Molded case circuit breakers are designed to provide circuit protection from overcurrent for low-voltage distribution systems. High-voltage circuit breakers may be: air-blast type, in which the arc is extinguished by a strong blast of air through an orifice across the arc; oil-type, which uses oil to extinguish the arc; or vacuum-type, in which contacts are separated in a vacuum.

Trip elements are either thermomagnetic or solidstate. Thermomagnetic (or electro-mechanical) trips, which are the industry standard in molded case breakers, use two components: bimetals and electromagnets. The bimetals are heated by the current flowing through the breaker. When overloading the breaker, the bimetal will bend and push the trip bar, causing the breaker to trip. Solid-state trips use current transformers and solid-state circuitry for reliability, accuracy, and repeatability. Once the tripping point is set, it remains constant at the set point.

An arc extinguisher confines, divides, and extinguishes the arc drawn between the breaker contacts each time the breaker interrupts current. A terminal connector joins a circuit breaker to a desired power source and load. Connection methods include bus bars, panel board straps, plug-in adapters, and reconnected studs.

Breaker selection is based on several site-specific parameters, including the following:

• Interrupting capacity is the maximum fault current the breaker can interrupt without damaging itself. The interrupting capacity of a breaker must be equal to or greater than the amount of fault current that can be delivered at the point in the system where the breaker is applied. This value can be in terms of symmetrical or asymmetrical, momentary or clearing current, as specified.

• Continuous current rating is the current the breaker will carry in the ambient temperature for which it is calibrated.

• Voltage rating is the maximum voltage that can be applied across its terminals.

• Frequency rating is the maximum frequency that can be applied without derating. High-frequency applications often require that the breaker be specially calibrated or derated.

• External conditions are the various (unusual) external conditions that the breaker may be subject to (temperature, altitude, moisture, etc.).

When a circuit breaker opens (while current is flowing), an arc is drawn out between the contacts (for a few cycles)

until the breaker action extinguishes the arc. Trip elements must have several protective modes, typically classified as follows:

• Long time functions allow overload currents of a low value to flow for several tens of seconds. This allows motors to be started without causing feeder disconnect.

• Short time functions allow higher currents to flow for up to 0.5 seconds. This allows transformers to be magnetized, for example.

• Instantaneous functions will cause the feeder to be disconnected immediately, isolating a short circuit in a feeder circuit before it takes the source down. Ground circuit protection will cause the feeder to be disconnected when excessive current flows from the feeder into grounded equipment.

In low-voltage circuit breakers, the trip elements are integrally mounted. In medium-voltage breakers, trip functions are provided by separately mounted protective relays. In addition to overcurrent, protective relays are available to protect against: unacceptable conditions in current flow direction, balance, and sequence; power flow for direction and magnitude; voltage and frequency for over- and under-conditions; voltage for balance and phase rotation; and other parameters.


Synchronization is a primary control function for operation of localized on-site generators in parallel with one another or with a utility-derived service. This is accomplished by matching the output voltage wave form of one ac electrical generator with the voltage wave form of another ac system. Paralleling ac generators can be likened to a car entering a highway which needs to synchronize its speed and position with the other cars.

There are two basic processes used for paralleling generator sets to a bus:

• Sequential paralleling. The generator sets are connected to the bus in a predetermined order. The lead generator set is connected first to the bus. When the second generator set is required and ready to be connected, it is synchronized to the previous generator and placed on the bus. This system is not the preferred approach because it delays power to critical loads. It is, however, an economical system because only one synchronizing device (automatic synchronizer) is used for all generators on the bus.

• Random paralleling. The generator sets are all started at the same time and the first generator to reach nominal voltage and frequency will be closed to the dead bus (in emergency systems). Dead bus logic is usually employed to prevent the possibility of connecting more than one generator to the dead bus at the same time. As the remaining generators approach proper operating parameters, their automatic synchronizers bring them into synchronism with the now live bus and cause their generator circuit breakers to close. This method is the most commonly used; it allows the generators to connect to the bus in the fastest time and lessens the possibility of failure of all generators to connect to the bus if the synchronizer fails, as may be the case in sequential paralleling.

