No overload protection for fixed load cable?

[JohnW2]

That regulation has been included in the regulations for a long time — certainly in the 13th edition which was current when I worked in contracting. I used it for just what Holmslaw says it's for – to connect between busbars and switchfuses. ..... Holmslaw has not said that 433.3.1 applies only to busbar/switchfuse connections, only that that's the usual situation. I agree with that. On reading the initial posts I kept thinking "that applies to busbars"; eventually holmslaw said so in a later post.

I didn't think much about this when I first read it, but now I have thought, I'm a little confused.

Just to clarify .... the discussion has been specifically about 433.3.1(ii) (when I've managed to type it correctly!!). In fact, 433.3.1(i) is just common sense (and hardly needs saying), and 433.3.1(iii) is just acceptance of an inevitability (since the has got to be some cable, no matter how short, at the origin of an installation prior to the installation's first protective device.

433.3.1(ii), in itself, simply says that, in the circumstances described, overcurrent protection can be omitted provided that the conductor is protected against fault current as required by Section 434. I would have thought that any device in place which adequately protected a busbar against fault current would inevitably also afford adequate overload protection, in which case 433.3.1(ii) (alone) would presumably be redundant in relation to a busbar - am I wrong?

Are you perhaps talking about not only invoking 433.3.1(ii) to allow the omission of overload protection for a busbar but also simultaneously invoking 434.2.1 to allow the omission of fault protection as well? If so, I guess that makes sense, and may explain some of the confusion in this thread - which has related to a conductor which, per the regs, appears to require fault protection but not overload protection.

Kind Regards, John.

 

[ricicle]

It is not in relation to the busbars themselves but any conductors which are tapped off the busbar and feed a switchfuse which may be a lot lower rated than the busbar.

Say a 800A busbar (protected by a 630A fuse)
Now a 100A switchfuse is fed off the busbar chamber and bolted to the top of it. The conductors feeding the switchfuse are only 25mm² so are clearly not overload protected from the busbar to the switchfuse but the fuses in the sf prevent the cables being overloaded.
They are also short, mechanically protected and cannot cause fire/burns as they are inside a metallic enclosure.
As long as the 25mm² cables are short-circuit protected by the 630A fuses then all is good.

 

[JohnW2]

It is not in relation to the busbars themselves but any conductors which are tapped off the busbar and feed a switchfuse which may be a lot lower rated than the busbar.
Say a 800A busbar (protected by a 630A fuse)
Now a 100A switchfuse is fed off the busbar chamber and bolted to the top of it. The conductors feeding the switchfuse are only 25mm² so are clearly not overload protected from the busbar to the switchfuse but the fuses in the sf prevent the cables being overloaded.
They are also short, mechanically protected and cannot cause fire/burns as they are inside a metallic enclosure.
As long as the 25mm² cables are short-circuit protected by the 630A fuses then all is good.

Thanks. Right, that makes much more sense, I thought people were talking about the busbar itself.

Having said that, I'm not sure that this is relevant to the discussion which was going on, because I don't think this is actually a case in which one would want/need to invoke 433.3.1(ii) to allow omission of overload protection for the 25mm² cable feeding the switchfuse. In fact that 25mm² cable does have overload protection (the switchfuse itself), albeit downstream, but as permitted by 433.2.2. So, whilst I understand that what you describe is compliant, I don't think it really explains why some people seem to think that 433.3.1(ii) is primarily relevant in relation to busbar situations - but maybe I'm still missing something. What do you think?

Kind Regards, John.

 

[ricicle]

433.2.2 is the regulation that is applicable to the busbar scenario.

 

[JohnW2]

433.2.2 is the regulation that is applicable to the busbar scenario.

Indeed, I think that's what I just wrote So do you agree that it's not correct that the reg we were discussing (433.3.1(ii)) is not primarily relevant to busbar scenarios (as was being suggested)?

Kind Regards, John.

 

[ricicle]

Whoever was suggesting such a thing obviously doesn't know their Siemens from their Farad

 

[JohnW2]

Whoever was suggesting such a thing obviously doesn't know their Siemens from their Farad

Thanks. It was at least two people, actually, (one of whom doesn't like being wrong!) but thanks for confirming that I'm not going mad!

Kind Regards, John.

 

[Electrifying]

Nothing wrong with having a 2.5mm cable feeding a double socket outlet - protected by a 32 A MCB, then.

 

[JohnW2]

Nothing wrong with having a 2.5mm cable feeding a double socket outlet - protected by a 32 A MCB, then.

