PNB query

[grumpyjohn01]

Hi All, Have been working on an isolated group of dwellings and had cause to read through a quote from DNO which states that a potential future supply would be provided with PNB earthing.Have to admit I wasnt sure what this was so went and looked it up.Now I know! Thing is it seems from what I read that its only used from pole mounted transformers to supply 1 customer,thing is here there are 3 single phase supplies and another 1 or 2 in the offing all to different customers,also all the service heads have PME type cutouts and previous sparks have connected as such and removed old earth stakes etc and bonded as for TNCS with PME.So do I reinstate earth rods? It seems from what I read that this is required,what about bonding sizes? what should max Ze be? any advice appreciated thanks John

 

[johnboy6083]

I would design to TNC-S if no information is forthcoming from the DNO. My experience of PNB, is on medium-large 11kv transformers feeding treatment works, and it is simply a term for earthing the transformer star point.
In this situation, i would design to the most onerous conditions, which is TNC-S. the cost difference is minimal.
As for the rods, there's no harm in leaving them in, but IMO, not neccesary.

 

[mechelec]

I would query the DNO regarding the PNB earthing arrangement. As you say it is only used where there is one customer from a pole transformer and the star point is tied to earth at an earth rod at the customers cutout and not at the base of the pole. Ask for PME or TN-S as an alternative.

 

[grumpyjohn01]

Thanks Mechelec,this is what concerns me ,that PNB is meant to be staked down at customers cutout and as I say because of PME type cutouts being used all METs are connected to neutral and rods removed therefore the star point isnt tied down anywhere! Think I will just stick some rods in anyway just to be safe

 

[johnboy6083]

I would query the DNO regarding the PNB earthing arrangement. As you say it is only used where there is one customer from a pole transformer and the star point is tied to earth at an earth rod at the customers cutout and not at the base of the pole. Ask for PME or TN-S as an alternative.

This is new to me. I see PNB all of the time in larger industrial settings, with the neutral/earth link just before the incomer ACB, in the termination section of the Panel. The transformer usually has multiple rods in and around the transformer pen. I didnt know that they earth the tx at the customers premises. Won't this cause step voltage to rise considerably in the event of a fault, especially as the rod is in the customers premises?

 

[mechelec]

Step voltage is not a great issue on Lv distribution networks. If you think about a TT system you have a rod at the customers premises and one at the TX and the neutral is separate from the cutout back to the TX. with PNB the earth rod is at the customers end and the neutral is connected to this usually at the cutout. So during an earth fault I would suggest that you would be less reliant on the general mass of earth as the star point of the TX is earthed closer to point of fault on the customers installation.
You will still have earth rods at the TX for the HV earth. If it is a hot site the HV and LV earths need to be kept separate.

 

[grumpyjohn01]

So I definitely should install rods then?

 

[mechelec]

If the DNO are supplying you a PNB supply I would get them out to have look. Good luck

 

[Knobhead]

Have a look at http://www.spenergynetworks.co.uk/newconnections/pdf/EART-01-002%20Issue%203%20-%20Low%20Voltage%20Earthing%20Policy%20and%20Appl..pdf

 

[Engineer54]

Never known a PNB system being provided to supply numerous supply points, that's what PME was designed to provide!!

Leave any existing rods in place and use them in this case. ...lol!! That also goes for PME supplies too, you are in effect providing another local point, of the protective multiple earthing of the neutral.... Actually, it's a ridged requirement in many European countries that's providing a PME supply to an installation.... Should be the same in the UK too!!!