Testing ze with armour as earth

[Chris86]

Hello all,

Quick bit of background, factory maintenance spark for 20+ years, covered some installation work over the years but not done a great deal of testing. New lines and cells are being installed over the next few months and all installation is being done in house. Decided it is time to focus on the testing, no chance of company sorting the proper qualifications so have decided I just need to train myself!

Looked at one of our distribution boards today (Schneider acti9) and it is fed via 4 core swa with armour used as earth. There is a fly/link lead connected from the banjo down to an earth connection on the side of the board. Another fly/link lead then runs from this connection bolt up to the main earth bar. Board is fitted to one of the main girders using unistrut clamps. Question is, when testing ze, will the armour/board/girder connection cause a parallel path? If I remove the fly lead from the earth connection bolt and test from there, the armour will still be in contact with the board. Is there something obvious/daft I am missing or over thinking?

Thanks and ATB

Chris

 

[Deleted member 26818]

Unless this is the origin of the installation, you will be measuring Zs not Ze.
As such you should leave all earths connected.

 

[westward10]

If this board is part of the installation, e.g. it is not at the source of supply you are testing Zs and not Ze.

 

[Chris86]

I feel it would be the origin of the installation, the supply to the board comes from an ancient mini form board which our landlord doesn't allow us near! Would the distribution board now be classed as the origin of the installation?

ATB

Chris

 

[Deleted member 26818]

No, the mini form board would be the origin.

 

[Chris86]

In that case, I stand corrected. Thank you very much for your quick replies.

I am sure I will be back with more questions at some point. I have bought books, gn3, osg etc and Christopher Kitcher is now my new best friend! But there will always be something that's crops up that the books cannot solve.

Thanks again,

Chris

 

[Deleted member 26818]

See if you can get anything by Brian Scadden.
He used to write the questions for the 2391 exam.

 

[Chris86]

Thanks for the recommendation, I will see what is on offer on the web. I would have much preferred to have gone down the recognised qualification route but we work shifts and time off for courses isn't possible. Even looked for somewhere that would do weekends and I would pay myself but drew a blank.

ATB

Chris

 

[David M]

Electricians should have the knowledge and skills to inspect and test, regardless whether they install or not. So its great that you recognise this and are prepared to learn!

I thoroughly teach my apprentices how to inspect & test, interpret results and understand why/what each test is done for.
As a maintenance spark this is very useful for fault finding & keeping a check on health & safety issues.

Keep asking the questions and the good folk on this forum will be happy to help you out.