Guide

How busbar insulation fails in service

Four modes, one published test each, and a tracking figure that four parts in this range carry and the PVC ones do not.

Four modes, and only one of them is quick

A busbar covering degrades in four ways. A conducting path creeps across its surface, the surface pits and loses material, the polymer ages until it no longer stretches, or the covering is damaged during work in the panel. Only the last is instantaneous; the other three run for years, and each has one bench test standing in for those years.

The manufacturer publishes no inspection interval and no end-of-life criterion for any part in the range. The nearest thing to one is the ±25% ageing band on the GBBS barrier board, and it is a laboratory acceptance limit, not a field limit.

Scope: this page is about the covering itself degrading over time. Arcing at an exposed bar end, and the cap fitted over it, sits on what a BUSCAP does at a live bar end.

Surface tracking

Tracking is a carbonised conducting path forming across the covering, not a puncture through it. The failure is the moment that path reaches the next phase or earth. Contaminants, moisture and animals are the three causes the manufacturer names for inter-phase flashover inside a cabinet.

ASTM D2303 is the method that reports it, and four parts record the same result against it: no tracking, erosion or flame failure up to 3.25 kV for 20 minutes. Those four are the BUSTUBE sleeve, the 66 kV GEHB sleeve, the BUSBOOT polyolefin shroud and the GBBS barrier board.

All four are cross-linked polyolefin, and that is the line the specification has to respect. The two PVC parts, the BUSBOOT PVC shroud and the BUSCAP, publish no D2303 figure at all; their published flame statement is a UL 94-V0 pass. A 3.25 kV line copied onto a PVC item is not supported by anything the manufacturer has published.

Erosion

Erosion is material loss from the surface, and D2303 reports it from the same run as tracking. Tracking draws a conducting line; erosion digs a pit. Once a pit has formed, the wall left under it is what holds the voltage off.

How much wall there is to lose is published, and it is set differently on each part. On the Heavy Wall sleeve it follows the size code: 3.20 mm at GHB 25/8, 3.50 mm from GHB 30/12 to GHB 85/32, 3.70 mm from GHB 100/38 to GHB 205/85 and 4.20 mm at GHB 250/120, each to ±10%. On the polyolefin shroud it follows the voltage instead, from 1.0 mm minimum at 12 kV through 1.4 mm at 18 kV and 1.8 mm at 24 kV to 2.5 mm at 36 kV.

So measure a pit against the code actually fitted, not against a nominal figure for the range. A pit 1 mm deep is 31% of the wall on a GHB 25/8 sleeve and 40% of the published minimum on a 36 kV shroud.

Where the cabinet atmosphere is the suspect, the published resistances are what to check against it. The sleeve is stated to protect busbar against strong acid, alkali and salt; the board is stated resistant to chemicals, solvents, alkali and salt, UV-stabilised and rated for outdoor use.

Thermal ageing

Ageing is measured as lost elongation, not as melting. The barrier board's accelerated-ageing check holds it at 90 °C for seven days and requires tensile strength and ultimate elongation to stay within ±25% of the unaged value. That is the only variation band in the range: the polyolefin shroud records a pass with post-ageing minima of 10 N/mm² and 250% instead, and no ageing figure is published for either PVC part.

The continuous limits are not uniform, and the differences are wide enough to matter in a hot cabinet. −40 to +115 °C covers the BUSTUBE sleeve and the polyolefin shroud; the 66 kV GEHB sleeve and the bus sheet stop at +105 °C; the PVC shroud and the BUSCAP start at −20 °C; the GBBS board runs −45 to +105 °C.

The cold end is checked separately and it is the one that catches an aged covering. Low-temperature flexibility holds the material at −40 °C for four hours and looks for cracking; the polyolefin shroud and the barrier board both record none.

Mechanical damage in service

This is the mode with no ageing curve behind it. It arrives at once, and it is the reason animals sit on the manufacturer's own list of inter-phase flashover causes beside contaminants and moisture.

One part is specified against it directly. GBBS board resists mechanical impact and wear, and cuts and drills without special equipment, so a damaged length is replaced on site with hand tools.

For the rest, what matters after damage is removability, and the published position is asymmetric. The BUSBOOT shroud and the BUSCAP are both stated to install, remove or replace in minutes, the shroud as often as required. No removal method is published for a sleeve, which is shrunk onto the bar with heat.

Which published test predicts which mode

ASTM D2303 — tracking and erosion: Recorded for the BUSTUBE sleeve, the GEHB 66 kV sleeve, the BUSBOOT polyolefin shroud and the GBBS board as no tracking, erosion or flame failure up to 3.25 kV for 20 min. Not published for either PVC part.
ASTM D2671 — accelerated ageing and low-temperature flexibility: Both thermal results on this page come from it: 90 °C for 7 days with ±25% on tensile strength and ultimate elongation, and −40 °C for 4 hrs with no cracking.
ESI 09-11 — heat shock: 90 °C for 30 min, no cracking or flowing, recorded for the polyolefin shroud. It is the only heat-shock figure published across the range.
IEC 216 — continuous temperature limit: The band the covering may sit in permanently, running from −45 °C on the board to +115 °C on the polyolefin sleeve and shroud. It is a limit on the polymer and says nothing about the current the bar underneath may carry.
UL 94-V0 — flame behaviour: A pass is recorded for the PVC shroud and for the BUSCAP. No UL 94 rating is published for the sleeves, the polyolefin shroud or the board, whose only flame statement sits inside their D2303 line.

