Singlephase centre-tapped transformer need NER? 2

[veritas]

Hi

I have a 30kVA 415V to 240V single-phase trfr as per the attached diagram. The trfr feeds a distribution board in a coal mine. The secondary is 240V and I have worked out a fault between the secondary terminals to be around 3480V using PTW (4166A if I neglect source impedance).

1. How do I calculate a A-E fault? I surmise that since the driving voltage is now 110V (half of 240V) and also only half the secondary winding comes into play, the impedance is about half of 3% so the fault current will again be around 23480V? Not sure how to calculate it.

2. If the fault current is indeed around 3480A or in the region of 1000A to 3400A, should there not be a 5A NER in the earth connection? The customer has it solidly earthed with 30mA EL relays on all the distribution board circuits. Can the EL relays handle such high current? Other implications?

Thanks in advance.

 

[stevenal]

Might not want to post those scans if the work is copyrighted. Thanks for confirming what dpc and I have been stating. I agree that test reports should provide more information, and it is unfortunate they don't.