be their lawyers' idea.

It is often said here that a 3A fuse will not discriminate with a 6A MCB ...

Yes, but as I wrote earlier today, one has to be careful about that argument, since it is effectively saying that, if a 3A fuse is deemed adequate to protect a cable with a CCC of 3A, then a 6A MCB would be adequate protection for the cable - which is certainly not something that BS7671 would agree with.

... so they must be selling dangerous products.

I don't think that really follows from the first part of the sentence. We've discussed this umpteen times before, and I don't think that many people believe that 'inadequate' fusing (internal or external) usually makes a product 'dangerous'. What is usually discussed is the small possibility that a low-rating fuse (whether internal or external) might occasionally limit the degree of damage to the product in the case of a fault - and that if the manufacturer does not include that fusing internally, they may 'require' it to be provided externally.

Is it really believed that they make products for Europe but for Britain they have a different design and production line using inferior innards?

I'm not aware of anyone who believes that to be the case - which, as you suggest (and as winston reminds us far more times than we really want to be reminded, even without your help!) is an apparent anomaly on the part of the thinking and/or behaviour of manufacturers which I certainly cannot explain.

Kind Regards, John

 

[securespark]

No.

Your kettle likely has 0.75mm² flex.

But we all know that the manufacturers "regs" allow a 13A fuse on 0.75 factory fixed to an appliance, but 7671 says 6A max.

Edited for clarity.

 

[JohnW2]

But we all know that the manufacturers "regs" allow a 13A fuse on 0.75 factory fixed to an appliance, but 7671 says 6A max.

Indeed (isn't it actually BS1362 and/or BS1363 that says that?) - and that is another apparent anomaly.

There's obviously something a bit odd about the way they conceive CCCs for flexible cables, since the figures are generally very much lower than for the same CSA of 'non-flexible' cable. In terms of heating (or even 'melting') of cable, it ought to be essentially the same for flex and non-flex, so I can but presume that other 'factors' are being taken into account - and that may introduce scope for the differing 'official' CCCs which exist for flex.

In the case of many 'portable appliances' (kettles, toasters, hair-dryers etc. etc.) one could rationalise it in terms of the likely (fairly brief) duration of current during 'normal use' - but I think the same 'rules' probably aslo apply to fan heaters etc., if not even tumble driers adn ovens (but the 'rule' may be restricted to 'portable' appliances - I can't remember).

Kind Regards, John

 

[securespark]

Could be that the CCC is reduced in 7671 for flex because it is a ''rough service'' cable subject to stress and movement etc, whereas T & E, once installed, is not.

However, that does not explain the difference between the CCC's.

 

[JohnW2]

Could be that the CCC is reduced in 7671 for flex because it is a ''rough service'' cable subject to stress and movement etc, whereas T & E, once installed, is not.

That's the sort of thing I meant by "other factors" (since it clearly is not simply a matter of conductor CSA) - and, as I said, that may well introduce scope for different bodies/standards/'rules' to come up with different CCCs for flex of a given CSA, since the application of factors such as you mention is not going to be a precise art.

Kind Regards, John

 

[EFLImpudence]

There has to be some reason other than "because they can". There's an essentially unlimited of things (nearly all of which would be silly) which they could demand "because they can", but they don't. You really would have to ask them 'the reason' - since, as I said, it can't really be for protection of cable (which might well have a much higher CCC than 3A, or even 6A). It could be their lawyers' idea.

May be - but they don't demand it where they can't.

Yes, but as I wrote earlier today, one has to be careful about that argument, since it is effectively saying that, if a 3A fuse is deemed adequate to protect a cable with a CCC of 3A, then a 6A MCB would be adequate protection for the cable - which is certainly not something that BS7671 would agree with.

BS7671 does agree when 433 is used.
Also, BS1362 fuses have a fusing factor of 1.9 - a point never mentioned - so, with a 3A fuse, the cable should have a CCC of 3.93A anyway, therefore 6A cable would have to be used.

I don't think that really follows from the first part of the sentence. We've discussed this umpteen times before, and I don't think that many people believe that 'inadequate' fusing (internal or external) usually makes a product 'dangerous'. What is usually discussed is the small possibility that a low-rating fuse (whether internal or external) might occasionally limit the degree of damage to the product in the case of a fault - and that if the manufacturer does not include that fusing internally, they may 'require' it to be provided externally.

Then that is irresponsible as there are places not able to do that.
I'm not saying that they are irresponsible because I believe that manufacturers DO provide internal fusing when required. If not provided then it must be assumed that it is not required.

I'm not aware of anyone who believes that to be the case - which, as you suggest (and as winston reminds us far more times than we really want to be reminded, even without your help!) is an apparent anomaly on the part of the thinking and/or behaviour of manufacturers which I certainly cannot explain.

Q.E.D. then?
It could be a myth which has grown up through ignorance and perpetuated because it is perpetuated.

 

[JohnW2]

BS7671 does agree when 433 is used.

I presume again that you refer to 433.3. However, my point was that if/when overload protection IS required (i.e. when 433.3 does not apply), BS7671 does not allow a cable with a CCC of 3A to be protected by a 6A MCB because winston says that there would be no discrimination between the two (i.e. {presumably} that a B6 would operate at least as fast as a 3A BS1362).

Also, BS1362 fuses have a fusing factor of 1.9 - a point never mentioned - so, with a 3A fuse, the cable should have a CCC of 3.93A anyway, therefore 6A cable would have to be used.

I'm not sure I understand that arithmetic - so perhaps you could clarify. I don't think that attempting to compare 'fusing factors' of fuses and MCBs is straightforward, since it depends upon what 'disconnection time' one uses for the comparison.

