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Topic Title: Earth Fault Current Calculation for BS 7430:2011 Compliance
Topic Summary: Earth Nests and BS 7430
Created On: 16 July 2013 04:43 PM
Status: Read Only
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 16 July 2013 04:43 PM
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Joined: 09 July 2013

I hope somebody can help or point me in the right direction.

I am trying to find out how to calculate the single phase to earth fault on the primary side of an 11,000/415V 1MVA transformer with an impedance of 5%. The transformer is supplied via a 33/11kV 30MVA transformer with 12% impedance 2km away. The question is ultimately related to determining whether the HV and LV earth nests can be combined as per BS 7430, and without knowing the earth fault current on the primary side of the 11kV transformer the step and touch voltages etc. cannot be calculated as far i can understand.
 17 July 2013 12:07 AM
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You really need further details of the earthing arrangements on the 11kV side of the 33 / 11kV transformer. Frequently the neutral-earth connection has an impedance included to limit the maximum earth fault current. Also relevant is the connection arrangement between the 33/11kV and 11/0.400 kV transformers (eg is this cable or overhead line). A cable connection can significantly reduce the earth fault current passing through the ground reducing the associated rise of earth potential.

I'd strongly recommend that you engage a suitably qualified consultant to perform a study. I might be able to give further pointers if you can get this information and include a general layout and a single line diagram to me in a PM.


 18 July 2013 04:08 PM
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Thank you for the response Jonno.

My concern is more that some Engineers still have a tendancy to quote 1 ohm when looking to combine substation HV and LV earth nests when in fact earthing design studies need to be carried out to ensure EPR and touch votlages etc are within the limits as per the new BS 7430 and BS EN 50522.

The difficulty I have is calculating the ground fault current flowing from the HV side of the 11kV transformer in my example back to the source. I have not found one working example of such a calculation anywhere, except in IEEE 80 it provides the formula required to calculate, although I was hoping for a worked example!

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