Material selection for MCB, MCCB and ACB housings

Three devices, three duty profiles

A miniature circuit breaker, a moulded case circuit breaker and an air circuit breaker all interrupt current, but they do it at very different energy levels and in very different package sizes. A miniature device protects a final circuit and fits on a DIN rail. A moulded case device handles distribution currents and has a sealed case that contains the arc. An air circuit breaker handles the largest currents and uses open air arc chutes with substantial insulation around them.

That difference drives the housing material. The small device needs thin walls, precise moulding and high tracking resistance in a compact shape. The large device needs a material that resists the heat and the arc products of a much bigger interruption, and that holds its shape over years at high temperature.

Designers sometimes carry a material from one device family to another because it worked before. That is the most common source of a housing that passes the type test and fails in service, because the duty profile changed and the material data was never revisited.

The requirement set

Six properties decide most housing grades. The first is dielectric strength, the voltage the material withstands through its thickness. The second is tracking resistance, usually expressed as a comparative tracking index measured to IEC 60112, which describes how the surface resists forming a conductive path under contamination.

The third is flammability. A housing near an arc must not sustain combustion, and the rating is reported at a specific thickness and often a specific colour. The fourth is glow wire performance, which describes how the material behaves when a hot element touches it, and which is often the requirement that actually governs an appliance or switchgear part.

The fifth is thermal. Heat deflection temperature tells you how the part behaves under load at temperature, and the relative thermal index tells you how long the material retains its properties at a temperature. They answer different questions and are not interchangeable. The sixth is mechanical: impact, stiffness and creep, including the creep that decides whether a terminal pocket loosens over a service life.

Arc resistance is a seventh property that matters at the high end. It describes how the surface resists forming a carbon track under an arc, and it is one reason thermosets remain common in large breaker cases.

The material families in practice

Glass filled PA66 is the workhorse for small breaker housings. It is tough, moulds thin walls well and can be supplied with flame retardant packages that reach demanding glow wire ratings. Its weakness is moisture absorption, which changes dimensions and electrical properties, and it needs a drying and conditioning routine that the moulder must control.

Glass filled PBT is the alternative where dimensional stability matters more than impact. It absorbs little moisture, holds a moulded dimension well and has good electrical properties. It is less tolerant of thick sections and can be brittle under impact at low temperature.

PPS and other high temperature semi crystalline grades appear where continuous temperature is high and the part must keep its shape. They cost more and need hot tooling, so they are chosen for a real thermal reason rather than for margin.

Thermoset polyester and vinyl ester compounds, often glass filled, remain common in moulded case and air circuit breaker housings. They resist arc tracking, hold dimensions under heat and do not melt, which matters when the failure mode being designed for is a high energy arc. The trade-off is a slower moulding cycle, a harder surface finish and less freedom to mould thin, complex features.

PC and PC blends appear in covers, barriers and low energy enclosures where impact and appearance matter. They are rarely the right choice for a part that sees arc products directly.

Matching material to device

The table is a starting point for a material discussion. It is not a specification: the flame rating, tracking index and glow wire result at the wall thickness in your design decide whether a grade is acceptable, and those results come from the supplier's test data and your own type tests.

What the filler does to the part

Glass filler raises stiffness and heat deflection temperature and lowers the coefficient of expansion, which is why it is almost universal in breaker housings. It also makes the material anisotropic. A 30 percent glass filled grade shrinks less along the fibre direction than across it, so a part with flow running in several directions warps in a pattern that follows the fill.

Weld lines are weaker in filled grades because the fibres align along the interface rather than crossing it. A housing with a weld line across a boss that takes a terminal screw is a crack waiting for a torque wrench. Move the gate or move the boss.

Filler also changes the surface. A glass filled part has a different gloss and feel from an unfilled one, and exposed fibres can show at a gate or a wear mark. On a visible cover, that may push the design towards a lower glass content or a painted finish, both of which change the approval basis.

Flame retardant packages interact with all of this. Some additives reduce impact strength, some increase plate-out on the tool and some affect the colour match. A grade change for a flame rating is a change to the whole part, not a swap of one line on the drawing.

The evidence behind an approved grade

For each material, keep the supplier's data sheet, the UL yellow card if the part is covered by a UL recognition, and the test reports that support the flame and glow wire claims at the thickness used. A yellow card lists the grades, the colours and the thicknesses for which a rating applies. A grade not listed at your thickness is not covered, whatever the data sheet headline says.

For the device, the relevant standard is the IEC 60947 series, with additional requirements from the installation and appliance standards that apply in the end product. The housing material is only one input: creepage and clearance distances, the arc chute design and the terminal arrangement all affect whether the device passes.

A material change after approval needs the same evidence as the original. Substitution by a distributor, a colour change or a move to a second source can all invalidate a rating. Ask for the certificate with the batch and check the grade designation against the approved list.

• Supplier data sheet for the exact grade and colour.
• UL yellow card showing the rating at the wall thickness used.
• Flame and glow wire reports at the production thickness.
• CTI report to IEC 60112 if tracking is a design input.
• Batch certificate matching the approved grade.

How to specify the material on the drawing

Name the polymer, the filler content and the flame rating, then add the specific grade if the design has been tested with it. A note that names PA66 GF at V-0 leaves the moulder free to choose any grade that meets the words, and the electrical results may not follow.

State the colour and the approval basis. A rating at natural colour does not automatically apply to black, and a black part may need a different additive package. Where the part is laser marked, check that the marking does not damage the surface tracking performance.

Add the conditioning requirement for hygroscopic grades. A PA66 part that is measured dry and installed wet will not hold the dimension the drawing assumes. State the moisture condition at which dimensions and mechanical properties are specified.

Finally, put the material and the approval documents on the first article report. A housing that measures correctly in the wrong grade is not a conforming part.

1. 01

   Fix the duty

   Write down the continuous temperature, the arc exposure and the tracking duty for the part.

2. 02

   Shortlist by evidence

   Check flame, glow wire and CTI results at the wall thickness you have designed.

3. 03

   Check the process

   Confirm the grade moulds the wall, the draft and the cycle the tool allows.

4. 04

   Lock the grade

   Name the grade, colour and approval documents on the drawing and the first article.