Engineering Circuit Breakers
Protective Devices for Appliances: Which Standard to Use?

by Ken Cybart, senior applications engineer, E-T-A Circuit Breakers, Inc.

Choosing circuit breakers to be used within appliances is not as simple as it appears. There are two UL standards for these devices - UL 489 and UL 1077. The following will explain the differences between the two standards and provide some guidance on how to apply them.

There are two Underwriters Laboratories (UL) standards that may apply to the circuit breakers that are installed in appliances. Some appliance specifications call out UL 1077, "Standard for Supplementary Protectors for Use in Electrical Equipment," while others refer to UL 489, "Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures." UL 489 can also be referred to as "branch circuit protection" or "UL-listed." Some appliance specifications now require UL 1077 specifically, although not all UL 1077 devices qualify. For example, UL 197, which covers commercial electric cooking appliances, was recently changed to allow only certain UL 1077 breakers.

Why is it important to find out if a UL 1077 device is allowable in an application? First, a UL 1077 device is often more cost effective than a UL 489 breaker. In addition, UL 1077 devices are generally available with a wider variety of designs, ratings, mounting arrangements, actuators, and tripping characteristics than UL 489 breakers - enough to fit almost any functional and design requirement.

Since UL 1077 devices are intended for use within an appliance that is certified to UL standards, their specifications cover a wide range. They may be tested and approved in several different ways; they may be overload tested at a 6x or 10x rating for a motor starting or at a 1.5x rating for general use. They may be short-circuit tested with separate overcurrent protection in series. They may be tested at different voltages (a.c. or d.c.), and their terminals may be designed for connection only at the factory or for installation in the field.

The most important difference between UL 489 and UL 1077 is what happens when there is a short circuit, rather than an overload. The two standards differ in their specifications for interrupting capacity and for surviving short circuits.

Short Circuits

The amount of damage a short circuit can cause is directly related to the amount of fault current available. In service entrance panels, available short-circuit fault currents can reach 50,000 A or greater. However, as power is distributed throughout a building, the available short-circuit currents diminish. If an electrical outlet is just 20 ft away from the power source, Ohm's law states that even with limitless short-circuit current available, AWG 14 copper wire will limit the maximum available fault current at the outlet to no more than 1,200 A at 120 V.

Table 1 shows the effect of different lengths of wire on the amount of short-circuit current available.

UL 489 Requirements

UL 489 covers circuit breakers "intended for installation in a circuit breaker enclosure or as parts of other devices, such as service entrance equipment and panel boards." Their main purpose is to protect branch circuit wiring, not necessarily the connected load. According to UL terminology, devices meeting this standard are considered "listed products."

Both UL 1077 and UL 489 require a series of calibration, overload, endurance, and short-circuit tests. For UL 489, the minimum short-circuit test must be performed at 5,000 A, and overload tests are performed at six times the current rating of the device, or 150 A minimum. Devices rated up to 600 V and 6,000 A are covered in this standard. During UL 489 testing, the device must survive short-circuit testing and work during future overload operations. Requirements for UL 1077 are different in this area. Table 2 gives a brief rundown on the differing requirements of the two standards.

UL 1077 Requirements

This seems simple, but like so many things having to do with regulations, it is not. Even the terminology can be confusing. While UL says that components recognized under UL 1077 are not called circuit breakers, many of these protectors are built to meet multiple standards like the European EN 60934 (which refers to them circuit breakers for equipment or CBEs) or even to UL 489 (see Figure 1), in which case they can legitimately be called circuit breakers.

UL 1077 breakers are backed up by a UL-489 listed circuit breaker, or suitable fuse, upstream. In addition, UL 1077 devices have specifications that vary between manufacturers or within the same series. They are approved for use as part of a product that has its own approval by a recognized testing agency.

Short Circuits and UL 1077

All circuit breakers are tested for short circuits, but the severity of a short circuit depends on circumstances, and it is important to know what the breaker will have to handle. Whether it is mentioned on the data sheet or not, every breaker has two ratings for interrupting capacity. One specifies the amount of current the breaker can interrupt safely and still work afterwards (sometimes referred to as Icn). The other (generally much higher) rating specifies the largest current that the breaker can interrupt safely (i.e., without starting a fire), but may be destroyed in the process ("not fit for further use" or "not recalibrated after testing"). Some manufacturers publish both ratings, but many do not.

The question of which rating to use depends on the maximum possible current available at the point of use. If shorts are expected frequently, it is a good idea to specify a breaker that will survive and is "fit for further use." If the breaker's main job is to protect against overloads, and shorts are infrequent or limited by other circuit components, then the breaker's interrupting capacity is generally not an issue.

Overloads

Overloads can be short-term or long-term. A protective device must not activate on momentary or short-term overcurrent events. Many types of appliances will experience short-term overcurrents in their normal operation. Some electronic devices, for example, will create inrush currents as their internal power supply and filter circuits start up. These inrush currents typically last only a fraction of a second and seldom cause a problem.

A major class of short-term overcurrents is motor starting surge. Most motors, especially those that start under load, draw several times their normal current when starting. A good example of this is a household washing machine going into the spin cycle, which may draw 27 A for several seconds before dropping down to its normal 5-A load. To avoid nuisance tripping, it is important to choose a breaker with the appropriate delay curve. Available delay curves range from instantaneous (used to protect sensitive semiconductors) to ones that will hold a 200-percent overload for a minute or more. Somewhere between these extremes is the appropriate delay curve for any application.

The two UL standards treat overloads differently. UL 489 requires breakers to withstand a 6x overload, while UL 1077 requires only a 1.5-times overload test, requiring a 6x overload test only in those cases that involve across-the-line motor starting. This means that it is often possible to use a cost-effective protective device under UL 1077.

Conclusion

When choosing protective devices, it is advised that a designer reviews both UL 1077 and UL 489 standards. UL 1077 makes it possible to tailor the appliance's protective devices more closely to the individual application than does UL 489. In addition, it can also be more cost effective. By taking a close look at the applicable standards, it is possible to design a safe and high-quality appliance with better styling, superior electrical protection, and lower cost.

About the Author Kenneth Cybart is senior applications engineer at E-T-A Circuit Breakers, a manufacturer of circuit breakers used in telecommunications equipment.