Star point earthing

Darrenm · Jul 4, 2014

Does anyone know how to correctly size the earthing conductor from the star point of a 11kV/433V distribution transformer to ground for its reference. I believe that I can use the maximum single phase to earth fault current and then give it a 1sec/3sec withstand time depending on client, Is this correct? I have asked around in my place of work and have been told half the size of the line/neutral conductor which I would accept if someone could tell me how you would work it out mathematically otherwise I think that there is a good chance that they would grossly over size this conductor.

TAke for instance the following example. HV LV earths are combined with the earth nest made up from 2x 70mm bare conductors surrounding the transformer. Now this is a maximum of 140mm. If there were 630mm tails away from the transformer and we halved the size of the line conductor that would give us a 300mm to use as the HV to LV link, now this to me seems daft as you are doubling the size of the actual conductors of the earth mat, that can't be right surely. To me there seems no logical point of having anything above that of the two tails coming from the earth mat.

Admittedly using single phase to earth fault currents to x seconds will also probably over size the cable but this could be used in a situation where the HV and LV need separation.

Any input is much appreciated.

Darren

Knobhead · Jul 7, 2014

It’s governed by the resistivity of the earth nest.

You need to talk to the DNO <1Ώ you can have a direct link between LV & MV. Above that you rate the cable as if it’s going through a NER using the adiabatic equation. With no direct LV MV link.

I’ve only worked on private MV systems and like the DNO’s we classified “hot &cold” sites.

As a rule of thumb I used 2 x 95mm singles for 1000KVA to the MET. But our systems were invariably <1Ώ IE cold sites.

Darrenm · Jul 8, 2014

Tony

Thank you for your reply. But doesn't the cable work as a reference to earth rather then taking any fault current? i can understand the sizing of a CPC but bringing an overly large conductor from the star point down to an MET seems over engineered.

Also where have you actually got this information from? i have not been able to find anything on the internet or in my books to work this out.

Darren

jimmymac · Jul 8, 2014

'Ours is not to reason why...ours is just to accept that Tony is always right'

Darrenm · Jul 8, 2014

Jimmymac

It seems that happens a lot in this industry with people doing something without really knowing why (this is not in response to Tonys answer). I have asked numerous people i work with about the sizing of this conductor with either blank expressions as a reply or some wishy washy answer meaning they don't really know themselves.
I have asked two separate earthing consultants who both have different opinions and Tony's is just another to add to the list. There must be a standard procedure to work to ie. take fault current X gained from ... and use this in the adiabatic to get the correct sized conductor.:stooge_curly:
Darren