What would you put on the cert?

[Dave 85]

Did a job today where I had a 6mm T&E on a 32amp retrofit MCB (re used old circuit) going into a RCD spur (13amp) then a 2.5mm going off to a shower pump. There is also an unswitched 3amp fused spur halfway down the 2.5mm with a 1mm going to a mirror light.

Sounds a bit of a muddle I know but was the only way I could do it. 3036 Fuseboard full and hard to get cable into, existing lighting circuit impossible to extend as it was buried in concrete ceiling.

My question is, on the cert, in the overcurrent protection box do I state the 32amp 60898, 13 amp 1361 or 3 amp 1361. And in the cable box do I put the 6mm, 2.5mm or 1mm.
Im guessing I'd mark it down as a 32 amp 6mm circuit as the other bits are essentially fused spurs but just wondered if anyone thought different.

 

[somersetsparks]

If you spur a 1mm of a 2.5 RFC Of course you only have to meet the disconnection time for the 1362, seen it done plenty of times.
I like the analogy of circuit of a circuit and can see absolutely no good electrical reason for it not to comply.

 

[Octopus]

Not trying to be difficult, but if you use the OCPD of 32A for a spur off a ring which has been fused down to 13A, and the cable to the "device" is now 1.5mm, surely you would need to change the OCPD to 16A to protect the cable, which would be daft at the extreme.

 

[brman]

yes, I know we discussed this recently in another thread and I still believe it is ok treat the FCU as the PD when looking at the circuit design. You do so for cable capacity so why not Zs?
To take it further, what if it is a RCD FCU? (which I guess it must be in this case as the main board is BS3036?) If you are to allow 1667 for max Zs then you are definitely treating the FCU as the PD for the circuit in relation to max Zs.

EDIT: which I meant to say means it is more logical to put the FCU on the cert, not the CU MCB as that is the PD for the circuit you have installed (and so are certifying).

 

[Guest55]

Not trying to be difficult, but if you use the OCPD of 32A for a spur off a ring which has been fused down to 13A, and the cable to the "device" is now 1.5mm, surely you would need to change the OCPD to 16A to protect the cable, which would be daft at the extreme.

Because the fuse spur limits the amount of current that can flow through the 1.5 , so no need to fit a smaller mcb at the CU.
i.e. - there is no chance of the full 32A passing thrugh the smaller cable.

 

[Octopus]

Because the fuse spur limits the amount of current that can flow through the 1.5 , so no need to fit a smaller mcb at the CU.
i.e. - there is no chance of the full 32A passing thrugh the smaller cable.

Yup and thats why the MWC should quote the OCPD as the fuse in the FCU and not the MCB in the board IMHO.

 

[brman]

Because the fuse spur limits the amount of current that can flow through the 1.5 , so no need to fit a smaller mcb at the CU.
i.e. - there is no chance of the full 32A passing thrugh the smaller cable.

and it also ensures the FCU fuse will go in less than 0.4s (or whatever) in the event of a earth fault, so meets the max Zs requirement. In what way is it not the PD for the additional circuit?
:lol:

 

[Guest55]

and it also ensures the FCU fuse will go in less than 0.4s (or whatever) in the event of a earth fault, so meets the max Zs requirement. In what way is it not the PD for the additional circuit?
:lol:

I've read that 3 times and i'm still confused.com lol.

 

[Geoffsd]

Well I'm hoping as this new cable would appear to be for a bathroom circuit that the Zs will be fine at 1667

Does no one else disagree with a max. Zs of 1667Ω for a 32A mcb?

The max. Zs should still be 1.44Ω even when the circuit is RCD protected.

 

[Guest55]

Agreed , poor design otherwise.

 

[Dave 85]

Christ, I was worried I'd asked a silly question but It seems like I've opened a can of worms. Problem is If you gonna treat one fused spur as a circuit, are you gonna treat them all as such? Cos I've never heard of anyone doing a cert in this way.
What I would suggest is that I take the Zs from the furthest point on the 6mm as anything downstream of a 13a fuse could be viewed in the same way as a 30 meter ext lead plugged into a socket.
Food for thought.

 

[Guest55]

What I would suggest is that I take the Zs from the furthest point on the 6mm as anything downstream of a 13a fuse could be viewed in the same way as a 30 meter ext lead plugged into a socket.
Food for thought.

Not quite Dave.
Anything that can be disconnected via a plug is not considered as fixed wiring , and can be ommited from the tests.

 

[sedgy34]

can of worms there alteration of the 6mm circuit would be a distribution circuit and then max zs on the end of the load of the fcu.

 

[brman]

I've read that 3 times and i'm still confused.com lol.

yeh, I just read my comment again and i don't know what I meant either :lol:

I think I read it that you were arguing previously that you shouldn't use the fuse in the FCU to assess max Zs. However you then pointed out that you do use the fuse in the FCU when assessing cable capacity. So I was suggesting that, if you use the FCU to assess cable capacity then why can't you use it to assess max Zs? And if you do that then you are affectively treating the FCU as a DB and circuit off that as separate final circuit so why not record it as such.

 

[Guest55]

Well , i cant help how others interpret wiring arrangements , its a free country , live and let live.
But.....
No-one actually writes out an EIC for a spur as the beginning of a circuit do they ? lol

 

[Octopus]

Well , i cant help how others interpret wiring arrangements , its a free country , live and let live.
But.....
No-one actually writes out an EIC for a spur as the beginning of a circuit do they ? lol

No. I'm getting the feeling that this thread has more pages to go...

lol