UFC 3 -520-01
June 10, 2002
5-4.1.10 Breaker Selection. Select breakers using the following criteria:
5-4.1.10.1 Rating Evaluation. The evaluation of the voltage rating, insulation withstand
voltage rating, frequency, continuous current, and interrupting duty provides the
required rating of the circuit breaker.
5-4.1.10.2 Selection Guide. Vacuum circuit breakers should be used with a suitable
enclosure for 1.5 kV through 15.0 kV up to 1,000 MVA interrupting duty. Consider use
of sulfur hexafluoride circuit breakers.
5-4.2
Low Voltage Conductors in Medium Voltage Switchgear.
5-4.2.1 Do not route low voltage cables or conductors, except those to be connected
to equipment within the compartment, through medium or high voltage divisions of
switchgear unless installed in rigid metal conduit or isolated by rigid metal barriers.
Insulate conductors entering switchgear for the higher operating voltage in that
compartment or separate the conductors from insulated conductors of other ratings.
5-4.2.2 Low voltage conductors routed from medium or high voltage sections of
switchgear must terminate in a low voltage section before being routed external to the
switchgear.
5-5
LOW VOLTAGE BREAKERS AND PANELS.
5-5.1
The following types of circuit breakers can be used for low voltage
applications:
5-5.1.1 Use molded case circuit breakers (MCCB) wherever possible. Determine if
electrical coordination is a design requirement for the electrical system or some portion
of the electrical system. If electrical coordination is a requirement for the installation,
the use of thermal-magnetic trip units might not allow coordination in the instantaneous
trip region.
5-5.1.2 Integrally-fused MCCBs can be used to protect loads if short-circuit currents
are high. Standard MC CBs are readily available with 65,000 ampere or higher ratings,
which reduces the need for integrally-fused MCCBs for most applications.
5-5.1.3 Select low voltage power circuit breakers only where the added cost incurred
by their use can be justified by operational considerations. Apply low voltage power
circuit breakers in enclosures in accordance with IEEE C37.13, L o w-Voltage AC Power
Circuit Breakers Used in Enclosures.
5-5.1.4 Insulated-case circuit breakers are rated in a manner similar to MCCBs and
can be used instead of low voltage power circuit breakers.
5-6
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