earthing

[abrahamJP]

Hi,

For a big shopping center, which has 30 LV Transformers(11KV/415V-system fault level-250mva) in Ground floor Transformer rooms-While designing earthing system how much fault current to be considered-
1)Only One transformer
or
2)Fault curent based on Probability of Failing, since 30 is a big number
or
3)Fault current of all transformers feeding to earthing network.

Earthing network is interconnected ring around perimeter of building,which is usual practice in designing of building earthing system.

Best regards

Abraham

 

[Parchie]

Do the math for each transformer!

 

[7anoter4]

If no connection it is between any transformer and other at low voltage side it is hard to say it could be more than one transformer involved in between one phase and ground fault. Take the maximum short circuit current of all these transformers.
However, since the high voltage and low voltage grounding is common you have to check the conditions as per BS EN 50522 .
For practical foundation Earthing you may consult NECA 331/2009 Standard for Building and Service Entrance Grounding and Bonding and for calculation BS 7430.

 

[waross]

Each transformer will be a separately derived system and may be considered independently.
However, a common conductor connecting more than one transformer to the grid is problematic.
It depends.
You will have to justify any decision to reduce the size of a common grounding conductor.
Do the feeders issuing from the transformers share any space, such that a fire or other event may involve damage to more than one feeder?
In the Canadian code we have a table for grounding conductor size based on busbar ampacity.
I would consider the combined ampacity of any group of transformers that will share a grounding conductor and be guided by the table results for a similar ampacity bus bar.
You may coordinate the grounding conductor ampacity with the instantaneous trip setting of a common primary breaker.
In any event, I would consult the AHJ for approval in writing of any reduction in the ampacity of a common grounding conductor below the maximum total current.
The physical arrangement and locations make a difference.
WE may be able to look at one layout and say that some reduction in ampacity is reasonable and defensible.
We may look at another layout and strongly recommend that the grounding conductor ampacity be based on the sum of the currents of the grouped transformers.
It depends.
What say the AHJ?

Bill
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"Why not the best?"
Jimmy Carter