Laying underground cable

[Haldane]

Hi all, I'm new to this forum. I'm currently retraining and studying for my 17th Edition Regs and stumbled across this (rather old) post.

I'm a bit confused by the following comment:

A twin and earth cable is never allowed to be buried underground, ducting or no ducting. It does not meet the requirements of regulation 522.8.10.

I'm looking at the reg. 522.8.10 and the way I interpret it is that twin and earth could be installed underground if it's enclosed in ducting or conduit but earthed armoured cable can be buried as is, without ducting.

Please could someone clarify as I've got the exam in a couple of weeks and although it may not come up It'd be good to know for sure.

Thanks

 

[ban-all-sheds]

The type of ducting normally used for underground cables does not provide equivalent mechanical protection.

 

[plugwash]

The ESQCR says that burried underground cables MUST be protected with earthed metal. I'm not sure if that applies to private cables though or only to DNO cables.

IIRC BS7671 does allow mechnical protection as an alternative to earthed metal but I wouldn't consider typical cable duct to be mechnical protection. Maybe concrete channeling would be.

Earthed metal conduit could in theory be used but maintaining a good conduit earth in the underground environment is likely to be difficult.

 

[westie101]

ESQCR requires that a DNO cable is surrounded by an "Earthed Metallic Sheath" which can be any metal!
Present cables we use have stranded copper as the sheath.

There is no requirement for mechanical protection.

Since 2002 they do have to have marker tape or be installed in a suitable duct, generally we use red plastic but the colour is not a requirement.
We do have access to a thick wall duct that does provide a reasonable degree of mechanical protection.

The reality is that even if you use SWA or STA a mechanical excavator or other mechanical plant will damage the cable with no problem, the "mechanical protection" IMHO is for when the cable is struck by a handtool.

 

[plugwash]

generally we use red plastic but the colour is not a requirement.

I thought the convention was black duct for LV and red duct for HV.

 

[bernardgreen]

I thought the convention was black duct for LV and red duct for HV.

It was black for my new mains supply.

 

[JohnW2]

I thought the convention was black duct for LV and red duct for HV.

Westie often thinks HV I'm surprised that the red has not been 'harmonised' to brown - since, as we all apparently 'know', brown is a colour whicch is generally well known to indicate danger!!

Kind Regards, John.

 

[Haldane]

Thanks for your replies. I think what you're all saying is that twin & earth could be used underground if it was run in earthed ducting or conduit which provided the same degree of mechanical protection as an earthed armoured cable.....Does anyone know if the regs ever actually quantify the degree of mechanical protection for different situations? (I'm thinking here of something like an IK code......although the highest IK code seems to be for an impact drop of 5 kg object from 40 cm height which I figure would be like someone going through a cable with a spade or axe.

 

[JohnW2]

Thanks for your replies. I think what you're all saying is that twin & earth could be used underground if it was run in earthed ducting or conduit which provided the same degree of mechanical protection as an earthed armoured cable.....Does anyone know if the regs ever actually quantify the degree of mechanical protection for different situations? (I'm thinking here of something like an IK code.

Not explicitly. However, 522.8.10 of the regs (which is the bit which talks about equivalent mechanical protection) has a footnote which says "BS EN 61386-24 is the standard for underground conduits" - so one suspects that Standard might proivide the answer to your question, if you could get access to it.

Kind Regards, John

 

[Haldane]

Well spotted John....I'll see if I can find that somewhere. Thanks very much for your help Best regards. Anthony