11kV/400V TX with no LV protection

To re-assure yourself, you only need to check, what is the upstream protection for the TX.

Given the TX itself will add an impedance of a couple of percent, it is safe to assume upto something between 20 to 50 times the nameplate rating,  that transformer action will be linear, and every  kilo-amp on the LV side at 230V reflects back to be 230/6400 of a kA, or about 35 amps on the primary. Depending on the design, there may be  expulsion fuses or current transformers and a more complex time integral electromagnetic trip, maybe even a boiling oil pressure operated  "Buchollz" trip as well, but there will always be something.

A really bad fault on the LV side of a magnitude that  might damage the transformer will simply cut the HT side off, and that includes shorted windings in the transformer itself, which no external LV fuse would guard against.

Mike

To re-assure yourself, you only need to check, what is the upstream protection for the TX.

Given the TX itself will add an impedance of a couple of percent, it is safe to assume upto something between 20 to 50 times the nameplate rating,  that transformer action will be linear, and every  kilo-amp on the LV side at 230V reflects back to be 230/6400 of a kA, or about 35 amps on the primary. Depending on the design, there may be  expulsion fuses or current transformers and a more complex time integral electromagnetic trip, maybe even a boiling oil pressure operated  "Buchollz" trip as well, but there will always be something.

A really bad fault on the LV side of a magnitude that  might damage the transformer will simply cut the HT side off, and that includes shorted windings in the transformer itself, which no external LV fuse would guard against.

Mike

mapj1 said:

To re-assure yourself, you only need to check, what is the upstream protection for the TX.

... and possibly whether there is an intertrip, where necessary, to prevent on-going energization from EM induction.