Three Phase Power supply

[dragonspark]

Greetings
I work for a client who has recently negotiated a bigger supply from the supply authority, the client has agreed with Scottish power to have a larger transformer and feeder cable installed, my part of the works is to upgrade the existing switchgear to accomodate for the new supply.
The existing supply was three phase and neutral plus earth fused at 200 amp from a transformer on a pole, the new supply is from a transformer on the same pole as existing supply and will be fused at 315 amps three phase and neutral, however the supplier says they can not provide and earth because the building is metal, therefore now apparently we require an earth rod and a large three phase rcd, which will be a nighmare given the existing equipment etc , welding etc, one fault and the whole site goes off.
All other buildings we go to are metal but have incoming earths, is the decision not to supply an earth justified, i am failing to see how the original transformer supplied three phase n&e but not the new tranny, any thoughts please.
many Thanks in anticipation

 

[snowhead]

Is the pole and existing transformer on the Clients land or on adjacent land?

 

[DPG]

Surely most new industrial or commercial buildings built these days are metal framed. They can't all be TT surely? Apologies for my lack of knowledge here.

 

[buzzlightyear]

has its a metal building scots power dont need to present a earth .
considering being TT to the building ,so its down to you.
or they might tap in to the trany and pme thier with a earth .

 

[westward10]

DNOs do not like giving a PME if a metal building has more than one DNO supply, not sure if this is the case here.
Reasoned if you had an open PEN at one service head it may not be noticed due to main protective bonds to the structural metalwork providing a return by way of another head, I believe.

 

[pc1966]

Is this transformer only used by your client?

If it is down to the TN-C-S argument on exposed metal can't the client pay the extra for a 4C+E cable so it is TN-S?

 

[Julie.]

That is allowed within the ESQCR, however you can usually get a PME service into a metal-clad building providing:

1) The metal cladding is bonded to the steel-frame.

2) The supply is three-phase with less than 40% unbalance and the frame to earth impedance is less than 20ohms.

The issue is for multiple supplies into the same building, in this case each additional supply will be TT, or one supply point of PME with each individual metered supply off this of SNE.

As for an up front rcd, don't forget, this could be time delayed, with the downstream ones still 30mA instantaneous

 

[pc1966]

As for an up front rcd, don't forget, this could be time delayed, with the downstream ones still 30mA instantaneous

That could still mean a near complete replacement of the DB(s) if many circuits are off MCBs so those would simply take out the delay RCD.

I guess if you can get the rod Ra down you might be able to clear some of the smaller MCBs within the delay time, but somewhere I think I read that the supply Ra might not be that low? Ten-ish ohms springs to mind? Then you really would not clear even a 6A MCB no matter how low your own Ra is and it becomes a case of all RCD/RCBOs, etc, to avoid site blackout.

 

[Julie.]

That could still mean a near complete replacement of the DB(s) if many circuits are off MCBs so those would simply take out the delay RCD.

I guess if you can get the rod Ra down you might be able to clear some of the smaller MCBs within the delay time, but somewhere I think I read that the supply Ra might not be that low? Ten-ish ohms springs to mind? Then you really would not clear even a 6A MCB no matter how low your own Ra is and it becomes a case of all RCD/RCBOs, etc, to avoid site blackout.

Yes, however I take it that the immediate concern is of nuisance tripping due to the starting/striking transients from equipment such as welders etc - the time delay could ride through these.

But there is no doubt transforming an old PME site into TT could be a big change if it already isn't substantially in line with the current regs

 

[plugsandsparks]

Recently had a new supply albeit 100A to a metal clad building. WPG installed 5C to just outside the building and then swapped to their normal cable 4C to the service head . On Paper it was described as SNE - TNS.
Anyway they just did it and i took the earth from the service head in the normal way. ZE was great as well.

 

[brianmoooore]

I've had DNO refuse to provide earth on metal clad, wooden framed farm buildings on a couple of occasions. No practical way of bonding the metalwork, as there's no guarantee on continuity between the steel sheets.

 

[DPG]

I've had DNO refuse to provide earth on metal clad, wooden framed farm buildings on a couple of occasions. No practical way of bonding the metalwork, as there's no guarantee on continuity between the steel sheets.

Wooden frame though - no steel going into the ground.

 

[brianmoooore]

corrugated sheets forming the sides go down and slightly into the ground