MIL-HDBK-1004/2A
3.4.11
Gas Pressurized Cable. Sulfur hexaflouride gas pressurized cable or
integrated gas spacer cable can be considered for use when used with either
XLP or EPR insulations. The jackets of direct-burial cables or field-
installed or factory-coilable conduits can be pressurized with this gas.
3.4.11.1 Sulfur hexaflouride Gas. This gas has five times the density of air
and acts as an "invisible liquid" as it stays in place even when exposed to
air. It is electro-negative with no oxygen or carbon; has a high arc
resistance; will not support combustion; and is odorless, tasteless, and
nontoxic. When used with XLP and EPR insulations, it prevents water vapor
diffusion, water treeing, and gaseous ionization. It provides monitoring for
indication of mechanical damage during shipment, installation, and during
operation. Sulfur hexaflouride gas improves the lightning and impulse
strength and can provide a rehealing of insulation after an electromagnetic
pulse insulation failure. The gas pressure protects against internal
corrosion of metal parts. The SF6 gas provides extra electrical strength in
splices and terminations.
3.4.11.2 Installation. Expand the requirements for integrated gas spacer
cable of NFPA 70 to cover the XLP or EPR insulation requirement. Determine
appropriate installation requirements for direct burial, in conduit, or as
submarine cable from manufacturers. In cold climates, indicate temperature
ranges, so a gaseous mixture which prevents liquification is achieved.
3.4.11.3 Optional Usage. Where adequate requirements are provided, gas
pressurized cable may be used as an option to cables covered in NFGS-16301.
3.5
Underground Transformers. Use vaults to house transformers and
associated equipment for underground distribution systems.
3.5.1
Equipment.
Use subway (submersible) type equipment.
3.5.2
Vault Design. Design transformer vaults in accordance with MIL-
HDBK-1008 and include the following provisions:
a) Provide adequate ventilation to prevent a transformer
temperature in excess of the values prescribed in ANSI C57.12.00, General
Requirements for Liquid-Immersed Distribution, Power and Regulating
Transformers, and C57.12.01, General Requirements for Dry-Type Distribution
and Power Transformers. This limitation requires that most of the electrical
heat losses must be removed by ventilation; only a minor part can be
dissipated by the vault walls. NFPA 70 recommends 3 inch (19 square cm) of
clear grating area per kilovolt-ampere of transformer capacity. In localities
with above average temperatures, such as tropical or subtropical areas,
increase the grating area or supplement by forced ventilation, depending upon
temperature extremes.
b) Provide adequate access for repairs, maintenance, and
installation and removal of equipment. Refer to working space requirements
covered in Section 5.
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