transformer question secondary fault and primary ocpd

[wperdue2]

Hello,
I was wondering if there is a fault on a secondary feeder or branch circuit supplied from a transformer, can it trip or blow the fuses or breaker that protect the primary?

for ex if it was a 277 480 primary then 120/ 208 secondary and there was a phase to phase fault on the secondary.

I believe the protection for the secondary ie the panel main/ disconnect/ fuses it feeds would blow first or trip.

Since a transformer only works from mutual inductance between the coils there is no real "connection" between the primary and secondary coils I dont think there would be a path back, only on the EGC, but I was told about a 20 amp 277v lighting circuit that tripped a 1200 amp gear in a building. Come to find out that was because the Ground fault relay on the gear was set too low but it got me thinking about transformers.

Thanks

Woody

 

[GoldDigger]

Yes, a line to line or a line to ground fault on a transformer secondary can cause a high current in the primary of the same transformer.

The question of which OCPD (or both) will trip for a given fault is harder to determine.

If you have an overload on the secondary instead and the transformer is sized for multiple circuits connected to the secondary, on the other hand, it is very unlikely that the primary OCPD will blow.

Even with just mutual inductance, any fault which causes excess current to flow in the secondary will reduce the impedance of the primary and cause excess current to flow there too.

A line to ground fault on the secondary which does not trip the secondary OCPD should not cause significant ground current on the primary side though, only line to neutral or line to line current depending on the transformer type.

 

[JFletcher]

Welcome.

Hello,
I was wondering if there is a fault on a secondary feeder or branch circuit supplied from a transformer, can it trip or blow the fuses or breaker that protect the primary?

for ex if it was a 277 480 primary then 120/ 208 secondary and there was a phase to phase fault on the secondary.

I believe the protection for the secondary ie the panel main/ disconnect/ fuses it feeds would blow first or trip.

Since a transformer only works from mutual inductance between the coils there is no real "connection" between the primary and secondary coils I dont think there would be a path back, only on the EGC, but I was told about a 20 amp 277v lighting circuit that tripped a 1200 amp gear in a building. Come to find out that was because the Ground fault relay on the gear was set too low but it got me thinking about transformers.

Thanks

Woody

Ive seen a 1/2 HP 480V motor fault trip the building's 2000A breaker. The electrician shorted a 277V phase leg to ground, and the entire building went offline.