What trips a primary circuit breaker in front of a transformer?

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sugg

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Hello, a theoretical question here...grounding related - at a transformer: primary= single phase 480 (two ungrounded conductors and one equipment grounding conductor) ... secondary= single phase 240 with a grounded neutral. This transformer has a system bonding jumper within: GEC, equipment bonding jumper (to transformer metal + first disconnect), and neutral from transformer = all bonded within the transformer.
If one of the secondary legs has a ground fault within the transformer, what is the electrical path which causes the primary breaker to trip? There is a low impedance ground path back to the primary panel.... The primary conductors energize one side of the transformer, which is isolated from the metal shell/frame of the transformer. However, if one leg on the secondary ground faults to the metal shell/frame of the transformer, I can't see the path which causes the primary to trip....Thanks, Gus Long
 
Thanks for that - SG-1.... when a normally grounded part within the transformer (w/ a primary and secondary coil - no direct connection) becomes energized via a secondary leg contacting the case, how does an overcurrent occur on the primary OCPD? From the energized case, the current then travels back to the primary panel on the equipment grounding conductor.... but then what? Ultimately, that EGC has a system or main bonding jumper somewhere - either in the panel with the primary or further upstream. OR, is the overcurrent caused by a sudden change in the transformer's induction, resulting in the primary OCPD pulling too many amps. It's the physical path I can't get my mind around!! - Gus Long
 
SG-1 said:
A short circuit on the secondary side will produce an overcurrent on the primary side & cause the OCPD on the primary side to trip.

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How much damage can occur before the primary OCPD trips? Never seen this, always wondered.
 
sugg said:
Thanks for that - SG-1.... when a normally grounded part within the transformer (w/ a primary and secondary coil - no direct connection) becomes energized via a secondary leg contacting the case, how does an overcurrent occur on the primary OCPD? From the energized case, the current then travels back to the primary panel on the equipment grounding conductor.... but then what? Ultimately, that EGC has a system or main bonding jumper somewhere - either in the panel with the primary or further upstream. OR, is the overcurrent caused by a sudden change in the transformer's induction, resulting in the primary OCPD pulling too many amps. It's the physical path I can't get my mind around!! - Gus Long
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There is not a physical path for a secondary ground fault back to the primary OCPD. There is only a magnetic path. The current in the primary has to increase to supply the current into the secondary ground fault. If the current in the primary is high enough, the primary OCPD will open.
 
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