Padmount transformer protection

You need the current-limiting fuse(or fusible link) to protect the Bayonet fuse. In general, the current-limiting fuse should only operate for internal faults in the transformer.

You can verify with ABB, but I doubt it is necessary to change the current-limiting fuse. But make sure the smaller bayonet expulsion fuse will not melt on transformer inrush.

"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)

Thanks DPC,

I already have checked the transformer inrush at 12x and it's ok.

I understand how the CLF can protect the bayonet, but the fuse link? How it's made and how it's working, i don't know.

So the bayonet in the switching device, without CLF isn't OK. i'm looking on drawing, i can clearly see the CLF for the transformer, but not for the switching device. I only see the bayonet.

I don't mean to throw this thread off, but reading this brings up another question about protecting padmount xfrmrs. When protecting transformers and primary feeds to them, there are some that leave the bayonet fuse to protect the transformer and feeder fusing to protect the wire.

I've seen others size fuses to the inrush of the total kva on the radial feed.

What's your philosophy?

tem1234,

The fuse link is just a small link of wire that will melt if the current get too high. Just a cheap substitute for a current-limiting fuse.

The main function of the fusible link or the CLF is to keep the transformer from catching on fire. If either operates, it means the transformer has failed and will be replaced anyway. These should never melt on a through-fault.

"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)

Much depends on your basic fuse coordination philosophy. Do you want the circuit breaker in the station to trip and save the fuse? Or do you want the fuse to melt so the breaker does not operate?
Here we use a combination of the two. Our trip levels on our breakers are set to the fault current levels at about 1/2 mile from the station. Anything inside that radius will cause the breaker to operate. Any taps from the main line feeder past that point are fused with 65 amp fuses. We allow the bay-o-nets fuses in each pad mounted transformer to protect the tranformer. Individual transformers tapped to the main feeder are fused according to the transformer size. 300 kVA = 20, 500 kVA = 30, etc. We have a 12,470 Gwye system.
I was always told by the guy who had my job before me that the tap fuse should protect the wire, the bay-o-nets will protect the transformer. I have seen up to 100 amp fuse on some of our tap lines, and they still coordinate with our breaker philosophy.
We use Cyme TCC to coordinate the time current curves of our fuses to our breaker relay.

I don't know if this helps or not, there are a lot of different ways and thoughts about how to protect wires and transformers.

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