NOTE: See TM 5-811-1/AF AFJMAN 32-1080 for guidance.
**************************************************************************
Pad-mounted transformers shall comply with ANSI C57.12.26 and shall be of
the [radial] [loop feed] type. Pad-mounted transformer stations shall be
assembled and coordinated by one manufacturer and each transformer station
shall be shipped as a complete unit so that field installation requirements
are limited to mounting each unit on a concrete pad and connecting it to
primary and secondary lines. Stainless steel pins and hinges shall be
provided. Barriers shall be provided between high- and low-voltage
compartments. High-voltage compartment doors shall be interlocked with
low-voltage compartment doors to prevent access to any high-voltage section
unless its associated low-voltage section door has first been opened.
Compartments shall be sized to meet the specific dimensional requirements
of ANSI C57.12.26. Pentahead locking bolts shall be provided with
provisions for a padlock.
2.9.2.1
High-Voltage Compartments
**************************************************************************
NOTE: Select transformer fuse type and
characteristics to provide protection of the
transformer and coordination with upstream
There are two types of fuses utilized in deadfront,
pad-mounted transformer protection: expulsion-type,
and current-limiting type. Expulsion-type fuses
contain a fuse link that melts when subjected to
either overload or fault current. The fuse-holding
device operates such that the arc of the melting
fuse link is extinguished under the transformer
insulating dielectric (oil-immersed applications).
Current limiting fuses create a physical circuit gap
by the melting of the fuse. The opening arc is
extinguished by the gases created by melting of the
fuse element (drywell applications).
Fuse mounting configurations for deadfront
pad-mounted transformers include: drywell mounting,
where a canister is provided in the transformer tank
(separated and sealed from the insulating
dielectric); and "oil-immersed bayonet" mounting,
where the fuse holder is mounted through the wall of
the transformer tank, such that the fuse element is
immersed in the transformer dielectric.
Drawout, dry-well mounted, current-limiting fuses
are significantly more expensive than expulsion-type
fuses and have more limited voltage- and current
range operating characteristics. When specifying
this fuse type, care must be exercised to ensure
coordination with upstream protective devices.
Although they operate on a more limited voltage
range than other fuses, they are the preferred fuse
type because the current limiting characteristics
provides enhanced fault protection for the
transformer.
SECTION 16375A
Page 32
Previous Page
