No overload protection for fixed load cable?

[Adam_151]

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

[/quote]

I never stated for one moment that the cable needs to be protected against overload by the >400 fuse... the 20/32A rated switchfuse is quite capable of doing that. What it does need to be protected against by the >400A fuse however is faults and normally something in the region of 16mm is appropiate subject to calculation (and also is much less likely to be subject to mechncial damage by not being small enough to slip into the pinch point of turn buckles on the trunking, like the 2.5mm is)

 

[holmslaw]

..

 

[EFLImpudence]

I agree - which makes me wonder why you seemingly deliberately omitted to quote my next sentence, which read:
However, if you asked me whether I would install an FCU, whether it was 'required' or not, my answer would be Yes.

I was agreeing with you.

I merely quoted the relevant wiring arrangements related to my subsequent comment.

It wouldn't have made sense had I included your reservations on the matter.

 

[Electrifying]

Oh my god, will you stop talking in riddles?

This thread has not really worked out as intended. I started it because BAS had, in the course of another thread, raised this issue of the regs theoretically allowing cables not to have 'adequate' overload protection in some circumstances. I was uncomfortable to have readers seeing that suggestion (whatever the technicalities of the regs) and thought that by starting this new thread, plenty of learned people would rapidly jump in and shoot the idea down - but it just hasn't gone to plan!

Kind Regards, John

There's nothing 'theroretical' about it.

If there is no possibility of overloading a cable - then 'overload' protection may be omitted - it's not 'theoretical', it's fact.

It's supported in the 'informative' Appendix 15, and it's supported in regulation 433.3.1 (ii).

No matter how 'uncomfortable' you may feel and no matter what 'DIY' users might think or do- it's allowed and perfectly acceptable!

Anyone who doesn't understand the concept should leave 'electricity' alone!

 

[Adam_151]

Why have you deleted your post?, Holmslaw

 

[holmslaw]

..

 

[ban-all-sheds]

Perhaps you could explain why you keep on about scenarios of small cables having to be connected to multi-hundred amp circuits inside busbar chambers when the questions actually under discussion in this thread are "does a 16A breaker provide adequate fault protection for a 0.5mm² flex in free air", and "does a 32A MCB provide adequate fault protection for 2.5mm² twin & earth".

 

[holmslaw]

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[ban-all-sheds]

And you can show where he was concerned about 433.3.1 in the context of small cables connected to multi-hundred amp circuits inside busbar chambers rather than thin appliance flex on a 16A breaker or 2.5mm² twin & earth on a 32A one, can you?

 

[JohnW2]

There's nothing 'theroretical' about it.
If there is no possibility of overloading a cable - then 'overload' protection may be omitted - it's not 'theoretical', it's fact. It's supported in the 'informative' Appendix 15, and it's supported in regulation 433.3.1 (ii).

I can't disagree with that. I should not have said 'theoretical'. What I should have said was something along the lines of 'per the word of the regs' - since I can't deny that the regs do permit what we've been discussing (in the circumstances specified).

No matter how 'uncomfortable' you may feel and no matter what 'DIY' users might think or do- it's allowed and perfectly acceptable!
Anyone who doesn't understand the concept should leave 'electricity' alone!

If I were absolutely certain that there was no possibility of overload, I would obviously not personally be uncomfortable at all. Even if I believed there was minimal risk of overload, I still would be pretty comfortable.

What I am uncomfortable about is the potential to confuse those who may not understand the concepts (fully or at all) but who nevertheless fail to take your advice to leave electricity alone. For such people, it's far better to instill in them the easy-to-understand concept that a cable should always be protected by an OPD with a rating no higher than that of the cable. You, I and anyone else who understands the princples and regs may deviate from that when appropriate and permissible, but I believe that it's the safest simple concept to try to impart to others.

Kind Regards, John

 

[JohnW2]

Would a 40A breaker provide adequate fault protection for a 2.5mm² cable?

OK, now that it's a new day, I've got out the back of a cigarette packet. As one might expect, provision of fault protection is very easy. It is, of course, dependent upon the design fault current in the installation, so one can only give representative figures,conditional on the fault current.

For a Type B MCB, disconnection time is <0.1 sec (often <<0.1 sec) for currents over 5In - hence currents >200A for a 40A MCB. With such disconnection times, the process can be regarded as adiabatic.

On that basis (and assuming my cigarette packet has provided correct answers!), for a copper conductor and 70 degrees C ('k'=115), if the design fault current is >200A, it seems that a 40A Type B MCB will provide adequate fault protection for a conductor with a CSA of about 0.55mm² or greater - hence very much OK for 2.5mm².

If the design fault current is >400A, then it seems that a 80A Type B MCB will give adequate fault protection for conductors with a CSA of about 1.09mm² or greater - hence, again, more than adequate for a 2.5mm² conductor

Kind Regards, John

 

[ban-all-sheds]

[DA]But what if the fault is at the end of a couple of m of 0.5mm² flex and the total fault loop impedance means that the fault current is < 5In?[/DA]

 

[JohnW2]

[DA]But what if the fault is at the end of a couple of m of 0.5mm² flex and the total fault loop impedance means that the fault current is < 5In?[/DA]

Whatever the [DA]...[/DA] was designed to achieve, the system clearly hasn't recognised it

Yes, as I said, everything is inevitably dependent on the fault current, and if it's going to be <5In, then one has to rework the calculations accordingly. However, a couple of metres of 0.5mm² flex (about 176m&#937 will often not make the difference between a fault current <5In and a fault current >5In. If, for example, the fault current would have been 240A without the flex in the fault circuit, with that flex it would fall to about 203A - but I agree it could if the FLI would not have been much above 5In (i.e. 200A for a 40A MCB) without the flex. Also note that my calculations were addressing your question in relation to 2.5mm² cable, for which a 40A Type B MCB provides far more protection than necessary with a fault current &#8805;200A - but that, with that fault current, the MCB wouldn't be quite adequate for 0.5mm² conductors (only for &#8805;0.55mm²).

There is, of course, also a caveat I should have added. In the case of a TT installation, the fault protection calculations I presented obviously only apply in the case of L-N faults. With an L-E fault on most TT installations, the fault current will not be high enough to operate even a 6A MCB (ever), let alone one of higher rating – so L-E fault protection in that situation has to be reliant upon an RCD. However, given the characteristics of RCDs, I would have thought that protection will probably always be at least as adequate as corresponding protection from an MCB would have been.

Kind Regards, John.

 

[ban-all-sheds]

Whatever the [DA]...[/DA] was designed to achieve, the system clearly hasn't recognised it

Nor you, it seems...

 

[JohnW2]

Whatever the [DA]...[/DA] was designed to achieve, the system clearly hasn't recognised it

Nor you, it seems...

Well, I did contemplate that it might mean Devil's Advocate (which one could hardly blame the system for not recognising!) but it wasn't really him who was responsible for that post - since it was purely reporting the results of calculations in response to a question from you, I wrote it myself

I suppose I also should have added that, because of my very limited experience of different installations, I really don't have much idea as to what 'typical' fault currents may be. FWIW, with my installation, measurements taken at the furthest point of relatively long RFCs usually give figures of around 600A for L-N and 500A for L-E**. That means that quite a length of 0.5mm² flex would probably be adequately protected (against fault,but obvioulsy not overload) by a 40A Type B MCB if hung off of one of my RFCs.

(** note that the L-E figures relate to my rather atypical installation. As previously discused, although it's actually TT, it appears that I am 'borrowing' a TN earth from neighbour(s) when bonding is in place).

Kind Regards, John.