The rated continuous current of a circuit
breaker is the designated limit of current in rms amperes at rated frequency
which it shall be required to carry continuously without exceeding any of the
limitations designated . For rated continuous currents, refer to the tables of
preferred ratings in ANSI C37.06-1979.
Conditions of Continuous Current Rating.
The conditions on which continuous current
ratings are based are as follows:
1) Breakers are used under the usual
service conditions.
2) Current ratings shall be based on the
total temperature limits of the materials used for breaker parts. A temperature
rise reference is given to permit testing at reduced ambient.
3) Breakers designed for installation in
enclosures shall have their ratings based on the ventilation of such enclosures
and a 40°C ambient temperature outside the enclosure.
4) Outdoor breakers and indoor breakers
without enclosures shall have ratings based on a 40°C ambient temperature.
Temperature Limitations.
Limitations on Insulating Material.
The temperature of materials used to
insulate the main power circuit conducting parts from phase to ground, from
phase to phase, or from terminal to terminal of an open breaker shall be limited
to the values listed in columns B and C in Table 2.
It is recognized that these limits are
generally less than those associated with the insulating class in IEEE Std
1-1969, General Principles for Temperature Limits in the Rating of Electric
Equipment, since such insulation may be subject to severe mechanical as well as
dielectric stress when used in high-voltage circuit breakers.
Where outdoor apparatus bushings within the
scope of ANSI/IEEE Std 21-1976, General Requirements and Test Procedures for
Outdoor Apparatus Bushings, and ANSI/IEEE Std 24-1977, Standard Electrical,
Dimensional, and Related Requirements for Outdoor Apparatus Bushings, are used,
the temperature limits in Table 3 do not apply to the bushings.
Limitations on Main Contacts.
The temperature of the main contacts used
in circuit breakers shall not exceed the values listed in Table 3. Contacts in
other than oil or air may be operated at other temperatures providing it can be
shown, by experience or tests acceptable to the user, that accelerated
deterioration will not occur. (Future consideration will be given to
standardized values.)
Conducting Joints Other than Terminals
for Insulated Cable Connection.
The temperature of conducting joints in the
main power circuit of a circuit breaker shall not exceed the values listed in
Table 4. Conducting joints in other than oil or air may be operated at other
temperatures providing it can be shown, by experience or tests acceptable to
the user, that accelerated deterioration will not occur. (Future consideration
will be given to standardized values.)
Terminals for Connection to Insulated
Cable.
Terminals of circuit breakers designed for
direct cable connection shall not exceed 45°C rise or 85°C hottest spot total
temperature when connected to 85°C maximum insulated cable, rated for the full
continuous current rating of the circuit breaker.
Limitations for Parts Subject to Contact
by Personnel.
Circuit breaker parts handled by the
operator in the normal course of his duties shall have no higher total
temperature than 50°C. Circuit breakers having external surfaces accessible to
an operator in the normal course of his duties shall have no higher total
temperature on the surfaces than 70°C. Circuit breakers having external
surfaces not accessible to an operator in the normal course of his duties shall
have no higher total temperature on the surfaces than 110°C.
Limitations on Other Materials.
Other
materials shall be chosen so that the maximum temperatures to which they may be
subjected shall not cause accelerated deterioration.
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