Bushing boots and termination boots for switchgear and transformers

Pick the construction on whether the connection is ever broken again, then confirm it against the measured bushing diameter.

Terminating into a switchgear panel

The decision, and the inputs it needs

Five inputs settle which boot goes on the requisition, and four of them are measured rather than looked up. Bushing outside diameter in millimetres. Connection geometry, right-angled or straight. Highest system voltage for the board. Whether the connection is broken again for testing or extension. And whether the cable arriving at it is PILC or XLPE.

Voltage settles the least of the five. All three constructions below sit inside the 1.1 kV to 36 kV band that the manufacturer's termination kits occupy, and none of them lifts the withstand class of the equipment they are fitted into. Reuse, sealing method and fixing method are what actually separate them.

A boot goes over a connection that is already made. No boot source in the catalogue claims a field-grading duty, so grading stays upstream with the stress control tube listed in the termination kit components. The boot's whole job is to occupy air that has become too short.

Three constructions, and the condition that selects each

Everything in this table is as published by the manufacturer. Where a cell reads not published, the figure genuinely does not appear in the source document — and the two size charts that used to be missing here are reproduced further down.

Property	Elastomeric bushing boot	Reusable insulating boot	Heat-shrink boot (GRB / GSB)
Material	Non-tracking elastomeric	Modified silicone/rubber shroud	Cross-linked non-tracking polyolefin
Inner surface	Not published	Not published	Water-resistant red mastic
Fixing	Fitted over the bushing	Fitted over the bushing	Recovered with heat at 125 °C
Broken again?	Not published as removable	Removable and refittable as often as required	Permanent once recovered
Voltage, as published	Up to 25 kV in one datasheet, 24 kV max system voltage in another, ANSI C37.20.2 to 36 kV in the feature list	AC 35 kV for 5 min, BIL 95 kV, DC 48 kV for 30 min	No kV figure against any code; the compound is qualified to 3.25 kV for 20 min [ASTM D2303]
Continuous temperature	−20 °C to 115 °C	Not published	−40 °C to +100 °C on the datasheet; −40 °C to +110 °C on the product page
Sizing basis	Bushing diameter 46–70 mm; GSBB-1 and GSBB-2 on cable-entry bore	Bushing diameter 46–70 mm; one code, GRRB-1	Recovered bore: GRB 16–72 mm, GSB 21–33 mm — full charts below
Sealing claimed	Not published	Cable, cable lug and bushing, against rodent ingress and high humidity	Water-resistant mastic lining
Cable behind it	Terminations generally	PILC and XLPE	Terminations generally
Selects when	Phase-to-phase and phase-to-earth insulation at the bushing has to be made up	The joint is opened for testing or extension during its service life	A sealed permanent cover is wanted in a box that stays shut

Reusable insulating boot

A modified silicone/rubber shroud fitted over a completed termination, published as installable and removable as often as required, on bushings of 46 to 70 mm diameter.

Withdrawn and refitted repeatedly without replacement, which is the one property neither other construction offers.
Direct voltage testing is permitted with the boot in place, so a routine test does not consume a new part.
Range-taking across 46 to 70 mm rather than one code for every bushing diameter.
Published for paper-insulated and extruded terminations alike, which matters on a mixed brownfield board.

Material	Modified silicone/rubber shroud
Bushing diameter	46–70 mm
Published dimensions	GRRB-1: bushing-entry bore 44.0 mm min, cable-entry bore 20.0 mm min, L1 175.0 mm min, L2 170.0 mm min
Published withstand	AC 35 kV for 5 min; basic impulse level 95.0 kV; DC 48 kV for 30 min
Cable types	PILC and XLPE terminations
Removal	Install or remove as often as required
Re-energising	Immediately after installation
Testing	Permits direct voltage testing
Sealing	Cable, cable lug and bushing, against rodent ingress and high humidity
Size taking	One published code, GRRB-1, range-taking across the 46–70 mm band
Published economic band	11 kV and 22 kV

Manufacturer datasheet (PDF)

Selection chart — elastomeric bushing boot (GSBB)

Product code	Total length	Bushing-entry bore	Cable-entry bore	Thickness
GSBB-1	230 mm (min.)	50.5 mm (min.)	17.5 mm (min.)	2.5 mm (min.)
GSBB-2	230 mm (min.)	50.5 mm (min.)	22.0 mm (min.)	2.5 mm (min.)

Two codes, separated on cable-entry bore alone: everything else in the chart is identical. Published on the manufacturer's bushing boot datasheet.

Selection chart — reusable insulating boot (GRRB)

Product code	Bushing-entry bore	Cable-entry bore	L1	L2
GRRB-1	44.0 mm (min.)	20.0 mm (min.)	175.0 mm (min.)	170.0 mm (min.)

