ZE at board/earthing alternatives

[StokieSpark]

Hi there... Couple of questions for you experts. Still learning.....

Been to a factory today... It has one main incoming board and 55 distributon boards scattered around there factory...

They asked me to fit/wire a couple of socket outlets under one dist board. Which only uses the armoured of the SWA as an earth but the phase conductors are 50mm2 each and i was under the impression that you can only use the armoured strands for an earth for cable upto 25mm2 (Is this right? )

As a result i measured the Ze and it measured in at 0.99ohm, (The factory is TNS) so the maximum i can have is 0.80ohm right?
I then measured it at the incoming board and it measured 0.19ohm, which is fine.

So I phoned my boss and told him we need to run a separate earthing conductor, he told me to just take a earth off the closest steel girder and that will be good enough..

Im not convinced... Surely the DNO supplied an earth for a reason and not used rods? And would this turn the installation into TT????


Give me your expert feed back. Thanks.

 

[Guest123]

Hi.

A bit confused, an installation can only have one Ze, at the origin....I assume this is your 0.19 value??? The 0.99 value is a Zdb I'm guessing (Zs of sub-main circuit).

What sort of length is the sub-main circuit, and are there earth leads present from the earth tags of the SWA glands to the earth terminals and are the glands sound???

The armour is fine to use as the CPC for the sub-main as it has a copper equivalent of about 32mm.

 

[StokieSpark]

Sorry I dont speak the lingo.

Yes Ze at the incoming is 0.19 and the Zs at the submain is 0.99.

It is rougly 120M away.

The gland is all fine, earth lead from the banjo and all nicely tight.

 

[Guest123]

Taking that into account, I can see no reason why you cant install your additional sockets, They will need RCD protection IMO, unless they are for specific items etc etc, even then the Zs may push you over the maximum for an MCB anyway.

 

[Deleted member 9648]

Your Boss might want to have a think about it,run an earth to the nearest steel girder?...is he for real?

 

[Guest123]

Your Boss might want to have a think about it,run an earth to the nearest steel girder?...is he for real?

My eyebrow did lift a little too!

 

[telectrix]

Your Boss might want to have a think about it,run an earth to the nearest steel girder?...is he for real?

maybe his boss didn't realise that there is a water pipe close by ( only joking)

 

[Deleted member 9648]

maybe his boss didn't realise that there is a water pipe close by ( only joking)

....or gas maybe?

 

[StokieSpark]

I was "taught" that all submain boards must be under the 0.80ohm(tns systems)
I didnt know that it would come under the 'max Zs'


Its like the blind leading the blind where i work!

 

[Sintra]

What type and size of overcurrent protection is on the sub main as this might make the 0.99 value exceed the max permissible Zs.

 

[StokieSpark]

It has got 150A fuses on the 3phase coming in. BS 88-2? (at a guess)

 

[StokieSpark]

150A

BS88-2? not sure...

 

[Guest123]

Are you sure......Stevie's asking about the fuses/MCCB protecting the 50mm SWA sub-main now, not the main incommer fuses.

Max Zs for a 160A BS88 is 0.25ohms.

 

[Sintra]

Indeed as Lenny says it's the protection on the sub main not the incomer.

 

[StokieSpark]

Right, the main incoming is a Giant MCCB is 800A which goes too a "cupboard distribution board" next to it, with a 160A MCCB which feeds the submain that has 150A BS88 main fuses.

 

[Sintra]

You can't get a 150A BS 88 as far as i know 125A then 160A

 

[olly w]

Going back to the bosses comments it is permissible to use an extraneous conductive parts as protective conductors.....I don't make the rules.
543.2.6.
I think there would need to be a bit more investigation than "just take an earth from the nearest girder"
P.S. I have never done it and hope never to have to.
I'm going to hide from the bullets now.

 

[StokieSpark]

You can't get a 150A BS 88 as far as i know 125A then 160A

Sorry, really long day... 125A

 

[Sintra]

No probs mate but regardless the Zs on your sub main is way to high for the 160A MCCB protecting it.