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Protection Riddle No.62 - Grounding of primary side of transformers
Why the primary side of distribution transformers usually is not grounded?
Author : Hamid - From: Iran
Thu, December 17th, 2009 - 19:10
The grounding of Primary side of electrical equipments such as distribution/load transformers or electromotor which connected to power sources is not recommanded; because:

1-If the branch breaker trips or opens, a portion of grounding system would be lost, therefore amount of system resistance would be conditional which is not desirable.
2-Relay coordination of system would be difficult. Please note following figure, when an earth fault occur in A point, the earth fault protection of healthy neighbor feeder can be front of maloperation if it is not equipped by directional element.

Author : Hamid - From: Iran
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