class R fuses


Thread Starter

Anthony Kerstens

I have a customer that has asked that R class fuses be used on LV building service distribution and lighting transformer primaries. They would also be inclined to extend that requirement to other transformers, such as what I use for step down to servo power. Their reason for this is to eliminate "nuisance tripping". My suspicion is that this arises from the customer previously having an improperly sized J (with an overburdened transformer), put an R in place, and "solved" the problem. So: I'm wondering, given that my fuse catalogue and other information indicates that a class J is the best choice for a transformer primary, what advantage a class R would give? The Gould catalogue I have in my library even goes as far as saying that class R's are generally not used on new installations. The differences that I can see between the J and R is that the R has a longer trip curve, and is physically much larger. Anthony Kerstens P.Eng.

Steve Myres, PE

Cost, I think, is about the only advantage. J classes have much better current limitation and short circuit interruption capacity, as I recall. I use mostly Bussmann fuses (when using fuses) and even in an application requiring a R-class dimensioned fuse, will use a LPS-RK or LPN-RK rather than a FRS or FRN (R-class). The LP (Low-Peak) type have electrical characteristics more like the J-class (LPJ) and fit R-class blocks. The LP's of either dimension, are a little more spendy, though. Steve Myres [email protected]
This is in response to Anthony Kerstens' Fri, Mar 23, 5:18pm query: Both the R-class and J-class are available as the Dual-Element Time-Delay (TD) type or as the Fast-Acting (FA) type. I believe you will find that the original "J" fuse was the FA type. The replacement "R" is probably the TD type. Please note that the TD and FA notation is mine only to simplify discussion! You should conduct a fuse-coordination study to insure selective tripping. A fuse manufacturer should be able to help. If unable to get the help then contact me. I can provide another "The Physics of..." discussion, presuming the List is amenable! Regards, Phil Corso, PE (Boca Raton, FL)

C. Thomas Wiesen

For us that are a little light on the application of fuses, can someone recommend a source of fundamental information for fusing? Things that I am interested in are: What types of fuses to use for what types of devices? How to rate a fuse? Where should the fuse go in the circuit? (I have seen power supplies use one or two fuses. Is a single fuse used on the ungrounded leg of a grounded system?) When should power circuits be isolated?
Responding to C. Thomas Weisen's query dated Tue, Mar 27, 10:21am on the subject of Fuse Applications: One excellent primer (pronounced primmer in the UK) is the National Electric Code (NEC). Produced by the National Fire Protection Association (NFPA), it is known as NFPA-70. Another is the American Electrician's Handbook. Both are considered as the "bible" of electrical system design and installation. They present the "minimum" standards that electrical professionals should use to attain safe, effective, and operable electric systems. In addition, the Electrical Construction & Maintance (EC&M) trade magazine published by McGraw-Hill also produces a series of Guide Books for Electrical Systems. Regards, Phil Corso, PE (Epsicon Inc)

Eric M. Klintworth

The Buss Electrical Protection Handbook is an excellent place to start. The only downside is it is rather biased against circuit breakers (although not without justification). You can get it from any Buss fuse dealer worth their salt, or download it (3 meg PDF) from: Eric M. Klintworth, PE Sharp Technologies, Inc. Columbus, Ohio

Michael Griffin

At 10:21 27/03/01 -0600, C. Thomas Wiesen wrote: >For us that are a little light on the application of fuses, can someone >recommend a source of fundamental information for fusing? > >Things that I am interested in are: >What types of fuses to use for what types of devices? >How to rate a fuse? Many fuse manufacturers have application handbooks. You should be able to download copies off their web sites if you can't get a printed one. >Where should the fuse go in the circuit? (I have seen power supplies use >one or two fuses. Is a single fuse used on the ungrounded leg of a >grounded system?) >When should power circuits be isolated? <clip> Some of these questions are best answered by consulting your electrical code (wherever it is you are). The electrical code should be quite specific about what is required. What size of fuse, and where one is required depends upon the application. Some of the details will vary from country to country. On a related question, I have seen some American designs with fuses in both the line and the neutral of a single phase AC circuit. This is not allowed here (you must fuse the line only). Does anyone know the reason for this fuse in the neutral? Is there a reason? ********************** Michael Griffin London, Ont. Canada [email protected] **********************

Anthony Kerstens

I use the electric code, and the fuse catalogue. Gould and Bussman also have pretty good websites where you can download spec sheets with curves. The printed Gould literature I have actually summarizes the code into a table of applications. I'm sure Bussman would have something similar. Your local Gould or Bussman rep should be able to put together a package for you, and maybe even arrange a seminar. I attended an excellent seminar put together by a Gould rep several years ago that include a video on catastrophic failures. (fuse boxes exploding, etc.) Anthony Kerstens P.Eng.

Bouchard, James [CPCCA]

My understanding of the Canadian Electrical code says that you are not allowed to put a fuse in the neutral conductor. Strictly speaking a neutral conductor requires at least three wires in the circuit since the neutral by definition only carries the unbalanced current and if you have only two wires in the circuit you don't have any way to balance the currents from two loads against each other. Both conductors carry the same current all the time. In a 120/240 three wire circuit the neutral will carry no current if the loads on each of the other legs is identical or if not it will carry at most the same current as the most loaded of the other two wires instead of the sum of the currents in the other two wires. The only times I have seen a two wire power circuit that had fuses in both sides was when neither side was grounded and there was a ground detector circuit that cut the power in case either of the wires was grounded. This was used on a hot wire sealer circuit where the sealing wire was mounted on insulating material for both thermal and electrical reasons. James Bouchard