Large amount of current (ground fault)

Miguel c

Member
Location
Barranquilla colombia
Occupation
Bilingual agent
Lets say we have a single phase 37.5 KVA transformer, (240,120 V ) the secondary coil can deliver 156 A total, the question is, when a ground fault occurs between a hot wire and the EGC, how come the amount of current is way to higher than the nominal current the secondary coil of the transformer can deliver 156 A? I know that the EGC is a low impedance path etc, but where does that super extra amount of current caused by the grount fault come from? Imagine we have 120 V line to neutral or EGC, 120 V ÷ Z 0.206 = 583 A.
583 A - 156 = 427 A, where do those 427 A come from ?
 

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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The transformer is rated to deliver 156A on a continuous basis. More than this and it is likely to overheat.

But the current is limited by the transformer impedance, not its current rating. Transformers are generally described as their % impedance at rated load, which makes the available short circuit current 1/%impedance * rated current.

Jon
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
Lets say we have a single phase 37.5 KVA transformer, (240,120 V ) the secondary coil can deliver 156 A total, the question is, when a ground fault occurs between a hot wire and the EGC, how come the amount of current is way to higher than the nominal current the secondary coil of the transformer can deliver 156 A? I know that the EGC is a low impedance path etc, but where does that super extra amount of current caused by the grount fault come from? Imagine we have 120 V line to neutral or EGC, 120 V ÷ Z 0.206 = 583 A.
583 A - 156 = 427 A, where do those 427 A come from ?
As Jon stated, that’s a rating only.
1 14 gauge wire is rated for 15 amps, but will carry five times more.
Yes, it gets hot... so does a transformer when a 37.5 delivers 300 amps for a while.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Just to be clear, in addition to the impedance of the circuit conductors and EGC, the transformer itself has impedance.

The transformer impedance is the ultimate limit for short circuit current. If you have a 37.5 kVA single phase transformer with a secondary voltage of 240V and an impedance of 4%, the rated current is 156A, but the available short circuit current is 3906A.

Jon
 
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