No overload protection for fixed load cable?

[ban-all-sheds]

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 .

As long as the person scribing the "red marks" could

1) Explain what electrical engineering principles make that arrangement no good unless a ring final or a ≥ 4mm² radial also originates at the same MCB

and

2) show which regulations merit a "red mark" unless a ring final or a ≥ 4mm² radial also originates at the same MCB

then that would be fine.

 

[holmslaw]

..

 

[JohnW2]

[As long as the person scribing the "red marks" could
1) Explain what electrical engineering principles make that arrangement no good unless a ring final or a ≥ 4mm² radial also originates at the same MCB
and
2) show which regulations merit a "red mark" unless a ring final or a ≥ 4mm² radial also originates at the same MCB
then that would be fine.

I totally agree (in terms of the regs) - but don't you think that many probably would (they might say 'intuitively') 'feel that there must be something wrong' and spend time trying to think of a regulation that had been violated?

... how about this one .... 3 separate 2.5mm² cables, each serving one single socket, all orginating from the same 40A MCB (with nothing else connected to it)?

Kind Regards, John.

 

[ban-all-sheds]

I totally agree (in terms of the regs) - but don't you think that many probably would (they might say 'intuitively') 'feel that there must be something wrong' and spend time trying to think of a regulation that had been violated?

Maybe they might, but having thought about it, and realised that they could do neither (1) nor (2), then it would be professional misconduct to mark it negatively on a PIR.

... how about this one .... 3 separate 2.5mm² cables, each serving one single socket, all orginating from the same 40A MCB (with nothing else connected to it)?

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

 

[echoes]

Is the commonly given advice to folks that an FCU is needed to connect a single < 3kW oven to a 32A radial where the final connection to the oven is < 4mm² incorrect? I would have thought so, provided the 32A MCB provided fault protection.

(Assuming MI's do not dictate)

 

[holmslaw]

..

 

[JohnW2]

... how about this one .... 3 separate 2.5mm² cables, each serving one single socket, all orginating from the same 40A MCB (with nothing else connected to it)?

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

To be honest, I haven't checked, but I was guessing that it probably would be.

Kind Regards, John.

 

[JohnW2]

Is the commonly given advice to folks that an FCU is needed to connect a single < 3kW oven to a 32A radial where the final connection to the oven is < 4mm² incorrect? I would have thought so, provided the 32A MCB provided fault protection.
(Assuming MI's do not dictate)

On the assumption that you could demonstrate that the "<4mm²" final cable was getting adequate fault protection friom the MCB, then, per all the discussions we've had here, I think that, as you say, it is not correct to say that an FCU is required. However, if you asked me whether I would install an FCU, whether it was 'required' or not, my answer would be Yes.

Kind Regards, John.

 

[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

FSS, it doesn't read 434.2.1.

I apologise - I was overlooking the fact about the 2.5mm feeding a fire alarm. In all other non-safety instances short circuit protection would still be needed.

 

[echoes]

Thanks John, but other than 'feeling' right, would the 13A fuse make it safer to any measurable degree? (I'm sure we could think of ways in which an oven could theoretically overload if we tried hard enough, but resist looking for a problem, given a solution.)

 

[holmslaw]

..

 

[JohnW2]

Thanks John, but other than 'feeling' right, would the 13A fuse make it safer to any measurable degree? (I'm sure we could think of ways in which an oven could theoretically overload if we tried hard enough, but resist looking for a problem, given a solution.)

I can't really answer that any better than you've done there yourself. If one feels that it is extremely unlikely that the oven will overload, then there's surely no sense in which it would be any safer with a 13A fuse in an FCU, is there? As I see it, only if you felt that there was a significant risk of overload would that fuse produce any significant increase in safety.

Kind Regards, John.

 

[ricicle]

The 2.5 could be feeding anything and it doesn't need any circuit protection as long as the installation method complies with 434.2.1.

That regulation doesn't say that short-circuit protection can be omitted altogether (unless it's one of the types of circuit listed as exceptions)

 

[EFLImpudence]

On the assumption that you could demonstrate that the "<4mm²" final cable was getting adequate fault protection friom the MCB, then, per all the discussions we've had here, I think that, as you say, it is not correct to say that an FCU is required.

In the past in related topics others have argued that the above is correct.

However, I have always given the advice to the OPs that the cable should be protected against overload.

I feel this is prudent on a 'DIY' forum when giving advice.

 

[JohnW2]

On the assumption that you could demonstrate that the "<4mm²" final cable was getting adequate fault protection friom the MCB, then, per all the discussions we've had here, I think that, as you say, it is not correct to say that an FCU is required.

In the past in related topics others have argued that the above is correct.
However, I have always given the advice to the OPs that the cable should be protected against overload.
I feel this is prudent on a 'DIY' forum when giving advice.

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 don't think we should be dishonest in telling posters what the regs say/technically require (or not), but I also think that (as per that extra sentence of mine) one should also give what we feel is prudent advice.

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