A circuit breaker is a device used for the protection of both circuits
and apparatus. According to the type of construction, circuit breakers
are classified in the following categories:
air circuit breakers, used at low and medium voltages; and
oil and gas circuit breakers, usually used at high voltages.
The most common circuit breaker used in medium voltage substation
applications is the air circuit breaker. There are a number of
exceptions when the use of oil type circuit breakers are required such
as coal mines, some chemical processes, bottled gas plants, refineries,
and explosive factories.
The basic construction of the breaker depends on the application in
which the breaker will be used and can vary according to the
manufacturer. Generally, every breaker consists of the following:
Circuit Breaker Inspection and Maintenance
The circuit breaker compartment contains the circuit breaker mounted on
a withdrawable carriage which permits the breaker to be taken out of
the enclosure. The front door of the compartment has a viewing window
for observing the status of the circuit breaker without opening the
door. The door also has a shutter through which the racking handle can
be inserted to rack the circuit breaker in or out.
The following items should be inspected during regular maintenance:
The functioning of the circuit breaker depends on the contacts. When
closed, practically the entire load current passes through these
breakers. As well, they must withstand complete overload or
short-circuit current. If the resistance of the contacts becomes high,
they will overheat which can cause damage or destroy the complete
Because of these factors, regular maintenance of the contacts is important and should include contact cleaning and smoothing.
The arc interrupters play a role in the dispersion and cooling of the
arc flame. Regular maintenance of arc interrupters includes cleaning,
checking for physical damage and performing an ac high potential test
across the arc chute.
The operating mechanisms function is to open and close the breaker
contacts. It consists of a series of linkages connected to a power
mechanism, either solenoid or a spring device. Inspection should
include cleaning, a check for wear and hand closing the breaker to make
sure all parts operate freely.
Auxiliary equipment includes control relays, switches and protective
relays. During regular maintenance, they should be cleaned and tested
for operation. Also, protective relays should be checked for setting to
assure they coordinate with other relays in the system.
Breaker Testing, Setting and Calibration
The most common relay used in the protection of the power distribution
system is the overcurrent relay. The function of this relay is to sense
the overcurrent in the system and, when operating correctly, provide
the circuit breaker tripping operation.
According to construction type, relay and trip units can be classified in the next two main categories:
The relay operating characteristic is shown as a logarithm function of
operating current and time. Such curves normally employ log-log scales
to cover a wide range of time and current. Similar curves are published
for overcurrent relays having different time-delay characteristics. It
is possible to adjust the operating time of relays. This is important
since they are normally used to selectively trip breakers that operate
in series on the same system circuit.
This adjustment of the operating time and current value is called
breaker setting. The electro-mechanical type is usually equipped with
two types of trip settings:
instantaneous current trip settings
overload current trip settings.
Additional adjustments of operating characteristics to allow for better
system co-ordination can be achieved by using a solid-state type of
relay or trip unit. These are usually equipped with the following type
long-delay current trip setting
long-delay time trip setting
short-delay current trip setting
short-delay time trip setting
instantaneous current trip setting
ground current trip setting
ground-delay time trip setting
A short description of the main types of trip settings is provided below:
Instantaneous trip settings
Instantaneous tripping without any intentional delay is used to provide
protection against short circuits. It is possible to select various
short circuit pickup levels according to the co-ordination needs.
Overload trip settings
Overload tripping is used to allow a breaker to protect system
components such as transformers, motors, conductors etc. that may fail
due to restrictive heating. When an overcurrent condition persists for
a specified length of time the breaker trips and breaks the load. A
vareity of trip times for different levels of current and curve
settings are possible in order to achieve the proper system
Ground fault settings
The ground fault current is often below the trip level of the overload
settings. It is therefore necessary to have separate ground fault
protection to prevent damage. Separate ground fault levels of 0.1-0.8
times the CT rating and trip times of no intentional delay are provided
with this type of setting. The level setting should be set low enough
to provide a trip under ground fault conditions and high enough to
prevent nuisance trips under normal conditions. Some ground current
will be detected from capacitive current or CT mismatch and spill
currents in residual ground sensing circuits. The level must be set
higher than this normally encountered value.
After the proper setting is achieved, calibration of the relay or trip
units should be performed in order to ensure the acting relay or trip
unitwork at the predetermined value. The relay calibration is the set
of complex tests performed on the relay or trip unit to confirm and
further adjust its trip characteristic - which should be in the limits
suggested by the manufacturer. Each manufacturer of relays has
established test and service procedures for the great variety of relays
offered. It is therefore impractical to attempt a discussion of
specific techniques. The manufacturer's instruction literature should
be used as a guide for the relay inspection and test being performed.
The calibration of the solid-state relay generally consists of primary
injection testing, testing of all the trip settings that each breaker
is equipped with and a final testing. It is important during these
testing that the manufacturer's instructions be carefully followed.
Test procedures for electro-mechanical relays usually consist of
adjusting the stationary contact, a minimum operating current test and
time curve calibration. If the minimum operating test shows that
calibration is necessary the damping magnet should be removed and
control spring tension should be adjusted.
The purpose of the time curve calibration is to ensure that relay
operating time for certain values of current be shown on the time
current curves, plus or minus five percent. If the operating time is
not within the tolerance for the low values of applied current, the
permanent magnet keeper should be adjusted until this operating time is
correct. If the operating time is not within the tolerance of the high
values of applied current, the electromagnet screw plugs require
Because the test results can vary with conditions and type of
instrumentation employed, the accuracy of test equipment should never
be taken for granted - it can become damaged, depending on the
frequency with which it is used and recalibrated. Instrument error may
be significant to the degree that adjustment of a device will be made
based on inaccurate readings. As you can see, recorded data is vital to
the evaluation of subsequent test results and decisions for
recalibration, adjustment and replacement of the protective devices.