Then that is irresponsible as there are places not able to do that. I'm not saying that they are irresponsible because I believe that manufacturers DO provide internal fusing when required. If not provided then it must be assumed that it is not required.

I agree that if a product would benefit from internal fusing, then reputable manufacturers will probably usually include it.

I think that the "demanding" is the real problem. If it were the case that the manufacturer is aware of things a UK user could do which might reduce the amount of damage to the product in the event of a fault, then it's not unreasonable for them to indicate that, even if they are 'things' which non-UK users could not do. To 'insist' on something being done with a product sold outside the UK which usually cannot actually be done outside of the UK is obviously a bit ridiculous.

Q.E.D. then? It could be a myth which has grown up through ignorance and perpetuated because it is perpetuated.

I don't think one can talk about "Q.E.D" when none of us really knows what, if anything, is behind all this.

You could well be right that it has been 'perpetuated because it is perpetuated' (there's plenty of precedent for that) - but, again, we really don't know. As you say, one might blame 'ignorance', but the thing I find hard to believe is that none of the manufacturers concerned have ever employed an electrical engineer who was sufficiently non-ignorant to enable him/her to detect (and correct) flawed thinking that was being perpetuated.

Kind Regards, John

 

[EFLImpudence]

Arithmatic:

MCBs, for which the CCCs are given, have a fusing factor of 1.45, BS1362 fuses 1.9.
Therefore the cable should be derated by 1.45 / 1.9 = 0.763; 3 / 0.763 = 3.93.

Just as BS3036 fuses have a fusing factor of 2, leading to 1.45 / 2 = 0.725 derating factor.



Ignorance:

Some manufacturers do not demand fusing for their appliances.
We can assume they do know what they are doing.

After all, the requirement for an isolation switch is nothing to do with the operation of an appliance nor an electrical requirement nevertheless it too is included.

 

[JohnW2]

Arithmatic: MCBs, for which the CCCs are given, have a fusing factor of 1.45, BS1362 fuses 1.9. Therefore the cable should be derated by 1.45 / 1.9 = 0.763; 3 / 0.763 = 3.93. ... Just as BS3036 fuses have a fusing factor of 2, leading to 1.45 / 2 = 0.725 derating factor.

OK, so you are talking about a 'fusing factor' which relates to operation in 1 hour or less.

As you say, this point is never mentioned, but its a bit different from the situation with BS 3036s.

Given that BS 3036 fuses are 'singled out' (in 433.1.202), any protective device other than a BS 3036 will satisfy 433.1.1(ii) if In≤CCC. In the case of a BS 3036, where the requirement (per 433.1.202) is that In≤(0.725 x CCC), it follows that, if 433.1.202 is satisfied, that I2≤(2 x 0.725 x CCC) - i.e. I2≤(1.45 x CCC) - so 433.1.1(iii) is automatically also satisfied.

However, in the case of a BS1362 fuse (or any other protective device which is not a BS 3036 and does not have a 'fusing factor' of 1.45), this is not the case. Such a device will satisfy 433.1.1(ii) so long as In≤CCC, but (as you have said above) it will not satisfy 433.1.1(iii) unless:

I2≤(1.45 x CCC)
hence (In x F )≤(1.45 x CCC) [where F is 'fusing factor')
hence In ≤(1.45 x CCC /F)

But, again, I doubt that this is ever thought about in the case of BS1362 fuses. I doubt, however, that this is a cause for concern since, as recently discussed, the CCCs for flex we work with are 'surprisingly low'.

Ignorance: Some manufacturers do not demand fusing for their appliances.
We can assume they do know what they are doing. After all, the requirement for an isolation switch is nothing to do with the operation of an appliance nor an electrical requirement nevertheless it too is included.

All agreed. A lot of small items of equipment do come with instructions that a 'small' (3A or 5A) fuse should be used in the plug, but I suppose that might be justifiable in terms of the size of the cable provided.

Kind Regards, John

 

[EFLImpudence]

OK, so you are talking about a 'fusing factor' which relates to operation in 1 hour or less.
As you say, this point is never mentioned, but its a bit different from the situation with BS 3036s.

Is it? Why?

Given that BS 3036 fuses are 'singled out' (in 433.1.202), any protective device other than a BS 3036 will satisfy 433.1.1(ii) if In≤CCC. In the case of a BS 3036, where the requirement (per 433.1.202) is that In≤(0.725 x CCC), it follows that, if 433.1.202 is satisfied, that I2≤(2 x 0.725 x CCC) - i.e. I2≤(1.45 x CCC) - so 433.1.1(iii) is automatically also satisfied.

I do not see why.
If 433.1.1(iii) is NOT satisfied then then (i) and (ii) are irrelevant. It must satisfy all three - 'and' not 'or'.

Why then are BS3036 fuses singled out as they are only 5% 'worse' than BS1362 fuses?

However, in the case of a BS1362 fuse (or any other protective device which is not a BS 3036 and does not have a 'fusing factor' of 1.45), this is not the case.

Why not?

Such a device will satisfy 433.1.1(ii) so long as In≤CCC, but (as you have said above) it will not satisfy 433.1.1(iii) unless:
I2≤(1.45 x CCC)
hence (In x F )≤(1.45 x CCC) [where F is 'fusing factor')
hence In ≤(1.45 x CCC /F)

For a 3A fuse and 3A cable, it isn't.
Therefore the derating factor applies.

But, again, I doubt that this is ever thought about in the case of BS1362 fuses. I doubt, however, that this is a cause for concern since, as recently discussed, the CCCs for flex we work with are 'surprisingly low'.

They are, so perhaps 3A flex (0.5mm²) is alright with a 6A MCB (given that 100A fuse wire has a c.s.a. of 3.14mm²).