In last month's article we discussed the importance of selecting the correct fuses for an application; not only when you first specify them, but also when you replace them. To aid in selection, manufacturers divide fuses into classes, specifying them alphabetically.

Classes H, K, and R are "full-size" fuses, often used in motor applications. You'll find all three classes with voltage ratings of 250V or 600V, while amp ratings range from one-eighth of an amp up to 600A. The critical differences lie in their interrupt capacities and current-limiting abilities.

Class H has a small interrupt capacity of 10,000A. Some are current-limiting, but only up to their interrupt capacity. You can't tell if they are current-limiting, because none of them are marked.

Class K fuses come in two interrupt ratings: 50,000A and 100,000A. All of them are current-limiting, usually either K1 or K5 (a K9 classification is now nearly obsolete). Despite their current-limiting abilities, none of them have current-limiting markings. Why? Because they are physically interchangeable with Class H fuses. This makes it possible to downgrade a current-limiting system by installing Class H fuses in place of Class K fuses.

Class R fuses help prevent system downgrading. In fact, this purpose was a main goal in their design. They have a fuse body allowing installation in a fuse clip that will reject both Class H and Class K fuses. Thus, with these fuses you cannot accidentally downgrade an electrical system designed to be current-limiting. Class R fuses, however, can replace either Class H or Class K, so you can easily upgrade installations with either of these fuses. In addition to having a universal interrupt capacity of 200,000A, all Class R fuses are current-limiting, with a rating of either RK-1 or RK-5. Of the two, RK-1 is more current-limiting, typically interrupting a short circuit in less than an eighth of a cycle; about two thousandths of a second.

Now let's see if you've been paying attention. You've just corrected a fault that blew a 250V/65A, Class K fuse in a motor starter. An identical replacement will not be available for 2 hr, but you must return the motor to service immediately. What are you going to do? A. Since you still have protection on the other two phases, you decide to jumper the fuseclips with a piece of wire. That gets the motor running, and you can replace the wire with the right fuse shortly. B. Joe says he's got a 600V/65A RK-1 fuse on the other side of the plant, but you'll have to run over and get it. C. You find a 250V/100A, Class K fuse in a nearby motor starter that's down for repairs.

Circle the answer that makes the most sense, considering production demands. Or if you are up to a little more thinking, rate these in order of best solution to worst.

Of course, these aren't the only kinds of fuses. For example, Classes T, G, and CC are popular. If you're ever in doubt about fuse selection, consult the manufacturer's guide. Many of these are now available online. You can find manufacturer's Web sites on Just click on the link for "other sites."


Now, let's see how well you did. The best answer is "C." The amp rating is a little big, but it'll get the motor running. It also provides good short-circuit protection for the system. Don't forget to replace this fuse as soon as the right fuse arrives. You should document this fuse as needing replacement.

If you said "A," you need to rethink your philosophy on electrical work. Jumping fuse clips puts you, others, and the electrical system in serious danger. You never know when a short circuit is going to occur. Jumping fuse clips is never an option. Besides the physical danger, there are legal ramifications that could result in tremendous personal liability for you.

"B" looks good, but after you run across the plant, you'll find out Joe's 600V fuse won't fit into a 250V fuse clip. Last month's article on fuse sizes explains why this is so.