Boots
Insulating boots at busbar and bus-duct terminations
Eight catalogued sizes, four right angle and four straight; the GRB-1 goes on over 78mm as supplied and closes to 35mm after free recovery.
Where the air clearance runs out
Inside a cable termination box the gap between phase and phase, and between phase and earth, is smaller than normal air clearance. The bushing, the lug and the bar end sit that close because the box was sized around the cable, not around the air. A boot puts solid insulation across the gap the air can no longer hold.
The manufacturer's datasheet names four things a boot in that position guards against: clearance below normal air clearance, flashover under high humidity, rodents, and surge impulse. Two of the four are environmental rather than electrical, which is why the moulding is internally coated with water-resistant red mastic and not left bare.
Three words get used for this position and they name three different parts. A boot is a closed moulded shape over the end of a run: a bushing, a take-off, a bolted joint, a bend. A sleeve runs along the straight length in between, which is not what is being sized here. A shroud is clipped over an open connection and lifts off whenever someone needs to get at it.
This page sizes the first of the three only. Continuous sleeving along a straight bar run is not sized here. No sleeve or shroud figure and no sleeve part code appears anywhere on this page. That subject sits on busbarsleeves.com, a sister reference published by the same team as this one.
Boot, sleeve, shroud
Held to for the rest of the page.
| Boot | Sleeve | Shroud | |
|---|---|---|---|
| What it covers | The end of a run: a bushing, a take-off, a bolted joint, a bend | The straight length between two ends | An open connection, from outside |
| Form | Closed moulded shape | Continuous along the run | Clips on |
| Comes off again | No, it is shrunk down onto the part | No | Yes |
| Sized on | Internal diameter at each end, as supplied and recovered | Not sized on this site | Not sized on this site |
| On this page | GRB and GSB, in full | Handed off | Handed off |
Right Angle Boot (GRB) and Straight Boot (GSB)
A closed moulded boot for the bushings inside a cable termination box, in cross-linked non-tracking polyolefin, internally coated with water-resistant red mastic. Four right angle sizes and four straight, catalogued on internal diameter at both ends.
| Material | Cross-linked, non-tracking polyolefin |
|---|---|
| Lining | Water-resistant red mastic |
| Shrink temperature | 125°C |
| Continuous operating temperature | -40°C to +110°C |
| Dielectric strength | 15 kV/mm (min.) |
| Tracking and erosion | Resistant to 3.25 kV for 20 minutes |
| Right angle sizes | GRB-1, GRB-2, GRB-2A, GRB-3 |
| Straight sizes | GSB-1, GSB-2, GSB-2A, GSB-3 |
Right angle boot: GRB selection chart
| Code | D, s (min) | D, f | d, s (min) | d, f | P, s (min) | P, f (min) | T (±10%) |
|---|---|---|---|---|---|---|---|
| GRB-1 | 78 | 35 | 34 | 16 | 160 | 130 | 4.0 |
| GRB-2 | 78 | 37 | 52 | 27 | 170 | 140 | 4.5 |
| GRB-2A | 90 | 37 | 67 | 27 | 170 | 140 | 4.5 |
| GRB-3 | 145 | 72 | 67 | 34 | 170 | 150 | 4.5 |
Transcribed from the manufacturer's selection chart, Part I. D and d are internal diameters; s is as supplied and f is after free recovery. All dimensions in mm.
Straight boot: GSB selection chart
| Code | D, s (min) | D, f | d, s (min) | d, f (max) | A, f (min) | L, f (min) | B, f (min) | T (±10%) |
|---|---|---|---|---|---|---|---|---|
| GSB-1 | 80 | 34 | 34 | 21 | 140 | 225 | 30 | 3.6 |
| GSB-2 | 80 | 35 | 57 | 22 | 140 | 225 | 30 | 3.6 |
| GSB-2A | 100 | 36 | 57 | 22 | 140 | 225 | 30 | 3.6 |
| GSB-3 | 140 | 65 | 90 | 33 | 155 | 250 | 40 | 4.0 |
Transcribed from the manufacturer's selection chart, Part II. Same s and f convention. All dimensions in mm.
Picking the profile, and the numbers it turns on
Two decisions in this order: which way the termination exits, then what diameter the boot has to close onto.
- The exit sets the profile. A bushing or take-off that turns through 90° inside the box takes the right angle GRB; one leaving in line with the cable takes the straight GSB.
- D is the end that passes over the bushing or the bar end. The s figure is what it will go on over as supplied, the f figure is what it closes to after free recovery.
- d is the cable end, and it carries the same pair. It is the end most often specified wrong, because the number taken in the field is measured over the lug rather than over the cable.
- Both ends have to work at once. A GRB-2 opens to 78mm at D and 52mm at d. A GRB-2A opens wider at D, to 90mm, yet sits on the same 67mm at d as the far larger GRB-3.
- P is the length the right angle boot occupies once recovered: 130mm on a GRB-1 and 150mm on a GRB-3. On the straight boots the equivalent recovered length runs 225mm to 250mm, which is the figure that decides whether the box has room at all.
- Wall thickness runs 3.6mm on the smallest straight boot and 4.5mm across the three larger right angles, and it is what a designer loses out of a fixed clearance.
- Shrink temperature is 125°C. The GHRB and GHSB harness boots shrink at 120°C and carry 10 kV/mm rather than 15 kV/mm. One letter separates the codes and the parts do not interchange, a trap right angle and straight boots sets out side by side. Neither GRB nor GSB appears in the harness index on the boot size chart.
- Once the size is settled, where the heat starts on a 90° profile is a separate question, and installing a boot or a breakout has it.
Three things the published chart does not give you
The GRB drawing labels a dimension R along the short limb. The selection chart in the sources this site works from carries D, d, P and T only, so no R figure is printed above. On the GSB chart the letters A, L and B head columns with no legible legend. Their figures are reproduced above without a claim about what each one measures. The min-or-max qualifier on the recovered f columns is legible only on the GSB d column, where it reads max. For the dimensioned drawing and the qualifiers as printed, the manufacturer's Right Angle Boot and Straight Boot page carries the sheet.
Asked before an order goes out
Is a GRB the same part as a GHRB?
Can one of these cover a straight length of bar?
Do I order against the s figure or the f figure?
What happens if the boot has to come off again for testing?
What is the red mastic actually for?
A size quote turns on four numbers
The diameter over the bushing or bar end, the diameter over the cable, whether the exit is 90° or in line, and the clearance available for the recovered length. With those four an enquiry comes back as a part code rather than as a question.