Potential Transformer Primary Fuse Basis for Sizing 2

[brainsalad]

Greetings:

Would be grateful for your thoughts regarding the origin of any bases used for the inrush capability of primary fuses for potential transformers.

I have a GE JVM-3 PT (2400-kV, 750-VA) with an EJ-1 fuse. Also an ancient ITE PT-6 PT (600-VA, 480-V) with Gould A6Y7E-96 fuses......

Can anyone recall the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?

Thanks for your help.

 

[oldfieldguy]

Guys-

The HV fuses for PT's aren't there to protect the PT. they're in the circuit to prevent a PT fault from affecting the source switchgear/buswork.

old field guy

 

[brainsalad]

All:

Thank you for your contributions.

I first set out to answer my original question, "[What is] the origin or basis of the requirement to have primary fuses tolerate at least 12x (or 25x) rated primary current?", but have been delighted in learning more than I have wondered.

After searching, one answer to my original question is found in IEEE-242 (Buff Book), which describes the basis for that requirement as derived from the "hot-load pickup" phenomenon, which requires magnetizing inrush points at 14x FLA at T=1s and 28x FLA at T=0.01s.

A nice article on the subject (but not necessarily for PTs):

http://www.electricity-today.com/et/issue0603/overcurrent_protection.pdf

Now wondering "why not" the same criteria for 480-V PTs (above is all medium voltage).....