breaker feeding a fused disconnect

[jlg]

Hi, I'm looking for a little input on a situation I have. I'm trying to update all our arc flash info/labeling and came across a situation I'm a little unsure about. I have a 200A breaker with a fault clearing time of 0.025s that is feeding a 112.5kVA transformer (480 to 120/208) with a 4.8%Z that goes to a fused disconnect with 150A RK5 fuses. Am I correct in my thinking that the fuse is going to dictate my arc flash rating. The slow acting fuse is greatly increasing the arc rating to beyond CAT 4, where if the breaker was controlling it looks like it would take it down to a CAT 1. Thanks!

 

[steve66]

are both the breaker and the fuse on the primary side of the transformer? Breaker at the panel, wire and conduit to the fused disconnect located near the transformer, and then the transformer primary?

Or is the fusible disconnect on the secondary side?

 

[jlg]

The disconnect is on the secondary side.

 

[charlie b]

The primary breaker is not relevant. We are not allowed to take credit for an OCPD on the primary side to protect anything on the secondary side. There are a couple exceptions, but they do not apply to your transformer.

This is not relevant to your question. But I am wondering why you are using a 150 amp fuse for a transformer that has a secondary current rating over 300 amps. :?

 

[jlg]

OK, didn't think the primary would make any difference, just wanted a second opinion.

This is a very old building (40-50 years) and all of this stuff was installed way before I got here, now I'm just trying to get all of our arc flash ratings up to date. There are a lot of things that I've seen done in this plant that don't make a lick of sense but probably won't be changed any time in the near future so I'm just trying to make the best of it.

 

[dkidd]

There is a forum dedicated to arc flash issues that is pretty good

http://www.arcflashforum.com/index.php

 

[steve66]

Just offhand, a 150 amp fuse with a 300KVA transformer behind it doesn't seem like it would give a Cat 4???