Three enclosures
Walking a panel, a bus duct and a transformer terminal box
Same five parts in all three. A different constraint holds each: panel width, phase centres fixed at manufacture, and an ambient that never comes down.
Walk the copper, not the catalogue
Start at the incomer and follow the bar to the last outgoing spout. Six things happen to it on the way and each takes a different part. Specifying by product code rather than by position is how a panel leaves the works with a shroud on a bar end and an open tee behind it.
The straight run is where enclosure width is won: 140 mm phase-to-phase on heavy wall at 36 kV against 320 mm bare to IEC 71-2. Every other stop exists so that saving survives to the drawing, because spacing follows the widest live geometry in the run and one open connection returns the section to the bare figure.
The walk below is written for a metal-clad medium-voltage panel. The bus duct and the dry-type transformer chamber that follow draw on the same parts under two constraints a panel does not have.
The order the parts go on
Sequence is not a preference here. Four of these six can only be done at one point in the build, and the last belongs to the cabinet rather than to the copper.
Sleeve on the bench, before the frame
A sleeve is a closed tube and can only arrive over an open bar end. Recover every straight length while the bars are still loose. A bar landed at both ends has lost the option, and buying it back costs sheet or a moulding at every position one sleeve would have covered.
Assemble the run and take the torque readings
Bolt up, tighten to the busbar designer's figure and record it before anything covers a fastener. Whatever is fitted ahead of that check is coming off again for it.
Sheet the joints that match no moulding
Adhesive-backed bus sheet is the part for a tee, elbow or landing with no catalogue shape. Cut it to run a minimum of 100 mm onto each leg before heat, and lap it 65 mm over the sleeve already on the run once shrunk. GMHS carries flashover protection to 17.5 kV, GHHS to 36 kV.
Shroud the joints that do, after the check
The moulded shroud goes on last of the covered parts because it is the one designed to come off again. Wall follows system voltage, not joint size: 1.0 mm at 12 kV rising to 2.5 mm at 36 kV in cross-linked polyolefin, 2.00 mm at 12 kV to 4.50 mm at 38 kV in PVC.
Cap the bar ends
Ends are capped once everything upstream is closed, so no later work disturbs them. BUSCAP runs GBC1 at 15 x 5 mm through GBC18 at 200 x 12 mm and is re-usable, which matters at a tap-off that will be opened again when the panel is extended.
Fit barrier board to the structure
GBBS never touches a conductor. It goes to the cabinet between phases or under a member with reduced clearance, cuts and drills without special equipment, and can be fitted at any point after the copper is in.
Six places in the panel
Positions in walking order, with the part that belongs at each and where it is specified.
1. Main horizontal run
The longest geometry in the panel and the only one measured in metres. BUSTUBE sleeve, GSC to 3.3 kV, GMB to 24 kV or GHB to 36 kV, sized on width plus breadth. Reel length varies more across the range than wall does, 100 m on the thinnest GSC codes to 1.5 m on GMB 250/100, and that decides how many joins a long run carries.
2. Riser to each cubicle
The run leaves the horizontal and climbs to the cubicle it feeds. The sleeve carries on through the bend, the material being specified as flexible enough for straight or angled bars without creasing, and stops one overlap short of whatever the riser lands on. Nothing separate is bought for the bend.
3. Tee to the breaker
A branch leaving the through bar at the feeder tap-off. A moulded shroud where the shape is catalogued, cut sheet where it is not. Wall is set by the panel's voltage rather than the size of the tee, so one shroud grade covers a 40 mm branch and a 150 mm one at the same kV.
4. Landing on a spout, CT or bushing
Where the copper stops being copper. Section and material both change at the landing, so nothing dimensioned against a bar fits, and joint geometry picks the part rather than position. The manufacturer's switchgear insulation index carries a bushing boot for the bushing case.
5. Exposed bar end
A spare tap-off, a provision for a future extension, or a bar that simply stops. BUSCAP is a flexible PVC cap tested for switchgear application to 36 kV, GBC1 at 15 x 5 mm to GBC18 at 200 x 12 mm. This is the position most often missed on a walk, because an end that ends nowhere reads as finished.
6. The gap between phases
Not a cover at all. GBBS barrier board is specified against inter-phase flashover started by contaminants, moisture and animals, and is bought as stock: 670 mm or 970 mm wide, 15 m long, at 2.0, 3.0 or 5.0 mm. It withstands a power arc without changing shape and wipes clean afterwards.
Bus duct and rising mains: the geometry is already decided
A duct is not a panel with a lid on it. Phase centres are fixed in the housing when the duct is built, so the clearance arithmetic that pays for a sleeve in a switchboard was settled by somebody else months earlier. Once the enclosure exists, insulation is the only variable left in it.
That changes what the sleeve is doing. In a fixed housing it is no longer buying width; it covers against insulation faults between adjacent bars in the duct, and against the acid, alkali and salt attack the material is separately specified to resist. The case is made on the fault, not on the panel drawing.
