Spur from shower switch

[aptsys]

433.3 has nothing to do with it. The cable is protected against overload by the fuse in the FCU. Anyway, this will not happen with one light and one fan.

The 3m. restriction only applies if the cable does not meet fault current requirements.

If this is the case then the cable must be installed in such a manner to reduce the risk of a fault AND the risk of fire to a minimum.
I would think that means installed in metal conduit.

However, I doubt that will be the case.

I would disagree with this. Unless the FCU is installed at the point where a cable with reduced CSA is connected to the circuit, 433.3 or 433.2.2 must be complied with. Do we know 434 can be complied with for an undefined length of 1mm² cable on a 40A MCB?

 

[DetlefSchmitz]

That's only permissible for lightning circuits. A shower circuit is not a lightning circuit.

I thought that lightning conductors had their own set of rules.

 

[EFLImpudence]

I would disagree with this. Unless the FCU is installed at the point where a cable with reduced CSA is connected to the circuit, 433.3 or 433.2.2 must be complied with.

433 is about omission of overload protection.

Overload protection has not been omitted. It is afforded by the FCU wherever it is.

Do we know 434 can be complied with for an undefined length of 1mm² cable on a 40A MCB?

It is not an undefined length.
We do not have the figures but it is extremely unlikely not to be and extremely unlikely to happen.

 

[Risteard]

I thought that lightning conductors had their own set of rules.

Stupid autocorrect.

 

[flameport]

Do we know 434 can be complied with for an undefined length of 1mm² cable on a 40A MCB?

Unknown without further information, that being the fault current.

434.5.2 - k²S² must be greater than I²t
For the 1mm cable, k is 115, so what matters is that I²t for the particular circuit breaker and fault current is less than 13225.
That could be true in some circumstances, but can easily not be in others.

This is for an MK MCB - complies for 500A or 1500A. Not for 6kA.

Other manufacturers are available and may be different.

 

[JohnW2]

434.5.2 - k²S² must be greater than I²t .... For the 1mm cable, k is 115, so what matters is that I²t for the particular circuit breaker and fault current is less than 13225. That could be true in some circumstances, but can easily not be in others. .... This is for an MK MCB - complies for 500A or 1500A. Not for 6kA.

I presume that what is plotted on the y-axis of that graph is I²t, in A².s - but "A_/sec" seems to be an extraordinary way of trying to express that!

I have to say that when I put on a mathematical hat, it has always seemed very odd to see people plotting I²t again I - it would seem far more logical to simply plot t again I (as per standard t vs. I plots, as shown in BS7671)!

In any event, the bottom line in practice is that, even if Ze were zero (which it obviously never will be), if the run of 1mm² cable is more than about 3.5m (which it very commonly will be), then the fault current (for a fault at the end of the cable) will be less than 1,500A. Indeed, even if the cable were of negligible length (which I suppose is the situation that, strictly speaking, one should consider if one wants to allow for a fault anywhere along the length of the cable), the Ze would have to be below about 0.15Ω for the fault current to exceed 1,500A.

Kind Regards, John

 

[JohnW2]

I have to say that when I put on a mathematical hat, it has always seemed very odd to see people plotting I²t again I - it would seem far more logical to simply plot t again I (as per standard t vs. I plots, as shown in BS7671)!

To iullustrate my point, I've read the figures (for the B40) roughly from the graph which flameport posted and re-plotted them as a straightforward "t vs I" plot. This differs from what we see in BS7671 only in as much as the BS7671 curves goes no lower than 100 ms disconnection time, whereas the below ones go no higher than 10 ms.

I think one way to think about this is that there will inevitably be a lower limit to disconnection time (governed by physical/mechanical factors), and that is more-or-less reached (with the B40) at currents of around 1,500A - beyond which the disconnection times remains in the range 2.5-4.5 ms (and I doubt that it ever gets much below 2.5 ms, no matter how high the current). By the same logic, I would suspect that the disconnection time never gets much below 2.5 ms for an MCB of any rating, at any current.

Kind Regards, John

 

[Jupiter01]

UPDATE: This is wired in and working beautifully
The extractor switch threw me slightly as it wasn't clear on which set of terminals expect the cable from the light versus the extractor. I guessed it and got it right!

Thanks again.