Are fuse classes interchangeable, or not?

[Shackled Designer]

My context is in industrial panel design where we purchase VFDs, servo power systems, PLCs, etc., and we attempt to assemble these on a properly protected panel.

By fuse class, I am not referring to bolts, 10-penny nails, or foil-wrapped wooden pegs , but UL classes such as T, RK1, G, L, H, and so forth.

My question has to do with cases where there is possibly an open design option (as is frequently the case with industrial panels), and so physical fuse sizing might be considered negotiable. For example, a drive manufacturer may call for the use of a class J fuse protecting the drive's branch. However, would there be anything wrong with using a class CC fuse if the fuse required is a 20A fuse? I realize, of course, that class J fuses cover a much larger range of OCP, so my options with CC types are limited (30A or less). On the other hand, it can be argued that a 20A CC class fuse has lower let-through values than the same current-rated J class fuse, and so it is safer. (At 30A, the J and CC class fuses have identical let-through values.) Also, CC class features smaller dimensions and so conserves panel space.

For branch circuit conductor protection, the NEC seems to treat several classes of fuses as interchangeable (see 240.4[D] in 2014). However, some fuse manufacturers claim that some fuse classes (such as J class) are not interchangeable with other classes. If we are able to leave aside the physical size considerations, what else beside the possible substitute fuse's protective characteristics needs to be considered?

Regards,
The Shackled Designer

 

[petersonra]

A lot of times the manufacturer of a device such as a drive will specify the class(es) of fuses allowed. Sometimes this has to do with testing by UL for a given short circuit rating. Otherwise, as long as they are branch circuit fuses it is a design decision.

UL508a has some further restrictions on what classes of fuses can be used if it is a UL508a design.

It is probably best to stick with class J fuses since they are the most common in the US. For 30A and smaller class CC fuses are pretty common as well. Generally speaking, class J and class CC fuses are also the most cost effective and are pretty space efficient.

 

[Shackled Designer]

Alright. So if the vendor says to use class J, then that is probably what has been tested and approved, in my case by UL. Looks like that's what I have to do then.

Too bad my bus bar only supports a single width per pole. I'll have to locate the class J fuse holder separately.

I appreciate your response in any case.

Kind regards,
Shak

 

[petersonra]

Alright. So if the vendor says to use class J, then that is probably what has been tested and approved, in my case by UL. Looks like that's what I have to do then.

Too bad my bus bar only supports a single width per pole. I'll have to locate the class J fuse holder separately.

I appreciate your response in any case.

Kind regards,
Shak

what bus bar are you using?

bussman has a system that can bus their class CC fuseholders - 1 and 3 phase versions available. Littelfuse has a similar system, also limited to class CC.

 

[Shackled Designer]

My company uses the Smissline bus bar from ABB. Like the Bussmann system you mentioned, the Smissline is physically size-limited to CC type fuses. Quite a few of the devices we supply through the panel are ideally suited for CC class fusing. Also, many miniature circuit breakers over a reasonably broad range (for our panel devices) of amperage fit these bus bar systems.

Vendor testing of device + OCP combinations as it commonly occurs, however, does not always include CC type fuses, and so the only documented combinations for UL compliance in many cases involve different class fuses. Looks like my panel will simply have to include one or two oddball fuse holders.

Time for a tall glass of "deal-with-it" and a straw to "suck-it-up."

Kind regards,
Shak