MIL-DTL-917F(SH)
3.13.1.6 Parts tolerances. In the selection of parts, the widest tolerances permitted by the individual part
specification, and as otherwise permitted herein and by the equipment specification, shall be used commensurate
with the particular long term stable equipment application requirement. However, in equipment fabrication, parts of
closer tolerances may be substituted as long as interchangeability is not affected (see 3.13.1.5).
3.13.2 Fuses and fuseholders.
3.13.2.1 Fuses. Fuses shall be in accordance with MIL-PRF-15160, except where protection of semiconductor
devices is involved. For protection of semiconductor devices, fuses shall be selected and applied in accordance with
the characteristics of the semiconductor devices and with the manufacturer's recommendations. Fuses for
semiconductor devices shall be considered non-standard parts (see 3.13.1.3). Fuses shall be chosen so that their
clearing I²t (where I is current, and t is time) is lower than the I²t of the items which are to be protected. The
integrated I²t of the fuse shall be at least 10 percent less than the integrated I²t of the semiconductor device they
protect. The use of two or more fuses in parallel is prohibited.
3.13.2.2 Fuse mounting. Fuses shall be mounted either in fuse clips in accordance with A-A-55507,
A-A-55507/1, A-A-55507/2, or A-A-55507/3, or in front indicating type fuseholders in accordance with
MIL-PRF-19207.
3.13.2.3 Fuse styles and characteristics. For circuits of 250 to 500 V AC or DC, the following style fuses of
blowing characteristics C shall be used:
Style
Specification
F60
MIL-PRF-15160/60
F61
MIL-PRF-15160/61 (May be used only on existing design equipment. Use of F60 fuses is preferred.)
F62
MIL-PRF-15160/62
F63
MIL-PRF-15160/63
F64
MIL-PRF-15160/64
F65
MIL-PRF-15160/65
F66
MIL-PRF-15160/66
3.13.2.4 Fuse selective blowing. Where fused circuits are connected in cascade, a degree of selective blowing
is required at each successive level closer to the power input (power supply source). The energy required for fuse
element melting shall be not less than two times the total energy required for circuit interruption at the largest of the
next adjacent load size fuses.
3.13.2.5 Fuse current ratings. Fuses shall have current ratings not less than 125 percent of continuous load for
transformer or non-inductive circuits and not less than the minimum value required to pass, reliably, the peak
currents of transient-producing circuits. The ratings shall be low enough to prevent damage to all equipment
conductors on the line side of any fault and to limit damage to the immediate area of the short circuit or ground
fault. For low-power systems, consideration shall be given to the minimum available short-circuit current to ensure
blowing. Fuses used shall be readily replaceable.
3.13.2.6 Fuse and fuseholder installation characteristics.
a. Fuses mounted in clips shall be placed in a readily accessible location and shall provide adequate clearance
to ground, ease of replacement, ease of testing, and safety to maintenance personnel.
b. In convection cooled equipment, the fuses shall be mounted in a vertical position to prevent obstruction of
cooling air. Fuses shall be so located as not to be influenced by excessive equipment heat flow or near a high
operating temperature part.
c. For fuses that are bolted in place, care shall be taken to minimize the loss of screws, washers, and nuts
during the removal or replacement of fuses.
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