Cutting busbar air clearance with insulation

Phase-to-earth is the binding gap on a sleeved bar. In every row of the manufacturer's clearance table the earth figure exceeds the phase figure — 45 mm against 35 mm at 12 kV on heavy wall, 190 mm against 140 mm at 36 kV. The uninsulated column is published as a single figure per voltage, so that split only appears once the sleeve is on.

The absolute saving climbs with voltage; the proportion does not. Heavy wall holds 35 mm phase-to-phase at 12 kV against 120 mm bare and 140 mm at 36 kV against 320 mm, so the gap is worth 85 mm at the bottom of the range and 180 mm at the top while staying between 29 and 44 per cent of the bare figure throughout. The published table starts at 12 kV and carries no LV row — the saving is a switchgear line item.

The shape penalty is a medium-wall problem

Round conductor clears tighter than rectangular at every voltage in both grades: a rolled edge concentrates the field, a cylinder does not. On heavy wall the difference is 5 mm at 12 and 17.5 kV and 10 mm at 24 and 36 kV — inside drawing tolerance, and no reason to change conductor.

On medium wall it opens up. The rectangular figure runs 10 mm wider at 12 kV, 20 mm at 24 kV and 50 mm at 36 kV phase-to-phase, and 80 mm wider at 36 kV phase-to-earth. A 36 kV medium-wall layout drawn on the rectangular rows and built with tubular bar is carrying 50 mm on each phase gap and 80 mm on each earth gap that the published data does not ask for.

Where the 620 mm goes

620 mm comes out of the enclosure at 36 kV before a millimetre of copper is counted. On a switchboard that is a section boundary moving; on a rising main it is a smaller shaft opening. Sheet metal, doors, gaskets and shipping volume all follow that one dimension.

Watch where the residue sits. The phase gaps give back 360 mm and the earth gaps 260, but the insulated side inverts: 380 mm is now going to the two side walls against 280 mm between the bars. Pulling the panel narrower past this point is a wall-clearance problem, not a phase-spacing one.

Copper follows the width. Phase centres set the length of every cross-connection, dropper and tee in the run, so drawing the phases in shortens bar that is bought by the kilogram. The manufacturer makes the same case from the joint end on the moulded shroud range, where covering a connection is offered as the route to a smaller panel and shorter busbar lengths rather than as a safety measure on its own.

The saving is only real if it survives the joints. Spacing is set by the widest live geometry in the run, so a sleeved straight length with an uncovered tee is spaced by the tee and the tables above have bought nothing. The shroud that covers that tee runs a 1.0 mm minimum wall at 12 kV, 1.4 mm at 18 kV, 1.8 mm at 24 kV and 2.5 mm at 36 kV.

What decides the number you may put on the drawing

The tables report what the material achieves under test. Not one of the six below is in them.

• The design standard the assembly is certified to. IEC 71-2 supplies the uninsulated baseline above; the switchgear standard the panel is signed off against is what an inspector reads.
• The type test. A reduced gap holds because the assembly passed power-frequency and impulse withstand with the sleeve fitted. The sleeve itself is qualified to ANSI C37.20.2 for switchgear application up to 36 kV — that is the material's certification, not the panel's.
• The purchaser's own specification. An end user or utility can set a gap above the standard minimum and can decline an insulation-based reduction outright. Settle that before the enclosure is drawn, not after.
• Access and working space. A gap that clears dielectrically can still be too narrow to land a torque wrench on the joint. Clearance sets a floor under phase spacing, not the final figure.
• Every joint covered. Tees, elbows and bolted laps need a shroud, cap or sheet of their own before the reduced spacing means anything.
• Pollution, condensation and vermin. A narrow gap behaves differently once the enclosure is dirty, which is a failure-mode question rather than a dimensional one.