UFC 3 -430-11
14 February 2001
cycle cost of the special enclosure will clearly exceed the life cycle cost of maintenance
of the air purge and the effort to provide continuous assurance of safe operation.
3-7.3.2 Specification Completeness. Include all requirements when specifying
explosion-proof enclosures. For example, specify the enclosure as Class I, Group D
explosion-proof, not just explosion-proof. An explosion proof enclosure is not
necessarily dust tight or suited for an outdoor location. Therefore, both the explosion
proof requirement and the weather or dust tight requirement must be specified to define
the required enclosure.
3-8
POWER SUPPLIES. Furnish a power supply that provides clean power to
the instruments. That is one that is free of disturbances and nuisance shutdowns. The
manufacturer should be able to provide equipment specifications and recommend
safeguards against severe power disturbances. Refer to NEMA ICS 1.1, Safety
Guidelines for the Application, Installation, and Maintenance of Solid State Control for
additional information. Include the following:
Provide power supplies that enable the controls, including combustion
safeguard systems and other control devices, to operate through an
electric power interruption of 20 milliseconds without affecting the operation
of the plant.
Provide an uninterruptible power supply (UPS) system to keep the
electronic instrumentation on line in case of a power interr uption. The
required time depends on the plant and instrumentation, but 30 minutes is
often specified. The UPS system must provide a safe plant shutdown in
case of a longer power outage. Small UPSs located inside of control
panels should be equipped with ventilation fans to remove unwanted heat.
Connect all trip circuits to the UPS system.
UPS system standards are defined by NEMA -PE 1, Uninterruptible Power
Systems.
3-9
INSTRUMENT AIR. Provide clean, dry instrument air as defined by
ANSI/ISA-S7.0.01, Quality Standard for Instrument Air. Test that the instrument air
control circuits are free of leaks per the same standard.
3-10
WIRING AND CONDUITS. Wiring must conform to NFPA 70, National
Electric Code. Run signal, thermocouple, and power wiring in separate conduits.
Wiring for alarm, shutdown, and interlock circuits of the same voltage as the power
wiring may be run in the same conduit as the power wiring.
Cable and thermocouple wire must conform to NEMA -WC55, Instrumentation
Cables and Thermocouple Wire. Provide high point vents and low point drains for all
conduits. Recommended practices for control centers are defined by ISA-RP60.8,
Electrical Guide for Control Centers.
3-6
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