Reaching for my Devil's Advocate hat again (since, as always, I'm no supporter of these ideas!) .... I suppose you might just about try to get away with that one if the cable were clipped direct (Iz=27A) and you were confident that the total load on that double socket would never be greater than 2x13A (i.e. assuming that no-one would try wiring their shower to it with a copper rod in place of the fuse in the plug!). However, I think that, even for those who want to do it, invocation of 433.3.1(ii) really requires that the loads are hard-wired - so that one knows exactly what they are and therefore can make proper judgements about the level of risk of overload.

Kind Regards, John.

 

[Adam_151]

The busbar arrangement discribed is all well and good... until some numpty sticks a fire alarm switchfuse on the end and feeds it in 2.5mm from the busbar.... not protected against shorts at all (the cable would go before the fuse!) and also small enough that it could slip into the trunking turnbuckles....

 

[Electrifying]

I suppose you might just about try to get away with that one if the cable were clipped direct (Iz=27A) and you were confident that the total load on that double socket would never be greater than 2x13A

However, I think that, even for those who want to do it, invocation of 433.3.1(ii) really requires that the loads are hard-wired - so that one knows exactly what they are and therefore can make proper judgements about the level of risk of overload.

Kind Regards, John.

You'ld better get on to the IEE (IET) then, because they allow it in appendix 15.

If I added a few words to what I suggested, would that make you accept the statement?

Nothing wrong with having a 2.5mm cable feeding a double socket outlet - protected by a 32 A MCB with a 4mm radial connected to it, then.

 

[holmslaw]

..

 

[JohnW2]

You'ld better get on to the IEE (IET) then, because they allow it in appendix 15.
If I added a few words to what I suggested, would that make you accept the statement?

Nothing wrong with having a 2.5mm cable feeding a double socket outlet - protected by a 32 A MCB with a 4mm radial connected to it, then.

I realised where you were coming from (the same would be true if you said 2.5mm² RFC), but I was trying to keep my answer in context of 433.3.1(ii) - i.e. to point out that one would not invoke 433.3.1(ii) in order to establish compliance of a 2.5mm² single-socket unfused spur/branch from a ring or radial final circuit. As you do above, people cite Appendix 15 (albeit that's only meant to be 'informative') to do that.

However, to be serious, it is interesting that eyebrows would probably raise very high (and 'red marks' would probably be put on PIR reports) if a double socket was fed with 2.5mm² cable directly and soley from its own 32A MCB - even though that's obviously not functionally different from the spur situation.

Kind Regards, John.

 

[Electrifying]

I realised where you were coming from (the same would be true if you said 2.5mm² RFC), but I was trying to keep my answer in context of 433.3.1(ii) - i.e. to point out that one would not invoke 433.3.1(ii) in order to establish compliance of a 2.5mm² single-socket unfused spur/branch from a ring or radial final circuit. As you do above, people cite Appendix 15 (albeit that's only meant to be 'informative') to do that.

However, to be serious, it is interesting that eyebrows would probably raise very high (and 'red marks' would probably be put on PIR reports) if a double socket was fed with 2.5mm² cable directly and soley from its own 32A MCB - even though that's obviously not functionally different from the spur situation.

Kind Regards, John.

Whether you cite 'Appendix 15' (which, by the way, is informative because it complies) - or you quote any other regulation - the bottom line is it is allowed to omit overload protection because it is not possible (if the circuit is un-altered) to overload the 2.5mm cable.

As you correctly stated though, conditions relating to fault protection - and, in particular, the size of the CPC, need to be adequate.

Indirectly, the two 13 A fuses in the plugtops provide overload protection - when they are plugged in.

 

[ricicle]

The busbar arrangement discribed is all well and good... until some numpty sticks a fire alarm switchfuse on the end and feeds it in 2.5mm from the busbar.... not protected against shorts at all (the cable would go before the fuse!) and also small enough that it could slip into the trunking turnbuckles....

So it sems despite my advice you still haven't bothered to read 434.2.1. All over the uk there are bus-bar chambers fed by >400Amp supplies, and most will have an unfused 2.5 cable supplying a fire alarm system installed by 'some numpty' in full compliance with 433.3.1 (ii).

Or perhaps you could explain another method of how to connect a >400a rated cable into a 20/30A rated switchfuse.

Or perhaps you could show us your calculations that prove a 400A fuse will provide short-circuit protection for a piece of 2.5mm²