One published code. L1 and L2 are the two leg lengths of the shroud. Both boots are published for bushings of 46 to 70 mm diameter, so the diameter band is not what separates them — the bore and the reuse are.

Published withstand, both elastomeric boots

Test	Value
AC high voltage	35 kV for 5 minutes
Basic impulse level	95.0 kV
DC high voltage	48 kV for 30 minutes
Bushing diameter	46 – 70 mm

The same three figures are printed against the bushing boot and the reusable insulating boot on the datasheet that carries them together. A second datasheet for the bushing boot alone prints a different pair — see the note below.

Elastomeric bushing boot: material data

Test description	Recorded value	Test method
Dielectric strength	16 kV/mm (min.)	ASTM D149
Tensile strength	12 N/mm² (min.)	ASTM D638
Elongation	350 % (min.)	ASTM D638
Density	1.23 g/cm³	ASTM D792
Hardness	65 ± 5 Shore A	ASTM D2240
Continuous operating temperature	−20 °C to 115 °C	—
Flammability	Pass	UL 94-V0

This table is published on a page covering the elastomeric bushing boot and the reusable insulating boot together, and the source does not say which of the two it describes. Treated here as the elastomeric figures. Ask before quoting any row of it against the reusable boot.

Selection chart — right-angle boots (GRB)

Code	D, s (min)	D, f (max)	d, s (min)	d, f (max)	P (min)	R (min)	T, f (±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

Printed as Selection Chart (Part - I) on the manufacturer's Angleboot datasheet. D and d are internal diameters: s as supplied, f after free recovery. All dimensions in mm.

Selection chart — straight boots (GSB)

Code	D, s (min)	D, f (max)	d, s (min)	d, f (max)	A, f (min)	L, f (min)	B, f (min)	T, f (±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

Printed as Selection Chart (Part - II) on the same sheet, against technical qualification report QR 1035. D and d are internal diameters: s as supplied, f after free recovery. All dimensions in mm.

Reading the boot charts

Two diameters do the selecting. D is the wide end, the throat that passes over the bushing; d is the narrow end, which closes onto the cable or the lug stalk. Each is published twice — as supplied, and after free recovery — and the recovered figure is the one that has to be smaller than the thing it grips. A GRB-1 goes on over anything up to 78 mm and recovers down to 35 mm at the throat and 16 mm at the tail.

The right-angle chart then gives P and R, the two leg lengths of the elbow, and the straight chart gives A, L and B along the taper. T is recovered wall thickness in both. Order against measured bushing diameter and measured lug stalk, not against the drawing's nominal — the recovered figures are minima and maxima, and a boot that will not close is a boot that has to be cut off.

GRB and GSB compound — physical properties

Property	Value	Standard
Tensile strength	12 N/mm² (MPa) (min.)	ASTM D638
Ultimate elongation	350 % (min.)	ASTM D638
Density	1.15 ± 0.2 g/cm³	ASTM D792
Hardness	45 ± 10 Shore D	ASTM D2240
Water absorption	0.5 % (max.)	ASTM D570

GRB and GSB compound — thermal properties

Property	Value	Standard
Accelerated ageing	120 °C for 500 h	ASTM D2671
Tensile strength after ageing	11 N/mm² (MPa) (min.)	ASTM D638
Ultimate elongation after ageing	300 % (min.)	ASTM D638
Low-temperature flexibility, −40 °C for 4 h	No cracking	ASTM D2671
Heat shock, 250 °C for 30 min	No cracking or flowing	ESI 09-11
Shrink temperature	125 °C	IEC 216
Continuous temperature limit	−40 to +100 °C	IEC 216

Tensile and elongation in this table are the post-ageing figures, not the as-supplied ones above.

GRB and GSB compound — electrical properties

Property	Value	Standard
Dielectric strength	15 kV/mm (min.)	ASTM D149
Volume resistivity	1 × 10¹⁴ Ohm·cm (min.)	ASTM D257
Dielectric constant	5 (max.)	ASTM D150
Resistance to tracking and erosion	No tracking, erosion or flame failure up to 3.25 kV for 20 min	ASTM D2303

What the boot is fitted to prevent

One failure, reached four ways. The published application is medium-voltage switchgear tested to ANSI C37.20.2, up to 36 kV, in a box where the air is already shorter than a clean design would give it.

Phase-to-phase or phase-to-earth clearance inside the termination box below normal air clearance.
High humidity, and condensation forming on the bushing and lug surfaces.
Rodents reaching the point where the lug meets the bushing.
Surge impulse arriving at a connection with no clearance margin left to absorb it.

