Clearances of the ANSI / IEEE set

The ANSI / IEEE set takes its yard clearances from RUS Bulletin 1724E-300 Table 4-7, by the level's basic lightning impulse insulation level (BIL). It adds the National Electrical Safety Code (NESC), 2007 edition, for the live parts above the surfaces where people stand and for the fence.

1The row of a levelCopy link to The row of a level

A level takes the row of RUS Bulletin 1724E-300 Table 4-7 with the smallest standard BIL at or above the level's BIL, from IEEE C37.04-2018 Table 6 (Insulation levels, The ANSI / IEEE set). Two rows list a BIL of 1050 kVp, and a level below 345 kV takes the 230 kV one. The row's values hold for altitudes up to 1000 m (Table 4-7, note 1).

The design reads three clearances from the row:

• phase to earth: the minimum distance from rigid conductors to grounded parts
• phase to phase: the minimum metal-to-metal distance of rigid conductors
• ground: the minimum height of bare overhead conductors above grade for personal safety

The yard's phase spacing is the larger of the row's rigid-bus spacing and the spacing of Table 4-8 for side-break or horizontal-break disconnect switches, since the yard's isolators are horizontal centre-break. Where Table 4-8 lists no switch spacing (the 345 kV row at 1300 kVp), the largest spacing of the rows below applies. The bases of the insulators and bushings of energized equipment are at least 2.44 m above the top of a foundation (Table 4-7, note 3).

This set has no sectional clearance. The NESC rules below take its place. The clearances of the set's levels are in the table.

2The NESC 2007 clearancesCopy link to The NESC 2007 clearances

NESC 2007 Rule 124A1 sets the clearance of unguarded live parts above a surface where people stand, by Table 124-1. The design takes Column 2, the vertical clearance, at the smallest listed BIL at or above the level's. Part A applies up to a maximum design voltage of 242 kV, and Part C above 242 kV.

Part B of Table 124-1 gives clearances by the switching-surge factor, and applies too where the switching-surge factor of the network is known. The design does not take it. The vertical clearance is checked against the equipment level of the high-voltage (HV) yard and of an outdoor collector bus.

Rule 124A3 keeps the bottom of insulators at least 2.6 m above grade, and the yard takes it as its insulator base. Rule 110A1 asks for a fence of at least 2.13 m overall, and the yard's fence is 2.44 m high.

Rule 110A2 keeps the live parts outside the safety clearance zone of the fence. The zone's dimension R comes from Table 110-1 by the BIL and is measured from the fence at 1.5 m above grade. The design makes two checks of it:

• the fence offset and the peripheral road, between the fence and the nearest live part, are at least R
• the gantry beam, where the line conductors cross the fence, is at least R above that point

3AltitudeCopy link to Altitude

Above 1000 m, the design derates the BIL of the rows by the factor $k$ of RUS Bulletin 1724E-300 Table 4-3, for the first listed altitude at or above the site. It takes the row with the smallest standard BIL whose derated value $k{\mathrm{BIL}}_{\mathrm{row}}$ is at or above the level's BIL. That row's clearances apply, and the yard's geometry grows to suit them.

The level's own BIL is not changed, and neither are the other insulation levels (Insulation levels, Altitude). Table 4-3 ends at 6000 m, and the design refuses a site above it. It also refuses a level whose row would need a BIL above 1300 kVp, the last row of Table 4-7.

A strung jumper over a bus is a nonrigid conductor. The design increases the metal-to-metal clearance by 50 % for it, in its level and in its check.