Circuit breakers are used in a variety of ways. They are mounted in panel boards to protect branch circuit wiring, and they are built into equipment to protect it. With this range of applications, it’s not surprising that a circuit breaker must provide both short circuit and overload protection.
Interrupting a short circuit current that’s limited only by the resistance of the wiring is a very severe test of a circuit breaker, and if the interrupting capacity of the breaker is not adequate, the device can literally explode. Overload currents that reach 2 to 5 times the normal rating of the breaker are handled differently, and very often the circuit breaker must carry the current for an appreciable time without tripping.
This white paper will give pointers on how to determine the main job a breaker must do and how to make an appropriate selection.
Protection against shorts and overloads is the largest concern when choosing a circuit breaker. Branch circuits fed from a 480V main need protection against short circuit currents measured in ten of thousands of amperes. For that reason panel boards 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 5000 to 50,000 amperes or higher.
Long years of experience in the field of circuit breaking with interrupting devices have revealed that under adverse conditions of circuit parameters, interruption may not be smooth. It may result in excessive voltage surges, as a consequence of restricting of the parting contacts. A wrong choice of interrupting device may result in insulation failure of the terminal equipment, such as a power transformer, an induction motor or interconnecting cables.
This situation may arise when:
- Interrupting small magnetizing currents, such as interrupting an induction motor or a transformer on no load, a situation, when the current may lag the impressed voltage by nearly 90 degree.
- Interrupting a charged capacitor bank, when the current will lead the impressed voltage by nearly 90 degree.
- Interrupting an unloaded transmission or distribution line or a cable, i.e. interrupting a line charging current, which is capacitive and may lead the system voltage by nearly 90 degree.
- Interrupting an induction motor immediately after a switch on, when the current is large and highly inductive.
- Interrupting fault currents that are mostly inductive and occur at very low power factors.
They are excessive in magnitude, and cause high thermal effects and electromagnetic* forces on the arc chamber, the contacts and the contact mounting supports.
Under the above conditions, the arc, as usual, will extinguish at the first current zero but will have a tendency to re-establish immediately again, after the current zero while the contacts are still parting. This is because the TRV across the parting contacts may exceed the dielectric strength of the contact gap achieved so far.
Restoration of the dielectric strength will depend upon the speed of the moving contact and the insulating medium of the arc chamber. There may be a number of restricts before a final extinction is achieved. The frequency of restricts may be extremely high, depending upon the L and C of the interrupting circuit, which would have the characteristics of a surge circuit on formation of an arc. In terms of actual rated frequency ( f ) , restoration of the dielectric strength may not take more than one half to two cycles, i.e. 10-40 ms (for a 50 Hz system). The behavior of circuit breaking thus depends upon the design and the quenching medium of the interrupting device.
Protection against short circuits
All circuit breakers are tested for short circuits but the severity of a short circuit depends on where it is used in the circuit. Not all devices will continue working after opening a short circuit. Standards UL 489 and UL 1077 have different requirements.
UL 489 requires that the breaker remains working after being subjected to a short circuit test, but UL 1077 and the IEC and EN 60934 allow for breakers to clear a short but be safely destroyed in the process. Whether a breaker will or will not survive a short circuit depends on the magnitude of current involved. Whether it’s 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 remain operable afterwards.
Protection against overloads
Overloads can be short-term or long-term. The protective device chosen must not trip on momentary or short-term over current events that are normal for the piece of equipment it is protecting. 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 class of short-term over currents is a motor starting surge. Most motors, especially those that start under load, draw several times their normal current when starting. Other over currents may last even longer, and still be part of normal operation. A piece of motor-driven equipment, for example, may draw 50% more than normal current for several minutes at a time and the breaker should not trip under these conditions. If the overload lasts longer than normal, the breaker should open to prevent overheating and damage. What gives the breaker the ability to discriminate between normal and damaging over currents is the delay curve.
There are four choices of delay curves in circuit breakers: 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.
Thermal 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. It is appropriate for machinery or vehicles where high inrush currents accompany the start of electric motors, transformers, and solenoids. There are some thermal circuit breakers with hot-wire elements, which provide faster switching. They provide a low cost solution for appliances and printed circuit board protection, among other applications.
Thermal-magnetic breakers combine the benefits of a thermal and magnetic circuit breaker: they have a thermal delay that avoids nuisance tripping caused by normal inrush current, and a magnetic solenoid for fast response at higher currents.
Both standard thermal and thermal-magnetic circuit breakers are sensitive to ambient temperature. However, they can be selected to operate properly in a wide temperature range.
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 over currents, but trip quickly on short circuits. Many hydraulic-magnetic circuit breakers are available in a selection of delay curves to fit particular applications. Hydraulic-magnetic circuit breakers are not affected by ambient temperature, but they tend to be sensitive to position. These breakers should be mounted in a vertical plane to prevent gravity from influencing the movement of the solenoid. If mounted in a different position derating may be needed.
Pure 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 equipment such as telecommunication equipment, printed circuit boards, and impulse disconnection in control appliances.