Supply with SWA

[bengasman]

For some odd reason one of my clients has the supply to the cut out in SWA and from all I could see the only earth is with a BS 951 clamp to the MET. Earth loop measures ok at about 1.5 Ω measured from socket nearest to supply.
Q1. Is a BS 951 acceptable on SWA?
Q2. How bad, in the real world, is earth supply from SWA mantle only?

TIA.

 

[EFLImpudence]

Are you sure it's swa and not concentric?

Anyway, see this thread for your query.
I don't think it is acceptable but few seem to care.
//www.diynot.com/diy/threads/tn-s-with-a-tenby-clamped-lead-sheath-for-earth.417039/

 

[bengasman]

I'm fairly sure it's SWA and not concentric although I wouldn't wager my live savings on it as I have only seen the outside.
Is there an easy way to tell from the look of the mantle? I'm not planning to open anything up to have a look inside.

Would a BS951 clamp be an acceptable (even if not ideal) way to connect the earth from a BS 7870 cable to MET?

I looked at the thread in the link and it doesn't seem to relate to my question as the cable here is definitely not lead. DNO here are usually happy to pop around and put a proper lead connector on to save the risk of more cost from damage to lead cable from BS 951 clamp any way.

 

[EFLImpudence]

Would a BS951 clamp be an acceptable (even if not ideal) way to connect the earth from a BS 7870 cable to MET?

As I said, I don't think it is but ...

I looked at the thread in the link and it doesn't seem to relate to my question as the cable here is definitely not lead.

No I realise that but the clamp is the same and it seems to be deemed acceptable nowadays.

 

[bengasman]

Thanks for that.
Just out of curiosity, is there a way to tell the difference between SWA and BS 7870 cable, just by looking at the outside?

 

[ricicle]

1.5 ohms sounds a tad on the high side for a TN system taken at a socket near the intake position.

 

[bengasman]

1.5 ohms sounds a tad on the high side for a TN system taken at a socket near the intake position.

That would give 150A with earth fault condition, isn't that enough?

 

[JohnW2]

1.5 ohms sounds a tad on the high side for a TN system taken at a socket near the intake position.

That would give 150A with earth fault condition, isn't that enough?

It's very marginal - you theoretically need at least 160A if the circuit is protected by a B32 MCB.

However, that's not really the point - as ricicle has said, for a socket near to the supply origin, and EFLI of 1.5Ω suggests that there is something not quite right with either the TN earth or the connection from it to the CU.

Kind Regards, John

 

[RF Lighting]

BS951 clamps on SWA were commonplace especially in private street lighting installations. Whilst not ideal it does the job, but I'd want to take a proper Ze reading to make sure it is ok.

 

[bengasman]

That impedance being on the high side could well be at least partially due to the incoming earth connection being a bit sloppy. It looked far from clean and the clamps wasn't particularly tight either, hence my question on here about a BS 951 clamp being acceptable before cleaning it up and tightening it properly. The reading was slightly less than 1.5, 1.4-something so all I need is to drop it 0.1 to get into the theoretically perfect range. Upgrading all the earth cables to one size bigger and making sure all the connections there are tight enough will give another one or two tenths gain.

Coming back to the original question, can I tighten that clamp without too much risk of damaging the mantle?

 

[RF Lighting]

No. If the Ze reading is too high call the DNO. They are obliged to repair it at their expense. Besides, it's their equipment and you are not permitted to work on it.

 

[JohnW2]

The reading was slightly less than 1.5, 1.4-something so all I need is to drop it 0.1 to get into the theoretically perfect range.

There would be nothing "theoretically perfect" about a reading of ~1.3Ω at a socket very close to the origin - that would indicate a TN earth which could well be iffy and, as has been said, you should get the DNO to check and, if necessary, rectify it.

Kind Regards, John

 

[bengasman]

No. If the Ze reading is too high call the DNO. They are obliged to repair it at their expense...

Is that regardless of who the DNO is in that area?
I've found BS 951 clamps on lead mantles and was quote money for them to come around and correct it.

 

[RF Lighting]

The DNOs are duty bound to maintain any earth they supply. That means if a Ze is too high they must repair it at their expense.

Just having a BS951 clamp does not mean any action needs to be taken, so it will be chargeable to replace it.

 

[bengasman]

Thank you, Good Sir, that's more or less what I wanted to know.