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 manual trip device that is connected to the tripping linkage. Automatic tripping is normally
performed by one of three methods:
o
Series-overcurrent tripping.
o
Shunt tripping.
o
Undervoltage tripping.
Series-overcurrent tripping is the tripping of a circuit breaker from a trip coil in series with the
circuit responding to an increase in the circuit current above a predetermined value. Series
tripping is normally used on low-voltage circuit breakers. These breakers are complete with
adjustable (long-time, short-time, and instantaneous) direct-acting series overload tripping
devices. Most manufacturers offer static trip units on large low-voltage air circuit breakers as an
alternate to magnetic overcurrent devices. These solid-state devices are more reliable, have
time-current curves with narrower performance tolerance bands, are easier to coordinate with
other protective devices, and are easier to calibrate and set. Shunt tripping of a circuit breaker
involves a trip coil energized from the same or a separate circuit or source of power, and
controlled by contacts of a protective relay, control switch, or other means. The tripping energy
for shunt tripping is either provided by a control battery or an AC control power transformer.
Where AC power source is used, the most common method of tripping is the capacitor trip
scheme. The AC supply is taken from the source side of the circuit breaker and the capacitor is
charged before the circuit breaker is closed. To ensure adequate voltage, the capacitor is used to
store tripping energy. Tripping a circuit breaker from a trip coil, responsive to a decrease in
voltage below a predetermined value of circuit voltage, results in undervoltage tripping.
(b) The closing mechanism is a manually operated or power-operated device that closes
and latches the moving contacts against the stationary contacts. Since the amount and speed of
application of power derived directly from manual effort is limited, there are definite limitations
to the size and type of circuit breakers that can be successfully operated by manual closing
mechanisms. All breakers with frame sizes above 1600 A must be electrically operated. There
are four general types of power-driven operating mechanisms. These are classified according to
the source of energy used to actuate them, as follows:
o
A solenoid mechanism is one deriving its operating power from the
electromagnetic effect of a coil on a movable part of a magnetic circuit. It
can either be AC or DC actuated.
o
A pneumatic mechanism is one in which compressed air from an air
receiver actuates a piston to provide the operating power.
o
A charged-spring mechanism is one deriving its operating power from
energy stored in a charged spring. In most cases the spring is compressed
by an electric motor. In some cases facilities are provided so that the
spring can be charged by hand in case of a loss of control power.
3-35
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