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

[HappyHippyDad]

Afternoon all..

I have just completed an EICR and came across the following :



1. Is the black tarry substance Pitch?
The homeowner is unsure how long it has been there, however it is not completely solid and is slightly sticky to touch indicating that it is still getting too hot at times and leaking? I will be contacting DNO but wanted to get some more information from those who are experienced with this?

Max demand is approx 50A so I would be more concerned with loose connections in the DNO's equipment.

2. TN-S is supplied by a BS951 clamp which I understand was done for a period. My concern is that the 6mm bare cable from the clamp to the MET is just connected to the sleeve of the clamp that is moveable. It is a firm connection to that part of the clamp via the bolt, but just looks awful. I'll tell the DNO about this as well but again wanted some opinions please?

Ze = 0.17Ω
Cut out fuse 100A (not actually seen - but stated on cut out)

Thanks.

 

[davesparks]

Why 7/029? It looks bigger than that I think. That equates to 2.98mm and it looks bigger, possibly 7/036?

Adiabatic shows that CPC needs to be 3.63mm or above!

I have a fancy pair of calipers that usually show me the CSA if in doubt, but with 7 old tangled strands it's difficult!

You might be right, I've got the old regulations at home so I can look up what the required size was at the time, it may be 7/044.

 

[davesparks]

Thanks Dave,

It does look slightly different to a 951. It is more they way it is connected. Can you see from the picture what I am getting at? The fact that it is connected to the flapping sleeve rather than firmly against the clamp as you would see with a 951?

I am familiar with that type of clamp yes, it's quite common in the SSE area. There is another one which is just that length of that band wrapped round the lead sheath with a bolt and two wedge shaped nuts to hold it tight, then a nut and bolt with a castle washer through the band in exactly the same way.
Is there a castle washer on that termination?

 

[HappyHippyDad]

But what is the copper equivalent csa of a 10mm x 0.5mm strip of flexible steel?

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!

 

[GB.Kayak]

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!

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

 

[HappyHippyDad]

I am familiar with that type of clamp yes, it's quite common in the SSE area. There is another one which is just that length of that band wrapped round the lead sheath with a bolt and two wedge shaped nuts to hold it tight, then a nut and bolt with a castle washer through the band in exactly the same way.
Is there a castle washer on that termination?

I'm sorry Dave, I dont know.

 

[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!