Cluttered!

[JohnW2]

Maybe the 0.4 seconds is arbitrary in the first place, but for aB circuit breaker that's how quick the magnetic trip operates under 5x rated current. Any lower and the magnetic part is not guaranteed to trip, therefore waiting for the overload trip to kick in, which would be a lot later. ... So once you're using an MCB, the decision on impedence is made for you, so don't think that slightly higher loop resistance would necessarily only give slightly slower disconnection.

That's a good point. However, there's no law saying that one must use MCBs. With a fuse, which is equally acceptable to BS7671, it is true that "slightly higher loop resistance would ... only give slightly slower disconnection".

Thinking aloud, maybe that would be a good reason for favouring fuses - not so that one creep over 'max Zs' figures without much consequence, but because the impedance of faults is not necessarily as 'negligible' as the regs would like to think. If the circuit was already very close to the Zs limit (for a 'negligible impedance' fault), just a tiny finite bit of impedance in the fault would be enough to reduce the fault current to less than 5 x In.

In passing, I have always assumed that, as you suggest, the relevant Standards require that, say, a Type B MCB should operate (by implication, magnetically) within the required disconnection time (0.4s for TN) at 5 x 1n. However, I've also always been a bit confused by the box which appears with the Type B curve in the regs, which seems to imply that BS EN 60898 requires disconnection at 5 x In in "0.1 seconds to 5 seconds". Does anyone know what the standard actually requires? (stillp?)

Kind Regards, John

 

[JohnW2]

According to the time current graphs, anything less than 5x results in at least 12 seconds disconnection time.

More like 10.2 seconds but, yes, this is a good point. See what I've just written in response to John D.

Kind Regards, John

 

[EFLImpudence]

I've also always been a bit confused by the box which appears with the Type B curve in the regs, which seems to imply that BS EN 60898 requires disconnection at 5 x In in "0.1 seconds to 5 seconds". Does anyone know what the standard actually requires? (stillp?)

Don't know about the standard, but as we are discussing, whether you need 0.1, 0.2, 0.4 or 5 seconds, the requirements are the same.

 

[EFLImpudence]

More like 10.2 seconds but, yes, this is a good point.

Don't forget, the next division on the graph is 20; not 11

 

[JohnW2]

Don't forget, the next division on the graph is 20; not 11

Good point

Kind Regards, John

 

[JohnW2]

Don't know about the standard, but as we are discussing, whether you need 0.1, 0.2, 0.4 or 5 seconds, the requirements are the same.

The Zs requirement (for an MCB) would, indeed, be the same - but the 'physiological argument' referenced to by John D would make no sense if the 'guaranteed' disconnection time at 5 x In was greater than 0.4 secs. If it is being assumed that ≤0.4 secs is required to save 95% of people from lasting damage or death, then goodness knows what percentage of them would suffer lasting damage (or death) if the disconnection time were allowed to be as long as 5 seconds!

Kind Regards, John

 

[EFLImpudence]

No, but some things are or were allowed to be five seconds but you cannot do that with MCBs.

 

[JohnW2]

No, but some things are or were allowed to be five seconds but you cannot do that with MCBs.

Yes, but it's not really the point. IF (which I very much doubt) the Standard does allow a Type B MCB (which is what the vast majority of people rely upon for their 'protection) to take as long as 5 secs to disconnect at 5 x In, then, in the context we are discussing, protection by a Type B MCB would presumably be unacceptable in terms of the physiological argument that has been presented, wouldn't it?

Kind Regards, John

 

[EFLImpudence]

They don't take 5s - even if that's all you needed. At 5 x In, it will be 0.1 or less.

Not sure what you are getting at.

 

[JohnW2]

They don't take 5s - even if that's all you needed. At 5 x In, it will be 0.1 or less.

Indeed, that would seem to be the case from the curves.

Not sure what you are getting at.

As I said, what I am 'getting at' is the fact that I have never really understood the tabulations in the boxes which appear in the top-right of all the curves in Appendix 4.

Taking the Type B MCB as an example, taken at face value it appears to be giving the maximum current required for a Type B MCB to BS EN 60898 to operate "in 0.1 sec to 5 sec", that current being 5 x In in all cases. How do you interpret that (and/or how do you think it is meant to be interpreted)??

Kind Regards, John

 

[EFLImpudence]

Indeed, that would seem to be the case from the curves.
As I said, what I am 'getting at' is the fact that I have never really understood the tabulations in the boxes which appear in the top-right of all the curves in Appendix 4.

Taking the Type B MCB as an example, taken at face value it appears to be giving the maximum current required for a Type B MCB to BS EN 60898 to operate "in 0.1 sec to 5 sec", that current being 5 x In in all cases. How do you interpret that (and/or how do you think it is meant to be interpreted)??

...because the graphs for fuses have a range of values for the disconnection time (0.1, 0.2, 0.4, 1 & 5s) you require.

These are all the same for MCBs so it says 0.1 to 5s and all are 5 x In.

 

[JohnW2]

...because the graphs for fuses have a range of values for the disconnection time (0.1, 0.2, 0.4, 1 & 5s) you require. These are all the same for MCBs so it says 0.1 to 5s and all are 5 x In.

Yes, I suppose that makes sense. Thanks.

Kind Regards, John

 

[John D v2.0]

5s is also fine as long as it's a lighting circuit or a boiler or something like that. Although I think in reality you'd only use slower mcbs/fuses on sub mains or equipment with a big inrush? The 0.4 only applies for circuits supplying hand held equipment.

 

[JohnW2]

5s is also fine as long as it's a lighting circuit or a boiler or something like that. Although I think in reality you'd only use slower mcbs/fuses on sub mains or equipment with a big inrush? The 0.4 only applies for circuits supplying hand held equipment.

The regs do not seem to differentiate between different types of final circuit - the 0.4s (or 0.2s for TT) appears to be required for any final circuit not exceeding 32A. The only situation of which I'm aware in which a 5s disconnection time is allowed is in a distribution circuit in a TN installation (1s with TT).

What I do find odd, particularly since ADS is in the chapter on "Protection Against Electric Shock", and also given our earlier discussion about the physiological basis for the 0.4s required disconnection time, is the footnote to table 41.1 (teh Table which specifies the required disconnection times) which reads "NOTE: Disconnection is not required for protection against electric shock but may be required for other reasons, such as protection against thermal effects". Any thoughts about that?

Kind Regards, John

 

[John D v2.0]

Oh fair enough maybe I misread which circuits it applied to, or maybe that was for RCD protection it varies. Anyway for the final circuits I'm extending into the new kitchen I'm planning to stay pretty standard so hopefully I won't confuse myself. Technically I am not qualified to change the MCB on any of them anyway, but even the lighting circuits should stay below around 1.2ohms.