MIL-DTL-24646B
3.5.9 Polarity and phase identification.
3.5.9.1 Polarity. For direct current (dc), like polarities shall be similarly disposed. Facing the box, the top bus shall
be positive (+); the center bus, where furnished, shall be positive and negative (±) (to indicate the voltage is at some
value in between + and -); and the bottom bus shall be negative (-). Should bus bars be vertical, the marking shall be
as specified, when the fuse box is rotated 90 degrees counterclockwise. Polarity identification shall be clearly
marked (see 3.8.3).
3.5.9.2 Phase sequence. For alternating current (ac), like phases shall be similarly disposed. Facing the front of
the panel, the phase rotation shall be C, B, and A respectively, from left to right or bottom to top. Phase identification
shall be clearly marked (see 3.8.3).
3.6 Performance characteristics.
3.6.1 Dielectric withstanding voltage. Fuse boxes shall exhibit no evidence of arcing (other than at test probes),
corona (visible, audible, or smell), flashover or punctured insulation when tested as specified in 4.6.2
3.6.2 Insulation resistance. The insulation resistance of the electrical circuits shall be not less than 10 megohms
when tested as specified in 4.6.3
3.6.3 Operation. When operated at rated load (see 3.1), the circuits shall be energized as specified on the wiring
diagram (see 4.6.4). Where switches are installed, they shall be operated through all positions to ensure there is no
mechanical binding and that they make and break the circuits.
3.6.4 Temperature rise. When fuse boxes are subjected to the tests as specified in 4.6.5, the temperature rise in
current carrying parts such as bus bar connections, terminals, and fuse clips, shall not exceed 70°C above an
ambient room temperature of 25°C. Also, the enclosure interior ambient temperature shall not exceed 65°C and the
temperature rise at the switch terminals (where applicable) shall not exceed 50°C.
3.6.5 Salt atmosphere. When fuse boxes are tested as specified in 4.6.6, there shall be no evidence of excessive
corrosion. Excessive corrosion is defined as that which interferes with the electrical or mechanical performance, or,
in the case of plated metals, corrosion which has migrated through the plating and attacked the base metal. Where
corrosion-resistance treatments are used, limited superficial corrosion such as would result from scratches will be
permissible.
3.6.6 Level of effectiveness of enclosure. The fuse boxes shall meet the specific degree of enclosure
requirements specified (see 3.1) when tested as specified in 4.6.7. Following the test, the fuse boxes shall pass the
dielectric withstanding voltage test (see 4.6.2).
3.6.7 Vibration. When fuse boxes are tested as specified in 4.6.8, the branch circuits shall remain energized and
there shall be no electrical or mechanical damage, nor shall any screws or nuts become loosened. Where switches
are present, there shall be no momentary opening or transfer of switch contacts during the test.
3.6.8 Shock. The branch circuits shall remain energized during and after the shock test as specified in 4.6.9 and
no portion of the equipment shall come adrift or otherwise create a hazard to personnel or other components nor shall
any screws or nuts become loosened. Where switches are present, there shall be no momentary opening or transfer
of the switch contacts. Following the test, the fuse boxes shall pass the level effectiveness of enclosure and dielectric
3.7 Finish.
3.7.1 Surface finish. Surface finishes shall be as specified in ASME B46.1.
3.7.3 Painting. Unless otherwise specified (see 6.2), steel and aluminum enclosures shall be painted and brass
enclosures shall be neither primed nor painted. Pre-treating, painting and finishing shall be per MIL-E-917.
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