For two systems to be synchronized, the following variables must be matched:

• The number of phases in each system

• The direction of rotation of these phases

• The voltage magnitude of the two systems

• The frequencies of the two systems

• The phase angle of the voltage waves of the two systems

The first two conditions are determined when the equipment is specified and installed. As illustrated in Figure 27-6, paralleling two ac generators requires that voltage, frequency, and phase angle be matched within close tolerances.

With synchronous generators, voltage, frequency, and phase must be matched each time before the breakers are closed. The voltage regulator controls generator output voltage by changing its excitation voltage. If two synchronous generators of unequal voltage are paralleled, the difference in voltage results in reactive currents and lowered system efficiency. When a generator is paralleled to a utility bus of unequal voltage, the generator will operate at varying power factor.

The frequency of a paralleling generator must be nearly the same as the utility system, usually within 0.2%. With a synchronous generator, the frequency match is normally accomplished by controlling prime mover speed.

The phase relationship between the voltages usually must be within 10 degrees. With synchronous generators, phase matching — like frequency matching — is accomplished by controlling prime mover speed. Synchroscopes and synchronizing lamps are used to measure the phase angle between two sources.

Paralleling is more easily accomplished with induction generators. No voltage regulator is needed because output voltage and phase angle will automatically match the system supplying its field voltage. Frequency is determined automatically by the field voltage of the utility system, but not until the tie-breaker is closed. Thus, the generator must be kept close to synchronous speed prior to breaker closure.

Synchronizing can be done manually or automatically. For the manual process, indicating lights, a synchroscope, a synch-check relay, or a paralleling phase switch may be used. Increasingly, however, manual systems are giving way to automatic synchronizing systems. Automatic synchronizers monitor phase voltage of an off-line generator and the voltage of the same phases of the active bus. The synchronizer compares the frequency and phase of these two voltages and sends a correction signal to the governor controlling the prime mover of the oncoming generator. When the outputs of the two systems are matched in frequency and phase, the synchronizer issues a breaker-closing signal to the tie-breaker, thereby paralleling the two systems. Figure 27-7 is a functional block diagram of automatic synchronizer operation.

Load Control

The most simple, low-cost system for load transfer between generating systems is an open-transition system. With open-transition transfer, the load is disconnected completely from one power source before being connected to the other. This causes a brief loss of power during transfer, with resulting surge when the load is reconnected to a power source. This disturbance is referred to as a bump.

A closed-transition system allows load transfer between generators and the utility system without inducing a surge. Closed-transition transfer thus has the advantage of a bumpless transfer between sources. Closed-transition transfer operates as follows:

• To initiate on-site generation, the generators start and parallel with the utility each through its own synchronizer. As the generators connect to the bus, the load will be transferred from the utility-derived

Fig. 27-7 Schematic Representations of Paralleling Switchgear System. Source: Woodward Governor

source to the generators. When the transfer is complete, the utility breaker is opened.

• To return to utility derived power, a plant synchronizer adjusts the frequency and phase angle of running generators to match the utility system. When the generator bus and utility are in synch, the utility breaker will be closed and the load will be transferred back to utility. When the transfer is complete, the generators will be tripped off-line.

Unlike the load transfer systems described above, import and export control systems are designed to allow for sharing of loads between the on-site generation system and the utility system. An import control system will limit the amount of utility-derived power by adjusting generator output. In a peak demand limiting application, the generators are started and paralleled with the utility when demand reaches a predetermined value. As the load fluctuates, utility power is limited to the demand set point by increasing or decreasing generator loading.

Consider a scenario in which the total facility load fluctuates between 1,000 and 3,000 kW, there are two on-

site generators, each with a maximum capacity of 1,000 kW, and the import level is set at 500 kW Table 27-1 shows the system response to different load conditions:

Table 27-1 Examples of System Response to Different Load Conditions

Load Condition

Load Value (kW)

Generator Load (kW)

Utility Load (kW) (Import = 500 kW)

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