S&C SM5 FUSE SIZING 1

[tulum]

Hey folks,

We are buying a 6.9kV - 600V 2/2.6MVA air cooled dry type transformer (7% Z min). The primary will have S&C SM5 fuses and the secondary will be a Molded case breaker (rated 35kA). The transformer is delta - Wye with a 5A NGR that is alarm only not tripped.

I have charted the SM5 fuse vs. a standard ANSI damage curve for a 2MVA transfomer and either a 250E or 300E would work. Is there any special consideration that I need to look at because the transformer is 80DegC and is actually capable of 2.6MVA continuous? Should I plot the SM5 curve against the 2.6MVA capacity curve instead or does this matter? Is the secondary Fault current calculated based on the 2MVA rating or the 2.6MVA rating?

Regards,
TULUM

 

[davidbeach]

Your impedance is on the 2MVA base and it will be 1.3 times higher if calculated on the 2.6MVA base. You will find that the two damage curves and the two let-through currents are identical. Typically all calculations except continuous current rating are made on the base rating. All fault protection is then on the 2MVA rating and overload protection is on the 2.6MVA rating.

 

[tulum]

Thanks Dave..

That was a very clear and concise answer... Love it!

Regards,
TULUM