Shrouding a switchgear panel or a transformer terminal box
A bolted joint in a 12 kV cubicle sits under 2.00 mm of PVC or 1.0 mm of cross-linked polyolefin. Four other points in the same panel take four other parts.
Where the arc actually starts
The exposure in a cubicle is rarely the straight bar run. It is the assembled connection — a bolted joint, a tee, an elbow, the fixed contacts a breaker racks onto, the point where a bushing meets a bar. Contaminants and moisture settle on those surfaces, the effective clearance across them drops below what the design assumed, and then a rodent, a lizard or a switching surge supplies the event. The manufacturer's own barrier-sheet literature names exactly those three agents — contaminants, moisture and animals — as the cause of inter-phase flashover inside a switchgear cabinet.
Ask which point went, and the answer is almost always the point that was awkward to cover. An odd-angle tee sitting behind a CT. A riser that clears the gland plate by a hand's width. The catalogue part did not close on it, so nothing went on it. That is the real failure mechanism of panel shrouding: not a cover that failed, but a cover that was never fitted because it would have had to be drawn.
A transformer terminal box changes two variables and no others. The connections are the same bolted lugs and the same bare tails, but the box is often outdoors and often warmer, so the cover has to survive ultraviolet and ozone as well as heat. Both shroud constructions are stated as suitable indoor or outdoor and resistant to UV and ozone, and the cross-linked polyolefin boot adds resistance to acid, alkali and salt attack. Where wildlife rather than contamination is the trigger, the moulded PVC insulating shroud offered for transformer bushing covers, substation connector covers, auto reclosers, surge arresters and single-pin insulators is the part that gets specified — moulded to size, shape and quantity, and carrying an anti-rodent, anti-termite formulation.
One cubicle, five exposure points
Walk it in this order. Each point has a different geometry, and geometry — not panel rating — is what decides the part.
The bolted bus connection
Joints, tees and elbows. A moulded shroud closes over the assembled connection along a fastener line. PVC walls are recommended at 1.25 mm for 3.3 kV, 2.00 mm for 12 kV, 2.50 mm for 18 kV, 3.00 mm for 24 kV and 4.50 mm for 38 kV; cross-linked polyolefin does the same duty thinner, at 1.0 mm for 12 kV rising to 2.5 mm for 36 kV. Dielectric strength is 16 kV/mm (min., ASTM D149) for the PVC and 22 kV/mm (min., ASTM D149) for the polyolefin. The PVC shroud is UL certified under File E335936 and tested to ANSI C37.20.2 for medium-voltage switchgear up to 36 kV.
The draw-out breaker
The breaker case and its contact assembly in an MV or HV power-plant or substation cubicle. The insulation boot for draw-out circuit breakers is moulded in non-tracking cross-linked polyolefin, meets ANSI C37.20.2, and is fitted or taken off in minutes complete with its fasteners. It shows no tracking, erosion or flame failure up to 3.25 kV for 20 min (ASTM D2303), holds −40 to +105 °C continuously (IEC 216), returns an oxygen index of 28 or better (IEC 93) and measures 45 ± 10 Shore D (ASTM D2240). It is halogen free and flame retardant, and it is also specified where acid, alkali or salt attack the racking gear.
The bushing interface
Where a bushing meets a bar or a cable lug, the shape belongs to the bushing, not to the bar, so a shroud drawn around a rectangular section will not close on it. The manufacturer's switchgear range lists a bushing boot and a heat-shrinkable gland for this point. Size it from the bushing profile and the tail it has to reach onto, not from the bar.
The cable entry box
Lugs, tabs and cut ends in the cable chamber are terminal-shaped rather than joint-shaped. They take a cap or a moulded terminal cover, which seals one end rather than wrapping a connection between two conductors. This is the point most often left to tape on a retrofit, and tape is what the thermographic camera later finds hot.
The underside with reduced clearance
Where the underside of a structure leaves less air than the design assumed, the part is a non-shrinkable insulating sheet cut and drilled to fit rather than a moulded cover, and busbarsleeves.com holds that datasheet and publishes it, together with the air-clearance arithmetic that decides how much of it a panel needs.
Surveying one cubicle before you order
Five columns on one sheet of paper. Most enquiries arrive with the second column filled in and the first, third and fourth blank.
Name the object, then measure it
Go through the cubicle with the panel drawing and list every place where metal is bare and live. Against each, record the shape — flat joint, tee, elbow, stud, lug — the bar section or stud diameter, and where a fastener line can physically sit. A cover is quoted from that, and from nothing else.
