Earthing a church

colin durning · Apr 26, 2011

Hi all,
I had been asked to look at the flourys in a church hall as they have problems getting new lamps.
When testing earth cont.(mettalic fittings) i detected 137v between fitting and MET.
I stopped there and inspected the system, free of charge

of course, and found 2 100A supplies, 4 sub DB's, 2 water and 2 gas supplies. No bonding whatsoever,only 1 main earthing conductor that feeds 1 sub DB.

I have advised them to get it earthed pronto and they would like me to give quote.

Would it be acceptable to run the bonding to a sub DB, or will i have to run them to the SOURCE???
Not sure if a sub DB is regarded as the source.
Will i have to run main earthing conductors to each sub DB.

Any help would be much appreciated

Adam W · Apr 26, 2011

Re: Earting a church

For TNS and TNCS main bonding should be wired back to the MET (OSG section 2, p15).

malcolmsanford · Apr 26, 2011

All bonding must be connected to the MET which is normally at the origin of the Installation. You can though have a MET anywhere within the installation, it means is that you will need to label it with a BS 951 label indicating that it is the MET this under Reg 514.13.1. So you could reposition the MET if you wish, but becasue of testing it better to be near the origin

Dont the sub DBs have CPC now

colin durning · Apr 26, 2011

Re: Earting a church

Yes. MET in the sub DB or at the source? if you catch me drift.
I'm not sure if the Sub DB is acceptable.

Engineer54 · Apr 26, 2011

Are all these Sub DB's at the same location?? if not you need to ascertain the main incoming supply DB and have your MET in that location. All other DB's will then have there earthing connection point as EMTs.

Is this an old church with a bell steeple or the like?? If it does have a highish steeple and is located in a fairly open area, it may also have a lightning protection system in place. So now we need to know what the supply earthing arrangement is TT, TN-S TNC-S(PME)

colin durning · Apr 26, 2011

Malcolm; at the origin there is no MET.
The 10mm T+E(tails) with earth snipped run from the meter to an isolating switch suppling 1x10mm and 1x6mm T+E feeding 2 sub DB's. Effectivley there is no earth, the supplies that feed the sub DB's still have CPC intact but they connect to nothing in the isolating sw.
My thougts were to run the bonding conductors back to the origin and install new MET and MEC to isolating sw as iam not sure if a sub DB's MET is regarded as the origin.
What do you think?

colin durning · Apr 26, 2011

Eng54;
Its old 150 ish, origonal supply feeds the church and vestry.
Supply 2 feeds newer (100yrs) church hallx2, kitchen toilets etc.
Tallest building about, 75-100ft, no sign of lightning cond.
Supply is TNS lead sheath
Origonal supply has a 2 way board adjacent and sub DB 15 mtrs away (8 way all rewireable)
Newer supply has a sub DB 8 mtrs away and the other sub is approx 40 mtrs away.

colin durning · Apr 26, 2011

I have a few photos(nice), dunno how you get them onto this. I'm not clued up that way

Engineer54 · Apr 26, 2011

Go through photobucket.com

Engineer54 · Apr 26, 2011

Choose one of the main supplies to be your MET point, and bond the other supply point to the MET. All other DB's earthing bars, will then effectively become EMT (earth marshaling terminals) All extraneous bonding conductors from water gas etc will then be taken to the MET.

colin durning · Apr 27, 2011

Ok i've got a few snaps on photobucket, the next big mystery is how do you get them on here ?
I'm sure somebody will tell me soon...

IQ Electrical · Apr 27, 2011

You can just post the links to the pics if you like.

Or hit 'Reply', then 'Go Advanced' and sample the joys of the file uploader by way of the paper clip attachment icon.