2.5 ohm lead sheaf

[unconditional]

Hi customer of mine currently has 2.5 ohms on a old lead sheaf, unfortunately its one like many where the braized earth has been cut off and a pipe clamp stuck on the sheaf, leading to the local networks to give their normal response- not our responsibility, can't be bothered, not enough profit in providing an actual service etc

My angle in this is 2.5 ohms a) suggests a fault somewhere at their end and b) if you have a fault between the service head and the street would it be enough to operate the ~600amp fuse on the network side, so yes they do have a responsibility to sort it.

Have told the client to ask for, in writing, the supply is safe but any other ideas or help in enticing them to do their job? For example are the any electric distro rules that specify the sheaf can only be TNS therefore require 0.8?

Thanks Ben

 

[Pete999]

Lead sheath, sorry but I just had to, no offence

 

[Wilko]

Hi - just my simple thought - get owner to email DNO to confirm earthing status of the service is "supplied as TNS". I think they all have 0.8ohm upper limit targets and they should maintain it. Maybe I'm wrong, but at 2.5ohms won't the the earth fault loop impedances be exceeded in the 16A+ circuits?

 

[westward10]

Earth has broken down outside and agree with Wilko if rcd protection isn't present some circuits will not be complying with fault protection.

 

[unconditional]

Hi thanks yes thats my or my clients immediate problem, but as the DNO like to say, its not their problem.

Fact is it's the middle of london and theres undoubtedly an earth available and for them to investigate/remedy it would be far preferable to the spike alternative but (not for the first time) they simply refuse to entertain the idea with the statement they're not obliged to provide an earth. Try - but their cable/supply must be in "decent order" of some sort, and poor earth readings on the sheaf suggest otherwise

Client will be pretty annoyed if I TT it which will mean replacing the CU - 20way/big house so RCBO's called for really - and how do you meet TT and fireproof CU regs? Under the strairs so plastic CU & line with fireboard? - so not small job - you can bet your last $ I do all that and whatevers causing the fault will go pop properly in six months and he'd get a new supply making all this work somewhat unnecessary.

 

[westward10]

Typical London always digging up roads and interfering. If the SHEATH of the cable is not effectively earthed, it is an exposed-conductive part and in my opinion they are responsible for it.

 

[soms]

Have a look at the Electricity Safety, Quality and Continuity Regulations 2002.
Part VII, section 24, regarding equipment on consumer's premises.

 

[Wilko]

Thanks Soms, that's spot on. I can see me quoting this at some point ...

Hopefully this link takes us directly there :

The Electricity Safety, Quality and Continuity Regulations 2002 - http://www.legislation.gov.uk/uksi/2002/2665/regulation/24/made

 

[unconditional]

Thanks for those, I was chatting with a guy from nic tech who suggested the same document.

13 - 2b looks usefull: - would a seemingly/somewhat compromised earth be a clear case of it not being met, enough to provoke the DNO into action?

Assuming a 600amp fuse back at the sub you need a 0.4 ohms to generate that current under short circuit.

Protective screens
13.—(1) Underground cables and associated equipment which contain conductors not connected with earth shall be protected in accordance with paragraph (2).

(2) The protection referred to in paragraph (1) shall comprise—

(a)in respect of joints or terminations of a conductor in a low voltage system, some form of mechanical protection; and

(b)in respect of any other part of any conductor, an electrically continuous metallic screen connected with earth,

so placed as to ensure that, so far as is reasonably practicable, any tool or device likely to be used in the vicinity will make contact with that protection or screen before it can make contact with any conductors not connected with earth.