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Transformer protection

Transformer protection

Transformer protection

(OP)
Dear colleagues,
The NEC 2005 Article 240.21(C)(1)permits to consider multiphase transformer secondary conductors to be protected on primary side of the transformer ONLY for delta-delta connection. How to explain this limitaion and why OTHER THAN delta-delta connections are not considered to be protected by the primary overcurrent protective device?

RE: Transformer protection

Save yourself a lot of time - it doesn't matter what the reason is, or even if there is a reason.  If it's in the NEC, it's the law.  

For delta-wye transformer, compare the current seen on a primary OC device compared to the current flowing in the secondary winding for a secondary line-to-ground fault.    

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