Minor works testing FCU

[dougquade]

Hi wondering if someone can advise me on something..

Im installing a few outdoor lights, will spur them off a ring via fused spur. question is when i’m testing do I take the max zs values of a 3/5a 1362 plug top fuse or zs at light fitting and use max zs for the mcb?

Please explain in detail, I’m a bit dyslexic

Regards

Big D

 

[pc1966]

You would use the max Zs figures for the fuse (3A fuse to BS 1362 has max Zs 15.6 ohms on hot circuit) and measure it at the furthest light, but ideally check every light if there are a few on the outside circuit.

That is assuming that the point where the FCU is in the ring already meets the Zs for the MCB, but that ought to be the case unless you have an extraordinary length of cable.

Also the supply that is feeding the outside light should be RCD protected, any new (or recent) ring would have such RCD protection but check it does, and also test that the RCD trips and resets OK using its test button.

RCD protection would allow a higher Zs value to still meet automatic disconnection, as you generally have to do with a TT earthing arrangement, but if practical I would always aim to meet disconnection on the over current protection protection (MCB/fuse).

 

[pc1966]

Realistically you should not be anywhere close to the max Zs for a 3A fuse.

For example, if you do the usual design and assume cold-measured cables at 80% of Zs then your maximum cable length for 1mm cable (44 ohm/km) is about 259m.

Ring final has max Zs which should be below 1.1 ohms to meet disconnection for a 32A B-curve MCB and that is your max "Ze" at the FCU
Fuse Zs hot = 15.6 ohms so 80% cold = 0.8 * 15.6 = 12.5 ohms
Max 1mm light cable R1+R2 = Zs - Ze = 12.5 - 1.1 = 11.4 ohms
Max new 1mm cable length = 11.4 / 0.044 = 259m

It is also doubtful that the light cable would run hot, certainly nothing like 70C, so you have some additional margin for disconnection.

Depending on the load you might run in to voltage drop problems before the maximum Zs length, but if only a couple of 30W or 50W lamps then probably not.

Basically your dead measurement of R1+R2 and/or live measurement of Zs at the furthest light is more above verifying the circuit has been properly implemented.