Fix the voltage class second
Class sets wall thickness and no other variable on the sheet touches it. PVC: 1.25 mm at 3.3 kV, 2.00 mm at 12 kV, 2.50 mm at 18 kV, 3.00 mm at 24 kV, 4.50 mm at 38 kV. Cross-linked polyolefin: 1.0 mm at 12 kV, 1.4 mm at 18 kV, 1.8 mm at 24 kV, 2.5 mm at 36 kV. If the two families disagree with each other on your class, they are not disagreeing — the polyolefin is simply the better dielectric per millimetre.
Mark every line re-entry or permanent
Beside each point write how often somebody has to get back underneath it: annual thermography, a re-torque interval, a connection you already know will change. That column decides material and fixing method, and it is the one column that never appears on the drawing you were handed.
Draw whatever the catalogue misses
Most panels contain at least one connection no standard part fits. The manufacturer moulds custom shapes and connections and states that a new design mould is produced within a week from a drawing, with an in-house tool and die shop, a formulation lab and mechanical and electrical testing to 50 kV behind it. One odd tee is not a reason to leave a point bare for the life of the board.
Check the temperature the enclosure really runs at
PVC shrouds are rated −20 °C to 115 °C continuous (IEC 216) and pass UL 94-V0 for flammability, at 65 ± 5 Shore A (ASTM D2240). Cross-linked polyolefin reaches −40 °C to +115 °C (IEC 216) with volume resistivity of 1 × 10¹³ Ω·cm (min., ASTM D257). An outdoor transformer terminal box in a hot climate is the case where the difference between those two temperature floors stops being academic.
What the annual shutdown needs from a cover
This is the half of the specification that gets left out, and it is the half you find out about two years later with the board dead and a window of six hours.
- A fastener line, not an adhesive line. The shrouds and the breaker boot are supplied complete with fasteners and are described as easy to install or remove as often as required. That sentence in a datasheet is the re-entry promise; its absence from a datasheet is equally informative.
- A part code written on the panel schedule. A cover taken off by a contractor who cannot name it is a cover that gets refitted with tape, and tape is a two-year part in a twenty-year board.
- Colour used as a state marker. The moulded PVC shroud is offered in grey, red or black, with many standard sizes held in stock. Choosing one colour for the permanent points and another for the ones that come off every shutdown costs nothing at order time and saves an argument at 03:00.
- Clearance for the tool, not just for the bar. A cover that clears the connection but not the spanner gets opened with a knife the first time, and after that it is not refitted at all.
- An honest count of which points reopen. If a cubicle has eleven covered points and the shutdown scope touches four, then four parts have to survive removal and seven do not. That is a cheaper bill of materials than covering everything as though it were permanent, not a dearer one.
- A note that a shrunk-on tube is not re-entry hardware. It is cut away and a fresh length shrunk on, which is a different maintenance decision with a different cost, and it is why this site treats covers and tubes as separate families.
What this does not do
Shrouding a panel is insulation enhancement around an assembled connection. It does not make an energised cubicle safe to open or safe to work inside, it does not stand in for the board's earthing, interlocking and access procedure, and it does not re-rate the switchgear to a higher voltage class. The wall thicknesses on this page are the manufacturer's recommendation for a given class; they are a cover specification, not a withstand rating for the assembly. Isolate, earth and prove dead exactly as you would if nothing were fitted.
Where to go next
- Insulating shrouds: the removable cover over an assembled connectionThe product page — PVC and cross-linked polyolefin, wall by wall.
- Termicap and Termicover: caps and covers for terminals and tabsFor the lugs and tabs in the cable chamber.
- Insulator covers and bus connection cover-upsFor the bushing interface, and for anything mounted outdoors on a structure.
- Fitting covers, shrouds and capsFour fitting procedures and one removal.
- How to read an insulating cover datasheetWhat each bracketed test method is claiming, and which figures are not comparable across materials.
- Why covers fail, and what each test method is trying to predictTracking, erosion and thermal ageing.
- Electrical insulation covers, indexed by where the bare metal isThe hub this page belongs to.
- Switchgear insulation products (PDF, manufacturer's own index sheet)The source for the switchgear range referenced throughout this page.
Send the survey sheet, not the panel rating
Shapes, bar sections, voltage class and a re-entry mark against each point comes back as a covered bill of materials in one reply. A rating on its own comes back as a question, and the question costs a week.