Transformer Protection Question 1

[hanksmith]

Ok,

Have a transformer rated for:
45kVA
Delta-wye
575-208/120
3 phase

I am limited to the use of J type fusing on the line side of this transformer based off Client standards,

Going through the calculations and using the Canadian Electrical code I come up with a 60A J-Type fuse.

When I look at the curves for this fuse I don't think it can handle the magnetizing inrush currents (25 times for 0.01 sec, 12 times for 0.1 sec)

Based off the in rush of this transformer I would need a 80A J-type fuse.

If I put an 80A fuse in the transformer won't be protected for over current, basically the fuse will supply 80A when the transformer should only ever pull 45A max.

What kind of transformer damage would I substain by putting in an 80A fuse and running the transformer at 80A, I would assume it would be very warm and would reduce the life of the transformer. How long could I expect the transformer to hold up like this?

I know this doesn't meet code but just a question on my part.

Any thoughts,

Thanks

 

[dpc]

First, 12x for 0.1 sec is very conservative. Generally 8X or 10X will work.

I don't know what the Canadian requirements are but per NEC, if the transformer has secondary protection, primary protection can be 250% of the rated current. Without secondary protection, which I'm guessing is the case here, it is 125%

At 575 V, 45 kV is 45.2 A. 125% is 56.5 A. My guess is that a 60 A fuse will probably work.

As to protecting the transformer from overloads - to do that you need to spend a few more pennies and put in a secondary main breaker or fuse. The primary protection keeps the building from burning down if the transformer fails internally. To truly protect for overload, some type of secondary protection is needed.

Good luck,

Dave

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)