NER & 3 phase short 1

[sibeen]

A friend has a site currently under construction which utilises a 22 kV incoming feed. This is then stepped down to 400 V for the LV. The site also contains four back-up generators which operate at 11 KV. This is stepped up to 22 kV and then stepped down by the normal feed transformers for the LV. All four generators have a Neutral _Earth Resistor (NER) and the step-up transformers 11/22 kV are also using an NER.

A question has been raised as to how the 3 phase short circuit currents are to be limited at the LV boards as with the four generators operating in parallel it seems to exceed the fault current rating of the LV boards. The consultant has replied that by using the NERs on the generators and step-up transformers the 3 phase fault current is also limited.

My initial reaction was, "no, that can't be right". As this is not normally an area I work in I'm beginning to doubt my sanity.

So, can an NER limit the 3 phase short circuit currents, and if so, how does it do it?

 

[rbulsara]

Your reaction is right. An NER does not limit the 3 phase fault current. It is not in the circuit of phase to phase or 3 phase faults.

Rafiq Bulsara

http://www.srengineersct.com

 

[sibeen]

Thanks, Rafiq.

My main concern was this was put forward by a rather large international consultancy and although I was 99% sure my gut reaction was right, there was a little nagging doubt eating away at me as surely this 'respected and august' organisation wouldn't be telling porkies.

 

[rbulsara]

Don't get me started on those big firms....But there is a brighter side to it..... the messes created by many of them keep the smaller smarter firms busy!

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

I worked for 'one of those big firms'. They have some exceptionally good old hands who have probably forgotten more than I know, and they have some average types like me, and some new kids who don't have the field experience and the benefit of a few years of learning after graduation. It depends who does the report.


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If we learn from our mistakes I'm getting a great education!

 

[bvc]

I have a small doubt about the NER. The NER available at my project installation has a ground connected to the resistor terminal. Also another ground is connected to the body of the NER. Is it correct to earth the body of the NER ? My concern is about a possible condition when the insulation of the resistance if at any place gets reduced and connected to body , then the effective neutral resistance at that time will be reduced if we earth the body. Kindly advise.

 

[ScottyUK]

Every NER I have seen is designed to have the outer casing bonded directly to the local earth electrode. The scenario you describe would otherwise cause the NER casing to be live, which is a safety hazard and probably a violation of your electrical regulations.


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If we learn from our mistakes I'm getting a great education!