Every method above is decoded once on this site, on the standards a busbar sleeve is tested to. The figures are quoted here and explained there.

Contamination and moisture: what each part does about it

Contaminants, moisture and animals are the named causes of inter-phase flashover inside a switchgear cabinet. Covering the conductor addresses none of them directly.

	Heat-shrink sleeve	Moulded shroud	GBBS barrier board
Where it sits	Shrunk onto the bar face	Over one joint, tee or elbow	Between phases, as an inter-phase barrier
Removal after damage	No method published	Removes and replaces in minutes	Cuts and drills with hand tools
Tracking, ASTM D2303	3.25 kV / 20 min, no failure	Polyolefin only; not published for PVC	3.25 kV / 20 min, no failure
Water absorption, ASTM D570	Not published	0.5% max. (polyolefin)	0.5% max.
Continuous limit, IEC 216	−40 to +115 °C; GEHB 66 kV −40 to +105 °C	−20 to +115 °C PVC; −40 to +115 °C polyolefin	−45 to +105 °C
After a power arc	Not published	Not published	Withstands power-arcs without changing shape; wipes clean with no visible surface damage
What it does about the phase-to-phase gap	Cuts the clearance the gap must provide — 140 mm at 36 kV on Heavy Wall against 320 mm bare	Cuts the clearance around the connection; no phase-to-phase figure published	Occupies the gap; the only part here whose stated application is an inter-phase barrier

Read the last two rows together before specifying against a contamination problem. A sleeve and a shroud raise the withstand of the surface they are fitted to and leave the air between phases as it was; the board is the only part in the range published as an inter-phase barrier, and the only one with a stated power-arc result.

Ageing and tracking, measured — BUSBOOT polyolefin shroud

Test description	Recorded value	Test method
Low temperature flexibility (−40 °C for 4 hrs.)	No cracking	ASTM D2671
Heat shock (90 °C for 30 min.)	No cracking or flowing	ESI 09-11
Continuous temperature limit	−40 °C to +115 °C	IEC 216
Accelerated ageing	Pass	ASTM D2671
Tensile strength after ageing	10 N/mm² (min.)	ASTM D638
Ultimate elongation after ageing	250% (min.)	ASTM D638
Resistant to track and erosion, 3.25 kV for 20 min.	No tracking, erosion or flame failure	ASTM D2303

Reproduced from the manufacturer's BUSBOOT Polyolefin datasheet.

Ageing and tracking, measured — GBBS barrier board

Test description	Recorded value	Test method
Accelerated ageing	90 °C for 7 days	ASTM D2671
a. Tensile strength	±25% variation	ASTM D638
b. Ultimate elongation	±25% variation	ASTM D638
Low temperature flexibility (−40 °C for 4 hrs.)	No cracking	ASTM D2671
Continuous temperature limit	−45 to +105 °C	IEC 216
Water absorption	0.5% (max.)	ASTM D570
Dielectric strength	20 kV/mm (min.)	ASTM D149
Resistant to tracking and erosion, 3.25 kV for 20 min.	No tracking, erosion or flame failure	ASTM D2303

±25% is the only variation band published in this range; every other part records a pass mark or an absolute minimum instead.

What “3.25 kV for 20 minutes” does and does not promise

It is a recorded D2303 result on a specimen, held for twenty minutes, and its value is that it lets two materials be compared on equal terms. It is not a voltage rating for the part: the same sleeve is rated for switchgear application to 36 kV and carries a minimum dielectric strength of 22 kV/mm to ASTM D149. It is not a service life. And it is not a figure the PVC shroud or the BUSCAP publishes, so it cannot be quoted against them.

What a thermographic survey can and cannot see on a covered bar

A hot bus bar is found with a camera, and a camera reports the outermost surface in its line of sight. On a covered bar that surface is polymer.

The wall between the lens and the joint is 3.20 mm on a GHB 25/8 sleeve and 4.20 mm on a GHB 250/120, each to ±10%, or 1.0 to 2.5 mm on a polyolefin shroud depending on the voltage it was moulded for.
No emissivity value for these coverings and no correction factor for a covered bar appears in the manufacturer's published data. Any figure applied is the surveyor's own assumption and belongs in the report as one.
The published continuous limit is a limit on the polymer — +115 °C on the polyolefin sleeve and shroud, +105 °C on the board and the 66 kV sleeve. Nothing in the range relates an outer-surface reading to the temperature of the metal underneath it.
A moulded shroud is stated to remove and replace in minutes, so a joint under one can be scanned bare at a shutdown. No removal method is published for a sleeve, which is one more argument for shrouding the joints and sleeving only the runs.
Joint quality is therefore an installation-stage control here rather than a monitoring one; those checks belong in installing a busbar sleeve, before the covering goes on.

Where each of these leads

What this page does not establish

None of these tests models a fault current. 3.25 kV for 20 minutes, 90 °C for seven days and −40 °C for four hours are bench conditions, and none of them reproduces an internal arc; the board's power-arc statement is the range's only claim in that direction and it is published without a test method or a rating. Nothing specified on this site raises a panel's arc rating or substitutes for the arc-fault classification the switchgear itself was tested to. And no figure here describes the bar under the covering: temperature rise, current-carrying capacity and derating of an insulated bar are busbar design questions, and no source on this site answers them.

Send the size code fitted and what the cabinet breathes

A failure question is answerable only against the part actually installed. Give the size code or shroud reference, the voltage class, and the ambient — damp, salt or dust.

Send a bar size and a voltage Five ways to insulate a busbar