armour sheath as main earth

[brs73]

morning and happy new year
i've just carried a pir and there are 6x 24way dbs all supplied by its own 25mm 2 core swa cable
probably around 25/30 years old.
i was looking for the calc to see if the armour was sufficient as the main earthing conductor.
all Ze readings at the boards and Zs readings on final circuits are well within any limits.
am i right in thinking that you need a seperate earth cable as well as the armour sheath???
also none of the boards have a main switch ,just an isolator in the basement, is this a code 1 or 2??
thanks in advance.

 

[Guest123]

Hi

A lot depends here on the type of insulation of the SWA i.e PVC-SWA or XLPE-SWA.

The armour of PVC-SWA is more than adequate to be used as CPC - 26.6mm copper equivalent of the steel.

XLPE-SWA is only 13.5mm copper equivalent of the steel so this would not be large enough.


Obviously if it is XLPE-SWA, you could carry out the adiabatic equation to check for compliance but my guess is it would not.


Your other querey puzzles me a bit...you say they have an isolator in the basement but no main switch in the DB end???

Can you clear up a bit??

Cheers.

 

[brs73]

thanks lenny
there's six switch fuses in the basement supplying various dbs around the property(its big)
none of the dbs have a main switch
the live just clamps on to the bus bar so you'd have to leg down to the basement to isolate this board
i've not seen this before

 

[Guest123]

Have to say thats a new one on me too.

Dont think I've ever come across a DB without a main switch of some description in it, what make is it???

As for coding it I'd hazard maybe a code 2, hard to say without digging through the books and having a feel of the job, providing all other circuit protection is up to standard???. Might be worth you checking out GN2, or give your scheme provoder a tinkle see what their views are.

 

[brs73]

they are old crabtree boards
i've attached a couple of pics(i think,i've not attached pics on here before)
going back to the armoured sheaths. i'm not sure if the cable is xlpe or not.
is there any way to tell the difference
theyre old without any markings on the outer sheath

 

[Guest123]

Well I've come across loads of the old C50 boards in my time....but never one without a main switch.

Baffled to be honest with you.

 

[trebor]

Have to say thats a new one on me too.

Dont think I've ever come across a DB without a main switch of some description in it, what make is it???

As for coding it I'd hazard maybe a code 2, hard to say without digging through the books and having a feel of the job, providing all other circuit protection is up to standard???. Might be worth you checking out GN2, or give your scheme provoder a tinkle see what their views are.

In my early contracting days (60s) it was commonn practice to install db`s without isolators, the isolation being provided by lockable equipment in the switchroom, obviously things have moved on and it is now common to have isolators on the board,but is it a requirement? I would hazard a guess this is a 14th edition installation and complies with the same.
This is tricky one as far as rec codes go, could be a 4 as does not comply with BS7671 2008
I cant find the requirements for a dis board but interestingly a consumer unit is required to have a dp isolator for use by ordinary persons, perhaps this is the difference?

 

[Sintra]

morning and happy new year
i've just carried a pir and there are 6x 24way dbs all supplied by its own 25mm 2 core swa cable
probably around 25/30 years old.
i was looking for the calc to see if the armour was sufficient as the main earthing conductor.
all Ze readings at the boards and Zs readings on final circuits are well within any limits.
am i right in thinking that you need a seperate earth cable as well as the armour sheath???
also none of the boards have a main switch ,just an isolator in the basement, is this a code 1 or 2??
thanks in advance.

Table for SWA using armour as cpc.

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[brs73]

cheers sintra
i found that table last night
but i dont know how to tell if the cable is xlpe or not
any tips??

 

[Guest123]

Well after a bit of googling based on Trebors post (60's), I believe you have PVC-SWA as xlpe was still to expensive to make back then due to the electron beam process needed to make it.

It wasn't viable until the mid 80's and was still expensive then.

To be sure I would do an adiabatic calc to cover myself.

 

[sparks1234]

I came across the DB with no isolator on a recent PIR and was unsure as to what to put on the form, so rang ECA and the response was, if the board has a means of lockable isolation somewhere in the building then this is acceptable, but make sure it can be locked off.