Not all resettable overcurrent protective devices are circuit breakers; some are supplementary protectors. So how do you know one from the other?

When is a circuit breaker not a circuit breaker? No, this isn't a trick question. Not all resettable overcurrent devices, even those that look very much like circuit breakers (CBs), can actually be applied as CBs under present requirements. You need to know the difference between overcurrent protective devices and supplementary protectors so you don't misapply the latter devices. Most major breaker manufacturers make both products, so you'll see both in their catalogs. In addition, foreign standards don't use these terms in the same way, so you'll often see imported equipment with these terms used incorrectly.

You'll see supplementary protectors in catalog information from most of the major overcurrent device manufacturers, but if you read the fine print, you'll see you're not supposed to use them in the field as CBs. Nevertheless, many often specify these devices in error.

The NEC (in Article 100) defines a CB as follows: Circuit Breaker: 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 rating.

This definition seems pretty clear, and most people understand what a CB is intended to do. You probably have a picture in your mind of what it looks like. And, you're probably saying right now you know a CB when you see it. A CB is a CB is a CB, right? But is it always what you think?

You base part of the picture on your experience in using CBs and part on your understanding of the intent of the requirements in the installation code-the NEC. But there are some devices identified as CBs meeting this definition that do not meet the intent of the NEC.

When the NEC requires a CB, its intent is an overcurrent protective device providing overload and short-circuit protection suitable for branch circuit protection. CBs meeting this requirement are tested to comply with Underwriters Laboratories Standard UL 489, Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures, and are listed as a "circuit breaker." For NEC field applications, if the device isn't tested to UL 489, it isn't a circuit breaker. No matter how much it looks like one, it can't be used for branch-circuit protection.

If it looks like a CB but actually isn't, then it's probably a "supplementary protector." The NEC and U.S. product standards identify these devices as such. UL 1077 covers these devices and defines the term "supplementary protector:" Supplementary Protector: A manually resettable device designed to open the circuit automatically on a predetermined value of time versus current or voltage within an appliance or other electrical equipment. It may also be provided with manual means for opening and closing the circuit.

When you compare this definition with that of a true CB, you see similarities as well as some key differences. Yes, both devices open automatically on a predetermined value of current, but beyond that, the definitions deviate.

* The supplementary protector definition states the device may be provided with manual means for opening and closing the circuit; but, manual operability is not required. A CB, on the other hand, is a device designed to open and close the circuit by nonautomatic means. Therefore, manual operability is required. *Supplementary protectors are intended for installation within an appliance or other electrical equipment. As such, this precludes you from using these devices as branch circuit overcurrent protective devices in distribution equipment, such as a panelboard or switchboard.

So why are these devices sometimes identified as CBs? UL clearly identifies these devices as supplementary protectors. They are not listed products; they carry only component recognition. This means they lack features making them recognizable as equipment capable of independent field installation. Instead, they're factory-installed as components of more complex equipment, which subsequently undergoes evaluations by a test lab for a more comprehensive listing. Nevertheless, some of these devices (see photos, in the original article's page 64) do meet the NEC definition (as far as it goes), but not necessarily the NEC intent for a device providing branch-circuit overcurrent protection.

Just to make things even more interesting, some of these devices carry international recognition under foreign standards identifying them as "circuit breakers." While UL has one standard that covers all circuit breakers, there are other standards covering CBs that are quite different. The IEC (International Electrotechnical Commission), for example, has a number of standards covering products named "circuit breakers." The IEC equivalent to domestic supplementary protectors complies with IEC 934, Circuit Breakers for Equipment. Thus, you can identify devices complying with IEC 934 as a CB, and devices complying with both UL 1077 and IEC 934 as both supplementary protectors and CBs.

So what are these devices for? As noted in Sec. 240-10, supplementary protectors are not intended as a substitute for branch-circuit overcurrent devices. As the name implies, their purpose is supplementary, or additional, protection to the branch-circuit overcurrent device. For example, note the allowance in Sec. 240-4 for these devices to protect fixture wire and flexible cord.

There are other differences between supplementary protectors and listed CBs. Since supplementary protectors are recognized components intended for use in equipment, they can be quite different and vary widely. They provide more specialized protection for a specific purpose; perhaps even for a particular type of equipment. Some have horsepower ratings and are for use with motors; some don't.

Supplementary protectors may have different ratings and functionality depending on their intended use. Some have manual switching means while others don't. Some trip on current levels while others trip on voltage levels.

There are many differences in the standards. For a general comparison in a few key specific areas, the following differences exist in the standards:

* Spacings: Spacing requirements in UL 489 and UL 1077 are quite different. UL 1077 spacing requirements are different depending on the application, such as general industrial use, household appliances, household kitchen appliances, or commercial appliances. For example, a 600V UL 489 CB requires a spacing of 1 in. through air and 2 in. over surface. A UL 1077 supplementary protector for 600V general industrial use only requires a spacing of 3/8 in. through air and 1/2 in. over surface.

* Overload protection: UL 489 requires CBs to be tested at six times rating; UL 1077 requires overload testing only at 1 and 1/2 times rating, unless they have a horsepower rating. In that case, they're tested at six times rating.

* Short-circuit interrupting ratings: UL 489 requires a minimum short-circuit interrupting rating of 5000A for CBs rated 250V and less, and 10,000A for those rated more than 250V. UL 1077 devices do not have short-circuit ratings, but the standard does require a limited short-circuit test at a maximum current of 5000A. (The actual test value varies from 200A to 5000A, depending on the rating.) The tests and acceptable results are also quite different. UL 489 requires the CB to interrupt the circuit twice; in the first test, the faulted circuit is closed on the CB and then the same CB is closed on the fault. After these tests, the CB must still be functional and pass a dielectric test. UL 1077, on the other hand, requires supplementary protectors be subjected to three operations. However, the supplementary protector can be wired in series with a fuse or CB (the branch circuit overcurrent protection), which is allowed to open during the test. While the device cannot become a hazard, it can become inoperable during the tests.

How do you tell what you have? To use a UL phrase, "Look for the Mark." The Listing Mark includes the information identifying what the device is. The Listing mark for a CB identifies it as a "Listed Circuit Breaker." (Identify an interchangeable trip unit device as a "Listed Circuit Breaker Frame or Trip Unit.") A supplementary protector identifies with the Component Recognition mark (the "mirror-image" reverse UR), which is not a listing mark. It also may include the words "Supplementary Protector."

These devices provide a good example of why product standards and installation codes must work in concert with each other. When the installation code (in this case, the NEC) specifies a CB, the device must be investigated to the applicable product standard based on that installation code. Even a device listed as a CB under a foreign standard won't meet the definition of "listed" under the NEC unless it complies with the "appropriate designated standards" per Art. 100. In this case, that's UL 489.

If the product meets another standard, such as UL 1077, you must install it based on installation rules for that product, which in this case, are the rules for supplementary protectors.