In my company's standard spec, we call for a type RK5 for 0-30A fuses.

Type RK5 are rated 0-600A, type CC are rated 0-30A

The contractor submitted type CC time delay.

Is there a reason to spec type RK5 for a 0-30A application?
Should I accept CC?
Should I change the standard spec to use CC for 0-30A?

I think we need a full listing of both fuse specs to advise - such as
voltage rating
current range
interrupt rating
Time vs current curves
Availiability to you locally
Last - cost.

Dan Bentler

It used to be that the fuse "type" was critical. These days, all that the "type" means is the packaging that it comes in. CC is one of the oldest fuse package designations, right up there with RK-9's which are almost impossible to find these days.

RK (actually originally R) is the old standard package for dual element fuses. RK-9 was the first and very large version. Then smaller fuses came out with the RK-5 designation. Now most manufacturers are pushing RK-1 if you stick with the RK series, but you need different fuse holders or extensions to use an RK-5 fuse holder (or else use the RK-5 fuses with RK-1 trip curves). If you read up on fuse info though the class J's are even better and are recommended for new designs. With AB contactors, they tend to come with RK-5 as the standard fuse holder but you can request a couple different versions when you order them including class J's.

With all this stuff, best thing to do is to study the fuse curves. Head over to Ferraz-Shawmut's web site or to Bussmann and you'll learn what the differences are.

Physically, the RK-5's are roughly 3-4 times as long as they are wide. Class CC's are about 2/3's of the length and much fatter. J's are shorter yet and fatter yet, and are just about the smallest fuse you can get for a motor starter.

If you are not dealing with inductive loads, you should be using strictly instantaneous trip fuses by the way and none of these fuse classes apply. If you are using semiconductor components such as drives, they often require very fast fuses to protect the SCR's and IGBT's since those components don't have very much physical mass and are highly susceptible to being destroyed by overloads.

It has been years since I went to "Fuse School" put on by Bussman but at the time I learned a lot of useful information I have recently not needed. I'd see if Bussman was still conducting these "fuse Schools" through their distributors and attend one or at least chat with your Bussman or other fuse manufacturer's rep.

Hey guys, long time.

I think maybe you're reading too much into my question, or looking at job-specific info. This is just a general question... in general, is there a reason to choose an RK5 for a 0-30A application. I guess my main question was, was it a typo in our spec to list an RK5 for 0-30A? Is there any application where you'd want to use an RK5 over a CC for a 0-30A application, or would a CC always be sufficient?

CC's would work. RK-5's have better characteristics and give your motor better protection in a fault situation (better chance of surviving a fault). Very little equipment comes with CC fuses as standard any more. Most motor starters default to RK-5 as standard with many manufacturers pushing for class J's. Why try to push for an older standard that is more or less being deprecated among fuse manufacturers? Plus, if you already have an inventory of RK-5 fuses, even though they aren't that expensive, why add more fuses to the existing inventory if you've got it covered with the existing RK-5 stock? It's just one more excuse the warehouse has for being out of inventory on a critical spare item like a fuse.

If it was my plant and I noticed class CC fuses being used in a new design, I wouldn't accept it. I'd give preference for class J's and accept RK-5's with justification. At least this has been my standard for the past 5 years.

Thanks Paul!