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 LANTNAVFACENGCOM A&E GUIDE
3/93
c. Indicate conductor sag values on plans. Provide initial sag values for
ambient temperatures of 30F, 60F, and 90F for required ruling
spans.
FUSE PROTECTION
A.2.2.4
a. Provide backup current limiting fuses for overhead distribution
transformers on voltage systems: (a) above 15 kV and (b) on lower
voltage systems that have available fault currents equal to or greater
than 7,000 asymmetrical amperes.
b. The purpose of the backup current limiting fuse is to protect the
transformer from high level fault currents that can rupture the
transformer tank and result in catastrophic damage. The expulsion
fuse link is required for overload and low level fault current protection.
The two protective devices must be coordinated to function together;
expulsion fuse links should be ANSI Type K to provide this
coordination.
c. Exercise care when using fused cutouts at line taps and at underground
terminations. The maximum interrupting rating of a fused cutout is
12,500 symmetrical amperes. Whenever system fault currents exceed
this value, protect the fused cutout with a backup current limiting fuse
or use an adequately rated power fuse.
d. In the design of overhead distribution systems:
(1) Provide appropriate detail on the drawings indicating the use of
backup current limiting fuses with the device being protected. If
the designer is using the LANTNAVFACENGCOM pole plates, it
is permissible to provide supplemental information with the pole
detail designation symbols instead of providing separate details.
The pole plates will be revised later to include the backup current
limiting fuses.
(2) Indicate on the project drawings the ampere rating of the ANSI
Type K expulsion fuse link as well as the voltage rating and
current designation of the backup current limiting fuse.
(3) Specify:
(a) ANSI Type K fuse links sized as indicated on the drawing.
Attachment A-13
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