Insulation parts for switchgear and busbars by voltage class

Switchgear insulation plastic has to do three things at once: block the voltage, resist tracking when the surface gets dirty and wet, and hold its shape under heat and load for decades. The right material changes as the voltage class climbs, and a grade that is perfect at 400 V can be a liability at 11 kV. This guide walks the classes and the properties that separate them.

It is written for buyers and mechanical engineers who need to choose a machined insulation part, not a moulding.

The properties that matter: dielectric strength, CTI, arc resistance

Three numbers carry most of the decision. Dielectric strength is the voltage a given thickness withstands before it breaks down, usually quoted in kilovolts per millimetre; because breakdown is thickness-dependent, compare grades at the same wall. The comparative tracking index, or CTI, measures how well a surface resists forming a conductive carbon path when contaminated and energised, and it is often the figure that decides a switchgear part. Arc resistance measures how long a surface endures a high-current arc before it turns conductive. A high dielectric strength does not imply a high CTI, so a material can be excellent through its thickness and poor on its surface. These three figures drive most of the parts we machine for the electrical and energy industries.

Low voltage: where POM and PA66 fit

At low voltage, POM and PA66 cover a large share of busbar supports, spacers and terminal blocks. Both are stiff, machine well and meet the usual low-voltage requirements at a modest cost. POM holds tighter tolerances than PA66 because it does not absorb moisture, which matters when a spacer has to keep a fixed creepage distance. PA66 brings better toughness and a higher continuous temperature, so it is the better pick where the part also takes a knock or sits near a warm busbar. For most low-voltage switchgear insulation plastic parts, the choice between them comes down to moisture and stiffness rather than to electrical performance.

Both grades machine to clean edges, which keeps the creepage path predictable, and both can be supplied cut to size so a panel builder can stock standard sections rather than wait on a moulding run.

Medium voltage: PPS, PEI and glass-filled grades

Once you move to medium voltage, tracking resistance and thermal stability start to dominate. PPS is the workhorse: glass-filled PPS 40 combines a high CTI, good stiffness and strong chemical resistance, and it keeps its shape to a high continuous temperature. PEI takes over where a part needs higher toughness, transparency or very high heat resistance, though it costs more. Glass-filled grades raise stiffness and lower thermal expansion, which helps a long support hold its creepage distance across a wide temperature swing. The filler also makes the surface rougher and a little more prone to contamination build-up, so finish matters as much as the grade.

Thermal expansion is the quiet variable. A long support that runs from a cold cabinet to a warm busbar will grow, and if that growth is not allowed for, the part either loosens or loads its fixings. Glass-filled grades expand less than unfilled ones, which is one more reason they suit long medium-voltage supports.

Tracking and contamination in humid service

Tracking is a surface failure, and it is where humid service catches people out. Dust, salt and moisture combine to form a film that lets a small leakage current flow; over time that current carbonises the surface and the path becomes conductive. Contamination-free cleaning helps, but the material choice matters more: a high-CTI grade tolerates contamination for far longer before it tracks. That is why a switchgear insulation plastic part exposed to outdoor or wash-down duty should be selected on CTI first, and why creepage distance on the drawing must be measured over the real, contaminated surface rather than the clean one.

Flammability and glow-wire requirements

Switchgear parts are usually required to pass a flammability or glow-wire test, and the rating is often written into the specification. Many glass-filled grades can be supplied to a V-0 rating at a given thickness, but the rating is thickness-dependent, so the certificate is only valid at the wall thickness that was tested. Glow-wire testing is a separate requirement that simulates a hot element rather than a flame. When both are called up, confirm that the grade holds both ratings at the thickness you actually machine.

Retention of the rating over time matters as much as the initial pass. Heat ageing and cleaning cycles can erode a surface, so a part mounted near a switching element should be specified with margin rather than at the edge of its rating. Where the layout allows, keep the insulation wall generous and the surface smooth.

Questions buyers ask about switchgear insulation

These are the questions buyers raise most often when they specify switchgear insulation plastic parts for a new panel.

Which material suits 11 kV busbar supports?

A high-CTI glass-filled grade such as PPS GF40 is the usual starting point, because tracking resistance and dimensional stability matter more than raw strength at that class. PEI is an alternative where higher toughness or heat resistance is needed. Confirm the CTI and the thickness on the actual drawing before you commit.

What CTI should I ask for?

Ask for the highest value your creepage and pollution assumptions require, and no higher, since the strongest grades cost more. For contaminated or outdoor duty the practical target is a material rated well above 600 V. State the test method and the conditioning, because CTI figures are only comparable when they were measured the same way.

Does thickness change the voltage rating?

Yes. Dielectric breakdown rises with thickness but not linearly, and flammability ratings are quoted at specific thicknesses. A grade qualified at 3 mm does not automatically carry the same rating at 1 mm. Always match the certificate thickness to the wall you machine.

How do I handle creepage distance on the drawing?

Set the creepage and clearance from the relevant standard for your voltage, pollution degree and material group, then add a machining allowance so the finished part still meets it after any coating or wear. Mark the critical surfaces on the drawing and note the finish, since a rough surface can track sooner. Send the drawing to our team through the contact page and we will confirm the grade and the achievable tolerances.