TN-S with a BS951 clamp and black tar covering supply cable?

[HappyHippyDad]

... well, a clamp is a ring ... therefore ... still way too small!

Not the clamp itself, the sleeve.. which is 10mmx0.5mm (approx)(so CSA of 5mm².. this is what the fault current will pass through. If the CPC was attached to the clamp in the usual way as it is with a BS951 then I wouldn't be as worried as there would be a much larger CSA for fault current to pass through as the current would utilise more of the clamp.

 

[Richard Burns]

Oh Richard!!!!

Well if copper is rated at 100% (because Copper is the standard by which electrical materials are rated), and we take steel as 15 (I'm being optomistic as it 3-15%) then steel is only 3/20 as conductive as copper.

If the clamp is 10mm x 0.5 it has a csa of 5mm.

5mm x (3/20) = 0.7mm (equivalent of copper)..

In otherwords 'way too small'!!

Of course thats all far too simplistic and ignoring many other factors that I dont understand.. A clamp MUST be a better conductor than 3/20 of copper otherwise we wouldn't use them as a bonding clamp!

k1/k2 * S
k1 for copper is 143, k2 for steel is 51, S= 3.63mm²
so S for steel needs to be (143/51)*3.63 = 10mm², you only have 5mm² (approx)

Hope they actually do something about it.

 

[HappyHippyDad]

Thanks Richard..

Of course!! I'd only just been looking at table 54.2!

I've never used the K factor for steel before.. and there was me trying to be all clever with conductivity ratios.. just looking foolish really! :smile5:

 

[Richard Burns]

Conductivity is for bonding conductors.
For a cpc you need to account for the short time of the fault and the high fault current to ensure they do not melt, deform, etc.

 

[davesparks]

The straps on those clamps aren't usually steel, they are normally tinned copper.

 

[davesparks]

Not the clamp itself, the sleeve.. which is 10mmx0.5mm (approx)(so CSA of 5mm².. this is what the fault current will pass through. If the CPC was attached to the clamp in the usual way as it is with a BS951 then I wouldn't be as worried as there would be a much larger CSA for fault current to pass through as the current would utilise more of the clamp.

There shouldn't be a cpc connected to the clamp at all!

 

[HappyHippyDad]

There shouldn't be a cpc connected to the clamp at all!

Sorry, poor terminology.

 

[Richard Burns]

The straps on those clamps aren't usually steel, they are normally tinned copper.

Hmm, not likely with the strain they need to be put under to screw them tight.

 

[davesparks]

Hmm, not likely with the strain they need to be put under to screw them tight.

There's not a lot of strain on them, its a curved plate which sits against the cable with the band held against the back of it.
It's not a bs951 pipe clamp.

 

[sam400]

I had a loose TN-S clamp on a CU change the other day - so I got the DNO out and they said it was fine,
different rules for them and us I suppose!