MIL-HDBK-1004/2A
a)
number of circuits,
b)
capacity of circuits,
c)
voltage and phase, and
d)
overhead or underground distribution required.
Finally, consider coordination of circuit protective devices between supply
and feeder circuits.
5.5.3
Incoming-Line Switching. Design the substation with a minimum of
incoming-line switching consistent with good maintenance and operation. For
rating of equipment, refer to MIL-HDBK-1004/3, Switchgear and Relaying. Also,
consider the methods described in paras 5.5.3.1 through 5.5.3.3.
5.5.3.1 Circuit Breakers. Use circuit breakers only when the circuit
interrupting or relaying requirements do not allow the use of switches.
Provide a disconnect and bypass switching features where drawout circuit
breakers cannot be utilized.
5.5.3.2 Switches. Switches are covered in Section 2 of this handbook. For
voltages of 15 kV or less, load interrupter or disconnect switches are
available. Load interrupter switches disconnect circuits under fully loaded
conditions and are therefore usually the most desirable choice. Use
disconnect switches only to interrupt transformer exciting currents. The use
of disconnect switches is not recommended, except for primary incoming lines
where secondary circuit breakers can interrupt loads. Assure that operators
do not open disconnect switches under load, either by interlocking with load
switching equipment or by operating procedures.
5.5.3.3 Current Limiting Protectors. Current limiting protection devices
are covered in Section 2. They are generally inappropriate for substation use
where metal-enclosed or metal-clad switchgear for 15-kV applications provides
a more desirable design.
5.5.4
Outgoing-Feeder Switchgear.
For ratings and selection of equipment,
refer to MIL-HDBK-1004/3.
5.5.4.1 600 V and Less.
For load circuits below 600 V, select from one of
the following:
a) metal-enclosed (low-voltage power) circuit breakers where
reliability and the longer withstand rating period are desirable; or
b) molded-case circuit breakers. Molded-case circuit breakers must
be further defined as to tripping and interrupting currents and whether fully-
rated capability or other features are desirable. The use of the term
"insulated case" conveys no minimum requirements in accordance with any
recognized industry specification and should not be used.
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