2.7.2.1
Vacuum Circuit Interrupters
Vacuum interrupters shall be hermetically-sealed in a high vacuum to
protect contacts from moisture and contamination. Circuit breakers shall
have provisions for maintenance slow closing of contacts and have a readily
accessible contact wear indicator. Tripping time shall not exceed [3] [5]
[8] cycles.
2.7.2.2
Sulphur Hexafluoride (SF6) Interrupters
SF6 interrupters shall be of the puffer type where the movement of the
contact plunger will initiate the puff of SF6 gas across the contact to
extinguish the arc. Breakers shall be provided with a
loss-of-pressure-alarm remote as shown on the drawings. Before the
pressure in the interrupter drops below the point where the breaker or
switch cannot open safely without damage, the breaker shall activate the
loss-of-pressure-alarm, open automatically, and remain in the locked open
position until repaired. The SF6 shall meet the requirements of ASTM D 2472,
except that the maximum dew point shall be minus 60 degrees C minus 76
degrees F (corresponding to 11 ppm water by volume), with only 11 ppm water
by volume, and the minimum purity shall be 99.9 percent by weight. Circuit
breakers shall have provisions for maintenance slow closing of contacts and
have a readily accessible contact wear indicator. Tripping time shall not
exceed [3] [5] [8] cycles.
2.7.3
Buses
Copper bus shall comply with ASTM B 188. Equivalent aluminum bus shall
comply with ASTM B 317/B 317M. Bolted or pressure joints for main and
ground buses, interconnections, and external connections to equipment shall
be of the silver-to-silver or the silver-to-tin high-pressure type. Bolted
connections shall have a minimum of two bolts, except for the ground bus
where one bolt will suffice. Each nut on any bolted connection shall be
secured with a belleville washer or other locking means torqued in
accordance with manufacturer's recommendations. Bus supporting elements
shall be bolted to switchgear enclosures and shall comply with IEEE C37.20.2.
2.7.3.1
Main Buses
Main buses and connections shall have at least the same short-circuit
current rating as circuit breakers. Buses may be copper or aluminum, but a
combination of both metals is not acceptable unless silver-to-silver or
silver-to-tin plating is used wherever aluminum and copper buses are
connected.
2.7.3.2
Ground Buses
Uninsulated copper ground buses, not less than 51 x 6.2 mm 2 x 1/4 inch in
cross-sectional area, shall be provided for the full length of a switchgear
lineup. Ground buses of aluminum are not acceptable. The short-circuit
current rating of the ground bus shall be at least equal to the short
circuit current rating of the primary bus. Compression indent type cable
lugs shall be provided at each end of a ground bus for connection of [No.
4/0 AWG] [_____] copper ground cables.
2.7.3.3
Control Buses
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SECTION 16311A
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