Equipment manufacturers usually regard any certified device that looks and seems to act like a circuit breaker as a circuit breaker per se. However, some devices that look like circuit breakers have limited application; their use in situations calling for a listed circuit breaker can raise safety issues. Known as supplementary protectors, they are recognized components. Supplementary protectors are not, as their name suggests, designed to provide primary power protection in equipment that supplies mains power to other equipment.

This article clarifies the often-confusing terminology and identifies applications for which circuit breakers and supplementary protectors are appropriate.

Devices for Overcurrent Protection

What exactly is a circuit breaker? The 1999 version of the National Electrical Code (NEC) defines a circuit breaker as "a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its ratings." Overcurrent in turn is characterized as "any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault."¹

The molded-case circuit breaker standard (UL 489) lists specific devices that are "intended to provide service entrance, feeder, and branch circuit protection in accordance with the National Electrical Code, NFPA 70."² Some applications of molded-case circuit breakers are outlined in Article 210-52 (Conductor Protection) and Article 430-52 (Motor Protection) of the 1999 code.

And what, then, is a supplementary protector? Article 240-10 of the 1999 version of the NEC establishes clearly the differences between circuit breakers and supplementary protectors: "Where supplementary overcurrent protection is used for lighting fixtures, appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for branch-circuit overcurrent devices or in place of the branch-circuit protection specified in Article 210. Supplementary overcurrent devices shall not be required to be readily accessible."

The supplementary protector standard (UL 1077) recognizes devices that are "intended for use as overcurrent, or over- or under-voltage protection within an appliance or other electrical equipment where branch circuit overcurrent protection is already provided, or is not required."³ It is important to note that supplementary protectors do not meet the strict NEC requirement for branch-circuit overcurrent protection quoted above. These devices are intended only to protect equipment components in specific applications.

The significant differences between listed molded-case circuit breakers and supplementary protectors are outlined in Table I. Despite the notable divergence in these devices' capabilities and purposes, supplementary protectors and circuit breakers are often confused. The confusion can be attributed to a variety of factors.

First, supplementary protectors are often referred to as circuit breakers for equipment (abbreviated as CBE) or miniature circuit breakers (MCBs). In related literature and advertising, these devices may be called circuit breakers, leading designers to incorrectly conclude that they will perform like molded-case circuit breakers. Supplementary protectors also look very similar to molded-case circuit breakers, and thus are thought by designers to provide equivalent protection.

Another problem is that manufacturers of supplementary protectors do not always clearly indicate that suitable branch protection or in-line fusing is required in certain applications. Such an indication explicitly shows the inappropriateness of relying on the protectors to fulfill circuit-breaker functions.

Finally, European manufacturers use supplementary protection for circuit-breaking in equipment. European equipment imported into the United States is generally used in industrial settings in which equipment wiring is provided remotely from the control panel. Such wiring is a violation of the NEC if it is protected by a supplementary protector.

Evaluating the Usability of Supplementary Protectors

The usability of a supplementary protector beyond its incorporation into a listed appliance or assembly is very limited. How can a design engineer determine whether a supplementary protector will be acceptable in an application? The National Electrical Code, NFPA 79 (the electrical standard for industrial machinery), and UL 508A (governing industrial control panels) are examples of product standards that provide guidance in determining the acceptability of a supplementary protector for a particular use. The flowchart in Figure 1 can help to evaluate its suitability. However, use of the flowchart should not be substituted for careful review of the applicable end-product standard.

Figure 2 diagrams a simple circuit that invites consideration of the possible application—and misapplication—of a supplementary protector. For example, a designer can apply the decision chart in Figure 1 and assume that the fuse in Figure 2 has not been installed originally as part of the circuit. The circuit diagram then provides guidance on the proper installation of the supplementary protector.

Figure 2. Schematic circuit diagram of a control panel with a supplementary protector.

If a short circuit or overload occurs on the load side of the supplementary protector in the diagram, then the only protection provided would be through the 100-A branch circuit. The 14 AWG conductor is not protected by the 100-A circuit breaker as required by the NEC.

The added fuse, sized appropriately for the voltage, supplies the needed branch-circuit protection. In this case, it would need to be sized at 15 A. With the addition of the fuse, the supplementary protector would be sufficient for the application in the figure. The fuse protects the branch-circuit conductor on the line and load side of the supplementary protector.

Conclusion

Supplementary protectors are circuit-protection devices that, like any other recognized component, must be used properly when incorporated into equipment and appliances. Engineers creating or reviewing a design that includes a device that looks like a circuit breaker (both on paper and in fact) must be sure to evaluate the circuit for proper branch-circuit protection. Supplementary protectors have very limited utility in applications involving appliances or assemblies that are not listed. Designers or manufacturers of an appliance or assembly should always request information regarding the conditions of acceptability for any supplementary protector being considered for use. This information helps confirm that the recognized component will be employed in accordance with its certification limitations.

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