Overload Ability of Power Fuses

[Mbrooke]

What does overloading do to power fuses? For example, a 300E fuse has a pickup of around 660 amps. I'd like to have 6 hours of 330 amps loading during contingency switching. This still leaves my with 200% for short load time inrush.

 

[Mbrooke]

I'm being told it will lead to annealing of the element, shifting the time-current curve.

 

[Palletjack]

And you don’t believe it?
Fuses TCC are perfect conditions.

Look in center of the center column...

https://www.sandc.com/globalassets/.../tcc-number-170-6-2.pdf?dt=637204140072133929

 

[Mbrooke]

True, they are perfect, and when a 300E fuse is subjected to 330 amps it will shift the curve- by all means. But, the part that confuses me is if that shift will become permanent over time from annealing.


S&C keeps hinting that their fuses are none damageable, and that they can tolerate overloading beyond their listed rating.

Because fuse links having silver element construction are not subject to damage by aging or transient overcurrents, it is unnecessary to replace unblown fuse links of such construction in single-phase or three-phase installations when one or more fuse links has blown.

 

[Palletjack]

Transient overcurrent and intentional overloading for 6 hours are two different things.

 

[Mbrooke]

True, there is a difference in that regard.

But let me ask you this: How is it that SS fuses can start their PX, PM and PF time current curves at 125% without link damage, but S&C starts them at 220-240%? How is this possible? Just wondering about the physics behind it.


I know what I'm thinking about is kind of ugly, but I'd like to maximize each part of the system during contingency switching. I mean some POCOs deliberately load 50MVA power transformers above 100% during the failure of its companion on the basis: 1) the peak load will be seen for only a few hours for a few days of the year 2) Mobile unit is getting brought in. 3) A reasonable loss of life is acceptable.

 

[Mbrooke]

Mobile unit:

 

[Palletjack]

I also know what a mobile unit is. We have three of them. I have one installed now in a drop down station..

Generally speaking, transformer overloads do not equate to fuse overloads.

 

[Mbrooke]

I know that you know, but just for the record lol

Usually they don't being two separate beasts- but being honest- a 220-240% pickup rating seems kind of high to me for a typical feeder. 200% typically works well- so its tempting the "maximize" the fuse.

BTW, what do you load your power trafos to during peak load?

 

[Palletjack]

We size ours so there is not an overload.
Do we need fans? Yes, and on one unit during hot summer peaks we need two stages.
It’s being changed next year.

 

[Mbrooke]

No overload for the loss of companion, meaning 50% of the second FA rating?