UFC 3 -520-01
June 10, 2002
5-4.1.4 Insulation Level Rated Impulse Withstand Voltage. Referring to IEEE
C37.04, the i mpulse strength of the breaker must be coordinated with the surge
protection of the system as follows:
5-4.1.4.1 Across breaker contacts.
5-4.1.4.2 Between breaker contacts and ground. Verify no increase in surge voltage as
a result of voltage reflectio n.
5-4.1.5 Frequency. For a frequency of 60 Hertz, compare the calculated ratings with
standard ratings. For other frequencies, check with the equipment manufacturer.
5-4.1.6 Continuous Current. Calculate the maximum current flow through the
breaker b y computing the current flow under normal and contingency conditions.
Provide for future load growth, if required.
5-4.1.7 Interrupting Duty. To select the proper interrupting duty using ANSI/IEEE
C37.010, Application Guide for AC High-Voltage Circuit B reakers Rated on a
Symmetrical Current Basis, it is necessary to perform a complete fault analysis to
determine the required interrupting duty of the circuit breaker under normal and
contingency conditions. Use the following criteria:
5-4.1.7.1 Provide for a future system design that might materially affect the interrupting
duty of the circuit breaker. Circuit breakers are rated on a symmetrical basis rather than
on an asymmetrical (total current) basis. Follow the criteria of ANSI/IEEE C37.010, and
IEEE C37.011, Application Guide for Transient Recovery Voltage for AC High-Voltage
Circuit Breakers Rated on a Symmetrical Current Basis.
5-4.1.7.2 If the operating voltage of the circuit differs from the rated voltage of the
circuit breaker, correct the final values to correspond with the rated values given in the
manufacturer's circuit breaker rating tables.
5-4.1.7.3 Determine the asymmetrical requirements based on the breaker contact
parting time.
5-4.1.7.4 Determine the actual operating duty and interruption time of the breaker from
the relay setting calculations.
5-4.1.8 Altitude Correction. Correct voltage and current ratings for altitudes above
1,000 meters (3,300 feet). Use ANSI/IEEE C37.20, Switchgear Assemblies Including
Metal-Enclosed Bus, and NEMA SG 4, Alternating-Current High-Voltage Power Circuit
Breakers, for correction factors.
5-4.1.9 Ambient Temperature. Derate circuit breakers in environments with ambient
temperatures higher than +40 C (104 F) or lower than -30 C (-22 F) in accordance
with IEEE C37.010.
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