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issue: March 2003 APPLIANCE Magazine

Electronics Report
Specifying Circuit Protection

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While a circuit breaker may seem like a simplistic component, there is enough complexity and confusion when it comes to specifying circuit protection that many engineers are designing devices with too little or too much protection.

Under-protected circuits leave equipment vulnerable to damaging electrical surges, and over-protected circuits add cost and can lead to nuisance tripping. The following are the most common mistakes appliance engineers make in specifying circuit protection and some tips that can prevent future mistakes, save costs, and protect your designs.

Specifying too high a rating to avoid nuisance tripping caused by in-rush or transient currents. Most engineers are concerned about nuisance tripping, as they should be, but they often specify a circuit breaker rated much higher than they should. Part of the reason is confusion between fuses and circuit breakers.

Engineers are used to oversizing fuses as a way to prevent nuisance tripping. However, there is no need to oversize a circuit breaker.

Unlike a fuse rating, a circuit breaker rating tells you the maximum current that the circuit breaker will consistently maintain in ambient room temperature. Thus, a 10-A circuit breaker will maintain a 10-A current without nuisance tripping. In fact, a typical 4-A circuit breaker with a slow trip profile will tolerate a temporary 10-A current surge without nuisance tripping.

Often times, nuisance tripping is caused by in-rush currents associated with certain electrical components - primarily motors, transformers, solenoids, and big capacitors. In such cases, the designer needs to specify a circuit breaker that has a delay. Thermal circuit breakers have a natural delay, and magnetic circuit breakers can have added hydraulic delays. Match the delay to the duration of the expected in-rush currents.

Over specifying or ambiguously specifying the degree of protection. Terms such as drip-proof, ignition protection, water-splash protection, and dustproof are commonly used, but may be misleading unless standard definitions are applied. When specifying, use the established standards as a measure, such as EN 60529/IEC 529, which defines Degree of Protection of Electrical Equipment. Using these standards, decide which protection is correct for the application.

For example, a combination switch-breaker installed in medical equipment might need a water-splash protection rating, but it probably does not need a rating for continuous immersion in water. Truly watertight and dust-tight circuit breakers are available, but they are expensive and usually unnecessary.

Specifying a fuse when a circuit breaker would be better. Although fuses provide inexpensive circuit protection, the cost savings should be weighed against the low total cost of ownership of circuit breakers. Foremost, circuit breakers can be quickly reset, enabling the circuit to be restored with a minimum of downtime. In addition, there is no assurance that a replacement fuse will be of the proper rating. If a fuse is replaced by a higher rated fuse, overheating and catastrophic equipment failure may occur. Circuit breaker performance is relatively stable over time, but as fuses age, their trip characteristics change. This may lead to nuisance tripping and increased downtime.

Circuit breakers offer designers more options than fuses do. An auxiliary contact may be added that can communicate an alarm condition to an LED indicator or software. In addition, a circuit breaker can be combined with a switch, saving space and adding overload protection. Remote trip is another option available with circuit breakers but not with fuses. Furthermore, unlike fuses, circuit breakers have a variety of types and trip profiles, and therefore, can be more precisely matched to loads and environment.
Finally, fuses cannot be tested without destroying them. How can you be sure the fuse you specify will open if there is an overload?

This information is provided by E-T-A Circuit Breakers, Mt. Prospect, IL, U.S.


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