Companion CLF with Cutouts Practices 2

[lansford]

Our utility is looking at its practice of installing a CLF with a NEMA rated link (K) Cutout at 12.47KV O/H. We install them wherever the fault current exceeds 5KA sym. Our standards say our lowest rated cutouts are rated 6.2KA sym. Hearsay notes we install them due to dist. tranformer withstand but we have no documentation to support this. An IEEE T&D survey states utilites use them for both cases without any information on levels of transformer withstand ratings.
What is your utility practice?

 

[rbulsara]

Not sure of the utility companies, but it is a common practice to protect commercial/industrial transformers. It is not because of lack of transformer withstand ratings, but the low SCCR (interrupting) rating of the K links, which can be as low as 2.5 kA. CLF are generally sized larger as a backup to the links.

Rafiq Bulsara

http://www.srengineersct.com

 

[jtester]

It's been a long time since I worked on transformer protection, but they have an (I**2)t withstand rating. The use of the CL fuse is to limit the energy pass through.
I've seen pole mount transformers that exploded, showering oil everywhere, due to too much energy.
CL fuse use is very common in padmounts where an event could be catastrophic.

Jim T