Sources disagree — three times on this page

Voltage. One datasheet says the elastomeric bushing boot is used up to 25 kV. A second datasheet for the same GSBB codes gives a maximum system voltage of 24 kV. The feature list on the product page says the part is tested to ANSI C37.20.2 for medium-voltage switchgear application up to 36 kV. Three documents, three answers, a whole voltage class apart. Impulse. The same two datasheets give the basic impulse level as 95.0 kV and as 125 kV. Those correspond to different classes, so the pair moves together with the voltage question and should be asked as one. Temperature. The heat-shrink boots' datasheet gives a continuous limit of −40 to +100 °C [IEC 216]; the product page for the same boots prints −40 °C to +110 °C. Nothing here is averaged. Get the applicable figures confirmed in writing against your Um and your box temperature before any of them reaches a schedule.

Not published — ask the manufacturer

Three fields are genuinely missing, and the dimensional tables are no longer among them. No creepage distance is published for any of the three constructions. No standard at all is named against the reusable insulating boot — the ANSI C37.20.2 reference belongs to the elastomeric boot's page, not to it. And no kV figure appears against any heat-shrink boot code, so a GRB-2A is selected on diameter and the voltage argument has to be made from the compound data instead. Send the bushing outside diameter, the lug size and the phase centre spacing, and ask for the code and the class against those three.

Where a boot stops

A boot is not a termination, and fitting it well does not make it one. It adds insulation across a gap that has become too short. It does not grade a field, earth a screen or shed rain, and it never substitutes for the kit that does.

A boot is not touch-proof either. Touch-proof means an earthed conductive screen over the accessible surface, and none of these three constructions carries one. Where the accessible surface has to be safe to touch, specify a screened separable connector; a boot pulled over an unscreened lug will not do it.

Bellows appear on the same manufacturer's page and are a different article altogether. They are flexible PVC for low-voltage cable ducting in switchgear and transformer equipment, in shapes and sizes matched to the mounting option. No dielectric figure is published for them anywhere, so they must never be written into a specification as insulation.

Two neighbouring parts get confused with boots on requisitions. If the exposed metal is a lug inside a compact entry box, the part is a cable terminal protector. If the bushing is extensible and carries nothing yet, it is an end sealing cap. Boots cover metal that is already connected.

Questions this page gets asked

Is a bushing boot a cable termination?

No. It goes on after the termination is finished, over a connection that has already been graded and made off. A termination kit carries stress control, a breakout, sheds where the location needs them and a screen earthing route; a boot carries none of those. The two are separate line items on a requisition and separate parts in the store.

Can the boot be taken off and refitted?

Only one of the three. Reusable insulating boots are published as installed or removed as often as required, and the connection may be re-energised immediately after they go back on. Heat-shrink GRB and GSB boots recover onto the bushing and are a cut-off-and-replace item. Nothing in the elastomeric boot's published text claims reuse in either direction.

Does a boot make the connection touch-proof?

No, and no published wording says otherwise. Touch-proof requires an earthed conductive screen over the accessible surface. That property belongs to a screened separable connector, where the screen is bonded to earth through a drain connection.

What bushing diameter does the reusable boot cover?

46 to 70 mm, published as one range-taking band rather than a code per diameter. Take the diameter where the boot will actually seat, and quote it with the connection geometry, because the band alone does not resolve a right-angled fit from a straight one.

How do I pick between GRB-1, GRB-2, GRB-2A and GRB-3?

On two measured diameters. The bushing has to pass through D as supplied, and the recovered D has to close onto it: 35 mm for GRB-1 and GRB-2, 37 mm for GRB-2A, 72 mm for GRB-3. The tail then closes onto the cable at recovered d — 16 mm, 27 mm, 27 mm and 34 mm respectively. GRB-2 and GRB-2A share both recovered diameters and differ only at the wide end as supplied, 78 mm against 90 mm, so GRB-2A is the one that fits over a larger flange.

Which boot suits a transformer terminal box?

The published application names switchgear and transformer boxes together, and the failure is the same in both: an enclosure that has taken clearance away from the connection. Selection still turns on diameter and on whether the box is opened again. For the transformer bushing boot itself the manufacturer's own page is the primary source: Bushing Boots & Reusable Insulating Boots. What changes on the cable side is set out under transformer and motor terminations.

What temperature do the heat-shrink boots recover at?

125 °C [IEC 216]. Service temperature is a separate figure and the two sources give it differently — −40 to +100 °C on the datasheet, −40 °C to +110 °C on the product page. Only the service figure applies once the box is closed. The order of work for the whole termination is in the installation procedure.

Is there a boot above 36 kV?

No. Nothing in the boot range is catalogued above the 36 kV ceiling that applies across the termination catalogue, and what that ceiling means is set out on its own page rather than glossed over here.

Getting one boot quoted, not three

Send the bushing outside diameter in millimetres, the connection geometry, Um for the board, whether the joint is ever reopened, and whether the cable is PILC or XLPE. Those five lines settle construction and size inside one reply.

What to put in the enquiry The bushing side of an RMU