Breaker protection�overloads vs. shorts
The engineer must know the different demands placed on circuit breakers from both dangers
BY KEN CYBART
E-T-A Circuit Breakers
Mount Prospect, IL
http://www.e-t-a.com
Circuit breakers are used in a variety of ways. They are mounted in panelboards to protect branch circuit wiring, and they are built into equipment to protect components and systems. With this range of applications, the location of the circuit breaker dictates which hazard it will be protecting against, a short circuit or overload.
Circuit breakers such as E-T-A's model 2210-S can offer high-current protection using different interrupting capacities.
Interrupting a short circuit�current flow limited only by the resistance of the wiring�is a very severe test of a circuit breaker. If the interrupting capacity of the breaker is not adequate, the device can literally explode.
In contrast to shorts, overload currents that reach 2 to 5 times the normal rating of the breaker are handled quite differently. In this case, it is often best for the breaker to carry the current for an appreciable time without tripping.
Specific uses
Branch circuits fed from a 480-V main needs protection against short-circuit currents measured in tens of thousands of amperes, and for that reason panelboards are equipped with circuit breakers for branch circuit protection that are listed under UL 489, Standard for Molded-Case Circuit Breakers and Circuit Breaker Enclosures , and rated to interrupt fault currents from 5,000 to 50,000 A or higher.
A circuit breaker installed inside a piece of machinery is generally there to protect the apparatus itself, and the applicable standard should be UL 1077, Standard for Supplementary Protectors for Use in Electrical Equipment .
In UL terms, UL 1077-compliant devices are called “supplementary protectors,” labeled as “recognized components” (not “listed component”), and identified with Underwriter's Laboratories' distinctive backward URsymbol.
These devices are often called “circuit breakers for equipment” (CBEs). While both UL 489 breakers and UL 1077 devices protect against shorts and overloads, UL 1077 devices tend to concentrate more on overloads�largely because they are always used downstream of a UL 489 breaker.
Protection against short circuits
All circuit breakers are tested for short-circuit activation, but whether a device continues to work after opening a short circuit depends on the severity of the event.
For that reason, standards UL 489 and UL 1077 have different requirements. UL 489 requires the breaker to be functional after being subjected to a short-circuit test, but UL 1077 and the IEC standard EN 60934 allow for breakers to clear a short-circuit condition, but become safely destroyed in the process.
Whether a breaker will or will not survive a short circuit depends on the magnitude of current. Whether mentioned on the data sheet or not, every circuit breaker has two ratings for interrupting capacity.
One specifies the maximum amount of current the breaker can safely interrupt and still continue working (officially known as “fit for further use” or “recalibrated after testing”). Under EN 60934, this is the PC2 rating, while under UL 1077 it's the SC 2 value.
For example, E-T-A's 2210-S thermal-magnetic circuit breaker can interrupt between 400 and 800 A at 250 Vac and still work afterwards. The other (generally much higher) interrupting rating specifies the maximum current that the breaker can interrupt safely (for example, without starting a fire) but may be rendered inoperable in the process (“not fit for further use” or “not recalibrated after testing”).
Under EN 60934 this is the PC1 rating, while under UL 1077 it's the SC 1 value The 2210-S breaker mentioned above can interrupt up to 3,500 A at 250 Vac�but only once. Some manufacturers publish both ratings, but many do not.
Overload protection
Overloads can be short term or long term. The protective device must not trip with a momentary or short-term overcurrent event that is normal for the piece of equipment being protected.
Electronic devices, for example, may create inrush currents as their internal power supply and filter circuits start. These inrush currents typically last only a fraction of a second, and seldom cause a problem.
Another short-term overcurrent condition is a motor's starting surge. Most motors, especially those that start under load, draw several times their normal current when starting.
Other overcurrent situations may last even longer, and still be labeled as normal operation. If the overload in such a case lasts longer than a few minutes, the breaker should open to prevent overheating and damage. What gives the breaker the ability to discriminate between normal and damaging overcurrents is its delay curve.
Delay curves
There are four kinds of delay curves in circuit breaker technology�thermal, thermal-magnetic, hydraulic-magnetic, and magnetic. Each has a different trip profile in relation to time and current, and each has distinct mechanical characteristics (see Fig. 1 ).
Thermal circuit breakers incorporate a heat-responsive bimetal strip or disk. This type of technology has a slower characteristic curve that discriminates between safe, temporary surges and prolonged overloads.
Fig. 1. Characteristics of different circuit breaker types are reflected in their delay curves
This characteristic is appropriate for machinery or vehicles where high inrush currents accompany the start of electric motors, transformers, and solenoids. Some thermal circuit breakers have hot-wire elements to provide fast switching. They provide a low-cost solution for appliances and printed circuit board protection, among other applications.
Thermal-magnetic circuit breakers combine the benefits of a thermal and magnetic circuit breaker. These devices have a delay to avoid nuisance tripping caused by normal inrush current, and a solenoid actuator for fast response at higher currents.
A caution when placing thermal circuit breakers in enclosed spaces�both standard thermal and thermal-magnetic circuit breakers are sensitive to ambient temperature.
A magnetic circuit breaker can be combined with a hydraulic delay to make it tolerant of current surges. These hydraulic-magnetic breakers are similar to the thermal-magnetic in that they have a two-step response curve�they provide a delay on normal overcurrents, but trip quickly on short circuits.
Many hydraulic-magnetic breakers are available with a selection of delay curves to fit particular applications. Hydraulic-magnetic circuit breakers are not affected by ambient temperature, but tend to be sensitive to rotational position.
These breakers should be mounted in a vertical position to prevent gravity from influencing the movement of the solenoid. If mounted in a different position, derating may be required.
Magnetic circuit breakers operate via a solenoid, and trip nearly instantly as soon as the threshold current has been reached. This type of delay curve is appropriate for sensitive installations such as telecommunications equipment.
Advertisement
