Preferred Method

[fattony]

Hi

I wanted to ask peoples opinion on a particular scenario,

Scenario 1
200A Supply into a busbar chamber, this supplies numerous distribution boards but someone has wired a 6mm cable into the busbar which feeds a 20A fused isolator, so potentially you could only draw 20A through the 6mm

Scenario 2
Similarly 100A supply with a 16mm cable which supplies a 63A fused isolator, so the maximum draw is 63A

Scenario 3
32A BS60898 with a 2.5mm cable supplying 1 x switched fused spur, maximum draw is the BS1361 fuse in the spur

Would you have anything wrong with any of the above?

Divided opinions between on our firm

 

[overskilled]

Scenario 3 Def no issue...same as a spur off a ring. Load is limited

 

[fattony]

Load is limited on all scenario's

 

[Leesparkykent]

434.2.2 and 434.2.1 off the top of my head as haven't got the book near me.

 

[davesparks]

How long are these cables, where are they located and what mechanical protection has been provided for them?

 

[Darkwood]

Depends on length of run really as well as the fact that Zs permitted values still need to be met ..haven't enough info for any of the scenario's to be conclusive but all could theoretically comply to regulations.

 

[fattony]

Depends on length of run really as well as the fact that Zs permitted values still need to be met ..haven't enough info for any of the scenario's to be conclusive but all could theoretically comply to regulations.

Well the 32A Zs was too high so it has to be changed and the length of run was 20m max, cable launched above ceiling in armour flex

200A supply on 16mm SWA limited to 63A load, length of run 35m cable tied down a riser, no idea on Zs as unable to test but would presume with the armour and the Zs would be fine

 

[davesparks]

Assuming that in 1 it is the usual arrangement if a busbar with steel trunking for the outgoing cables and the switchfuse attached to that then I see no issue
Assuming that in 2 this is a switchfuses located at the origin with short 16mm tails feeding it then I see no issue
3 however sounds a bit dodgy, if it's a spur from a ring then it'll be ok, but a 2.5mm radial fed via a 32A mcb is unlikely to be ok

 

[davesparks]

Well the 32A Zs was too high so it has to be changed and the length of run was 20m max, cable launched above ceiling in armour flex

200A supply on 16mm SWA limited to 63A load, length of run 35m cable tied down a riser, no idea on Zs as unable to test but would presume with the armour and the Zs would be fine

In which case I would say both are unacceptable
When you say armourflex what exactly do you you mean?

 

[Darkwood]

Well the 32A Zs was too high so it has to be changed and the length of run was 20m max, cable launched above ceiling in armour flex

200A supply on 16mm SWA limited to 63A load, length of run 35m cable tied down a riser, no idea on Zs as unable to test but would presume with the armour and the Zs would be fine

I doubt it will meet Zs values TBH ...you have the cable length and size so it should be too hard to calculate the Zs ...unless you mean 100amp supply as in your original post?

 

[fattony]

In which case I would say both are unacceptable
When you say armourflex what exactly do you you mean?

SY cable, to be honest I thought that was mainly used as control wires not final circuits...

 

[davesparks]

SY cable, to be honest I thought that was mainly used as control wires not final circuits...

I thought that might be it.
It should be restricted to control circuits or similar where emc shielding is required. But it is increasingly being used for general installation work which is not right

 

[fattony]

I thought that might be it.
It should be restricted to control circuits or similar where emc shielding is required. But it is increasingly being used for general installation work which is not right

Quick google offers mixed opinions on this, could you expand as to why its not right in your opinion?

 

[davesparks]

Quick google offers mixed opinions on this, could you expand as to why its not right in your opinion?

The braid does not meet the requirements of an earthed metal sheath/screen in terms of resistance to penetration and it's ability to carry fault current in the case of such a fault.
The sheath is not mechanically strong or resistant to abrasion or other damage.
With specific regards to outdoor installations it is not UV stable or particularly water resistant as you can see once water has got in and green stuff starts to grow under the clear sheath.
Manufacturers data sheets often include words to the effect of 'light duty control and signalling cable not suitable for connection to the public mains supply'

There is another problem which is not the fault of the cable, but another sign of the incompetence of many people claiming to be electricians these days. The cable is often not terminated correctly with the screen often remaining unearthed

 

[davesparks]

And of course I always seem to catch my fingers on the sharp ends of the braid on the rare occasions I work with it