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A look into the fuse boxes of any facility that has been in place for more than a decade will reveal fuses that no one would specify today. For example, Class H, renewables or slightly-newer Class K5 types. And even modern facilities may have fuses like Class RK5, which have been superceded by more modern types. This article will take a look at the consequences of retaining these older fuses and suggest modern replacements that provide multiple benefits to electrical contractors and their customers.

A renewable fuse, as the name implies, can be "repaired" and put back into service. To do this, unscrew the end caps, put in a new element, reassemble the case and you're back in business. The renewable fuse's perceived advantage is economy, because fuse elements cost less than entire fuses. In addition, an electrician can carry a handful of replacement elements in a pocket.

But renewable fuses have many drawbacks. First of all, they have a low short circuit interrupting capacity of just 10,000A. They provide no current limitation (defined as opening in 8ms or less during a short circuit). Their cases can deteriorate over time, and there is no way to control what someone may use as a replacement element.

Doubling fuse links may prevent nuisance opening, but it greatly diminishes the protection the fuse was supposed to provide. People have even been known to replace a blown link with a piece of wire when no replacement links were available. Because of potential danger and lack of protection, the National Electric Code (NEC) prohibits these in any new applications.

Renewables aren't the only Class H fuses; so-called "one-time" fuses (now designated Class K5) were a considerable improvement over the original Class H. They can't be tampered with, for one thing, and they're available with an interrupting rating of 50,000A. But they still provide no current limitation under short circuit conditions.

Class RK5 fuses are a bit better as they fit Class H and K5 fuseholders, are rated to interrupt 200,000A and provide current limitation. However, while they provide current limitation, it's not to the high degree of the newer Class RK1. Because RK1 fuses have superior performance, it's good to substitute them in most applications currently using Class H, Class K5 and Class RK5.

Standardizing on RK1 fuses has a lot of advantages for electrical contractors as it reduces inventory requirements (fewer types to keep in stock) and the chance of using the wrong fuse type. In addition, if services are charged at the flat rate, having fewer fuse types to worry about can speed the job and increase profits.

Industrial control panel safety

Fuses and circuit breakers are generally marked with an interrupting rating (IR) or ampere interrupting capacity (AIC). This rating, sometimes referred to as the withstand rating, is the maximum short-circuit current that the device can safely interrupt without causing a hazard. The same goes for industrial control panels, which have a short-circuit current rating (SCCR).

The SCCR of a panel must be greater than the maximum possible fault current available at its input. If the line feeding the panel can deliver more short-circuit current than the equipment inside can safely handle, then the result of a short-circuit can be a fire or even an explosion. Most electricians have seen dramatic training videos showing the results.

Usually the SCCR of a panel is determined by the "weakest link," or lowest-rated device within the panel, but it's often possible to increase the SCCR of a panel by replacing just a few components with current-limiting fuses.

Let's consider a panel with Class H or K5 fuses. Since it was installed — perhaps many years ago — the feed from the utility may have been upgraded (unknown to the facility) and the short-circuit current increased many times. Replacing the fuses with current limiting fuses can increase the SCCR of the panel to match the available fault current, improving safety at almost no cost.

Selective coordination

Putting in current-limiting fuses has another benefit: it can aid in selective coordination. Selective coordination is the selection of overcurrent protective devices (OCPD) in such a way that an overload or short circuit on one branch circuit will cause only the OCPD feeding that circuit to open, without causing "upstream" OCPDs to open. NEC requires complete selective coordination for emergency circuits and legally required standby systems, and in places like hospitals and elevators where losing power could be hazardous. However, best practice dictates that selective coordination be applied wherever it can reduce downtime and improve safety.

The usual way to achieve selective coordination is to "overlay" the system's fuse or circuit breaker time current (TC) curves to insure that the upstream fuse or breaker does not open under any circumstance before the closest device to the fault has a chance to open. This insures that the branch circuit with the overcurrent condition is isolated from the rest of the branches, which allows the remaining equipment to continue to function. Fuse manufacturers make this task easier by publishing coordination ratios for various fuse combinations.

By keeping a minimum ampere rating ratio between the "upstream" and "downstream" fuses, it is very simple to guarantee coordination. However, if the ratio isn't maintained, especially if there is a current-limiting fuse upstream from a noncurrent-limiting fuse such as Class H and K5 then all bets are off. The faster reacting fuse upstream may open and unnecessarily shut down additional pieces of equipment.

Reduce arc flash hazards

One of the most important benefits of current-limiting fuses is that they reduce arc flash hazards. Arc flash is an unexpected sudden release of heat and pressure produced by electricity traveling through air, usually caused by accidental contact between live conductors. Arc flash temperatures can reach 35,000° F. The incident thermal energy (heat loading) on a person standing in the Arc-Flash boundary can exceed the capability of personal protective equipment (PPE).

NFPA 70E lists the requirements for protection against arc flash, including the use of personal protective equipment. A current-limiting fuse will reduce the severity of an arc flash by opening in a half cycle or less. Figure 1 shows the difference in energy delivered to the arc.