Bonding Services Linking 2 Buildings

[gselectrical]

Good Evening all. Happy New Year. I hope everyone is keeping busy and all the best for 2014.
 
I have a question regarding bonding services that link two buildings that have different supplies.
 
We are currently installing the electrical system at a factory. The client has purchased another unit directly opposite their main unit. They have taken a steam, hot water and cold water supply from their existing factory into the new unit (overhead). 
 
The main unit has a three phase supply, tails to panel board are 2x240mm and main earth is 2x120mm. The gas & water are currently bonded using 50mm earth. 
 
The new unit has a three phase supply, 25mm tails, 16mm earth. 
 
I am assuming we need to bond the service where it leaves the main unit and where it enters the new unit?
 
Both buildings are PME and both are fed by different transformers. 
 
I assume bonding the service within 500mm of where it leaves the main unit with a 50mm earth and where it enters the new unit within 500mm with a 10mm earth would be adequate?
 
As always, your help and advice is much appreciated.
 
GS

 

[NozSpark]

Why are you bonding it where it leaves the building. .. you only need to bond extraneous services within 600mm of where they enter the building...

IMHO just treat them as seperate buildings

 

[Sidewinder]

Dunno, I'd be tempted to bond for the largest fault current, i.e. largest supply TBH.

 

[gselectrical]

Why are you bonding it where it leaves the building. .. you only need to bond extraneous services within 600mm of where they enter the building...

IMHO just treat them as seperate buildings

Cheers for the reply. 

Could you not argue that the services enter both buildings? If we where to bond the services with 10mm where they enter the new unit then we will have an extraneous conductive part within the main unit that is not bonded back to that buildings MET? 

Cheers

GS

 

[Sidewinder]

Surely the metallic services are continuous in the original building?

Thus the "original" bonds on these would cover this, however, as they could bring "in" a potential from the other building, then you could cover this with a bond within the main building also.

Thus bond within 600mm of the "entry" to each building, sized for the larger supply, I think is my best guess without really thinking about it.

 

[Steve3948]

I would agree with sidewinder, bond sized to larger supply.

 

[apprentice87]

I am DEFINITELY with Paul on this one.... So far as i know all bonding needs to be done in relation to the size of the Neutral conductor of the largest supply, so far as I am concerned 50mm it is!! Two reasons for this; 1, that is what you would have to do if you were "exporting" PME [yes i know] to this other shed. 2, if you link earthing systems, the bit that does the linking has to be able to carry the total fault current that  might arise. There is a reg that covers this somewhere!!

I will try to post a piccy of the nearest thing i can find....

Oh well, appears it is impossible to upload pictures now...

Still, 50mm it is..!!

john...

 

[gselectrical]

50mm bond within the new building also?

Even though the incoming supply is only 25mm? (TP&N)?

Cheers 

GS

 

[SLIPSHOD & SLAPDASH]

Just checking picture attachment



Carry on.

 

[apprentice87]

As Canoe said; "yes" No doubt about it...

john

See how building "B" has the same size bonding conductors as building "A"

john..

 

[SLIPSHOD & SLAPDASH]

Nice link, where is that from, it looks familiar ?

 

[gselectrical]

As Canoe said; "yes" No doubt about it...

john

See how building "B" has the same size bonding conductors as building "A"

john..

 Cheers. Thanks for the reply. 

The buildings in the diagram are all supplied from the same place (Building A). The 2 buildings in question both have different supplies (and sizes). Although I am sure this is probably irrelevant. 

GS

 

[NozSpark]

But that diagram shows the 3 buildings supplied from 1 service... the op has 2 buildings supplied from 2 different services, but sharing some extraneous piping

If I've read it correctly

Nice diagram BTW

 

[apprentice87]

Hi all!

Not sure where i got the piccy from! It has been on my computer for ages!! The link i gave is to my photobucket thingy i had to set up to post the piccy!!

john..

 

[apprentice87]

Thinking on it, logically i suppose, what you must think is, what are these pipes??? They are extraneous conductive parts. And what are extraneous conductive parts?? Parts liable to introduce a potential blah blah.

So what potential could we have?? either a slightly different "earth" potential, ok, not great, but not really a problem, or a full scale earth fault potential from the first building with the wacking great supply. Therefore, in the linked shed, do we not need, as well as the bonding conductor, the earthing conductor too, to also be sized for the PFC from the first great big [compared to what i fiddle about with!] supply????

If you follow me!!!!

john...

 

[Steve3948]

If the service pipes could have insulated couplers installed to break continuity, then you could treat the smaller installation as separate, but probably cheaper to install earthing to match larger installation.

 

[Sidewinder]

DNO might not be impressed with linking the LV CNE conductor from 2 HV transformers either!

 

[Marvo]

Surely if both supplies are PME'd then the LV CNE from the two HV Tx's is effectively linked anyway.

By our local regs you'd be required to bond at each building only according to the supply cable size for that particular building and tbh I don't see any mileage in bonding according to the larger supply specification when both bonds combined will handle far beyond the largest possible fault.

 

[Sidewinder]

It is now by the metallic services Marvo, that's my point!

 

[apprentice87]

Hi Paul,

See what you mean about linking the CNE conductors, but then, that is their own fault!!! If you linked the earthing, so what, at the end of the day, what is the problem, they are only ultimately spikes at either end of a cable, banged into the suface of the same planet As for the CNE bit, Hmmm, see what you mean, but this is their fault for coming up with a stupid system like TNCS. Cannot see there would be a problem though, both transformers are earthed at the star point presumably, [from where the neutral originates]. The neutral is connected to the consumers MET and that is earthed too, so, if you run out a bit of cable and earth it at the end, cannot see it matters, all just spikes banged into the same planet.

Could you not have all funny currents shuffling up and down the "wrong" neutral though?? Wonder if it matters if the transformers are vastly different sizes.. Dunno, not a clue!!!

john..