Duct and rising-main runs are also where round and tubular conductor turns up, and the chart treats it as a different bar entirely.
A tube is not a flat bar of the same number
One size code covers both, but through two separate columns that do not meet. A 100 x 10 mm flat bar sums to 110 mm on width plus breadth and sits inside GMB 100/38. A 100 mm diameter tube clears neither GMB 100/38 nor GMB 120/45 and takes GMB 150/60, two codes further up the same series.
| Size code | Wall (mm) | Rectangular, W+B (mm) | Round bar diameter (mm) |
|---|---|---|---|
| GMB 100/38 | 3.00 | 83 to 126 | 53 to 80 |
| GMB 120/45 | 3.00 | 104 to 150 | 66 to 96 |
| GMB 150/60 | 3.30 | 132 to 200 | 84 to 127 |
| GHB 100/38 | 3.70 | 83 to 102 | 53 to 72 |
| GHB 120/45 | 3.70 | 94 to 125 | 60 to 85 |
| GHB 150/60 | 3.70 | 122 to 168 | 78 to 105 |
The dry-type transformer terminal chamber
The box on the side of a dry-type transformer is the tightest of the three and the warmest. HV and LV terminals sit exposed in it, the chamber is sized around those terminals rather than around a person working on them, and its ambient rises and falls with the load rather than the room.
So the figure to check is not a peak. It is the margin between the part's published continuous limit and the ambient in that chamber with the transformer loaded. Sleeve, both shroud grades and the end cap are published to 115 degrees Celsius continuous; bus sheet and GBBS barrier board stop at 105. Ten degrees decides whether the flat products belong in the chamber at all.
The cold end is not uniform either, and it governs an outdoor kiosk more than a chamber. Cross-linked polyolefin parts are published from minus 40, the barrier board from minus 45, the PVC shroud, end cap and Termicap from minus 20.
Cable terminals in the chamber are a separate problem from the bar. Termicap covers and colour-codes a crimped wire-end terminal, TCV 12 through TCV 3251; a bar landing on the transformer pad is position four of the panel walk.
ANSI C37.20.2 on a submittal
The sleeve, the PVC shroud, the end cap, the bus sheet and Termicap are each published as tested to ANSI C37.20.2 for medium-voltage switchgear application up to 36 kV, which is why one document, the switchgear insulation products datasheet, can be attached against all three enclosures here. GBBS barrier board is not: it is published against ASTM methods and IEC 216 alone, so listing it under the same heading in a submittal overstates its data sheet. The reference establishes a component tested for switchgear service, not an assembly. Every standard named here is decoded once, on the standards page.
What has to be true before the copper goes in
Five practical constraints settle more panel builds than any electrical figure above.
- A sleeve needs an open bar end. Once both ends are landed, sheet and shroud are the only parts left, and they cost more per metre of bar than the sleeve that was skipped.
- A heat gun needs to get all the way round. A bar sitting 40 mm off a back plate in a closed cubicle cannot be sleeved in place, which makes it a general-arrangement decision taken months before anyone is on site with a torch.
- Heat and wiring should not share a cubicle. Where CT secondaries, auxiliary looms or moulded supports fall inside torch reach, specify the parts that fit cold: the PVC shroud and end cap are both published as installing, removing or replacing in minutes, and GBBS cuts and drills with ordinary tools.
- Order the yield, not the running length. Bus sheet is cut to the joint, barrier board comes 670 mm or 970 mm wide by 15 m, and sleeve arrives on reels falling from 100 m at the thin-wall end to 1.5 m on the largest codes. Reel length sets how many joins a 12 m duct run carries.
- Record the size code against the position. The engineer replacing a shroud in eight years has no other way to identify it, and a wrong replacement puts the panel back on the bare clearance figure.
What this page does not establish
None of the three enclosures is certified by anything written here. A panel is proved by its own type test, a bus duct by the current rating its housing manufacturer assigns it, and a transformer by the insulation system its designer specified for the winding and terminals. A component test report travels with the component; it does not transfer to the assembly, and no material certificate has ever passed a temperature-rise test on a panel's behalf.
Where each decision is worked out
- Which cover fits which jointSeven geometries sorted by shape, for positions three and four above
- Busbar sleeve size chartEvery GSC, GMB and GHB code with both bar columns in full
- Air clearance, bare against insulatedThe 12 to 36 kV figures behind the panel-width argument
- Installing a busbar sleeveThe bench work that step one above compresses into a line
- Busbar end cap, BUSCAPGBC sizes for the exposed ends found on the walk
- GBBS barrier board sheetThe one part here fitted to the cabinet rather than the bar
Send the enclosure, not just the bar
A bar section and a system voltage return one size code. A panel section drawing, the phase centres being worked to and which of the three enclosures it is return a parts list for the whole walk. Every part named here is manufactured by Gala Thermo Shrink Pvt. Ltd.