Transformer protection

[thinker]

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?

 

[dpc]

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.