MIL-HDBK-1004/2A
2.5.2
Current Limiting Protectors. These fusible type devices developed
under an Electric Power Research Institute (EPRI) project, provide current
limiting on up to 15.5-kV systems for up to 1,200 A continuous currents. Use
them only where higher continuous ratings are required than are available from
standard fused cutouts or power fuse disconnecting units.
2.5.3
Circuit Breakers. Use a circuit breaker rating adequate for the
load interrupting duty and which provides selectivity with circuit breakers
and fuses ahead of or after the circuit breaker.
2.5.4
Automatic Circuit Reclosers. Use the criteria in NEMA SG-13,
Automatic Circuit Reclosers, Automatic Line Sectionalizers and Oil-Filled
Capacitor Switches for Alternating Current Systems. Use of automatic
reclosing for other than overhead lines serving residential or commercial
loads may cause problems. In selecting the type of automatic circuit
recloser, consider the reliability and continuity of service. Reclosers may
consist of a circuit breaker or a multiple switching device. Reclosers
operate so that a faulted circuit may be opened and then, either
instantaneously or with deliberate time delay, reclosed. Up to three
reclosures with varying time intervals may be used. Coordinate automatic
circuit reclosers with fuses or circuit breakers on the same circuit.
2.5.5
Nonload-Break Switches. Use nonload-break switches only for the
interruption of circuits that carry no appreciable load. Select the type
applicable, depending on circuit importance, load, voltage, and fault circuit
duty. The types available are porcelain disconnect fuse cutouts, plain or
fused single-pole air disconnect switches, and disconnect fuse cutouts of
various types. Refer to manufacturers' catalogs and NEMA SG-2, High-Voltage
Fuses. Disconnecting and horn gap switches covered by ANSI C37.30, Definitions
and Requirements for High-Voltage Air Switches, Insulators, and Bus Supports
and ANSI C37.32, Schedules of Preferred Ratings, Manufacturing Specifications,
and Application Guide for High-Voltage Air Switches, Bus Supports, and Switch
Accessories are also nonload-break switches.
Load-Break Switches Load-break switches are provided with an
2.5.6
interrupting device capable of disconnecting circuits under load. Fuse
cutouts, (covered by NEMA SG-2) which are designed to be load-break are
available, as are load interrupter switches which conform to ANSI C37.30 and
C37.32. Vacuum switches provide load-break features. Vacuum switches can
provide a wide variety of operators and should be considered as an economical
method of providing automatic or remotely controlled switching.
2.6
2.6.1
Requirements. Lightning protection can be provided by installing
surge (lightning) arresters, open or expulsion gaps, or overhead ground wires,
or by all three methods combined. Also, consider the weather. For most
distribution circuits, distribution surge arresters protecting transformers
and aerial-to-underground transitions are adequate. Overhead ground wires are
rarely considered to be an economical installation for distribution lines, but
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