2 pole breakers on a single phase NER circuit.

[TerryOL]

Recently we installed NER's on three 11/0.433kv power transformers supplying a coal processing plant. This modification was performed to overcome a design fault that was not picked up till 2 years after commissioning. I have been informed that I require 2 pole circuit breakers on all single phase circuits. I have consulted various Australian Standards, Acts and Regulations but can not find any guidance on this matter. Can anyone help?

 

[ScottyUK]

I am surprised that you are permitted to have single phase loads connected line-neutral on an impedance earthed system. Normally impedance earthed systems are only permitted for loads supplied line-line.

On the assumption that this is allowed in Oz, you must treat the neutral as a 'live' conductor and switch it because it could reasonably be expected to be at a potential greater than earth. On system with a solidly earthed neutral, there is no mutual interaction between circuits which can cause the neutral to be at a potential significantly greater than earth.


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

 

[rbulsara]

Using L-N loads on a NER grounded systems defeat the purpose of the NER! What was the "fault" of the original design?

It is a matter of proper engineering not standards.


Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Use of 2 pole breakers for L-N loads on an impedance grounded system would be merely stupid. L-N loads without the 2 pole breakers would assinine. Anybody who would seriously propose L-N loads on an impedance grounded system needs a sanity check. To misquote Nike "Just Don't Do It."

 

[waross]

May we suggest using two pole breakers connected line to line to feed single phase circuits. A transformer may be required to adjust the line to line voltages down to 250 Volts for your loads. Ground one side of the 250 V winding or the center tap of a 250/500 volt winding if you go that route.
The codes that I work (CEC) under mandate solid grounding of the neutral if any line to neutral loads are present. I believe that other posters here work under similar codes.

Bill
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"Why not the best?"
Jimmy Carter