A busbar support has one job that matters and several that are easy to forget. It holds copper or aluminium bars in position, it insulates them from the enclosure and from each other, and — on the worst day of the panel's life — it has to survive the mechanical forces of a short circuit without letting the bars move, touch, or tear free.
Those forces are the sizing criterion most people skip. When fault current flows through parallel bars, the bars repel and attract each other with a force that rises with the square of the peak current. A support that is perfectly adequate as a spacer at load current can crack or pull out its inserts at 50 kA peak. This is why busbar supports and insulators are specified with a cantilever or bending strength figure, and why the support spacing in the panel is part of the calculation: halving the distance between supports dramatically reduces the bending load each one sees.
Material choice comes next. Most modern low-voltage supports are moulded from glass-fibre reinforced polyester (DMC/BMC) or similar thermoset compounds, which combine good dielectric strength with high mechanical strength and resistance to tracking. Porcelain still appears in older and outdoor designs. What to compare is not the headline material name but the published values: comparative tracking index (CTI), dielectric strength, tensile and bending strength, and flammability class — UL 94 V-0 is the usual expectation inside a panel.
Electrical clearance is the third check. The support's height sets the standoff distance, and the ribbed profile of the insulator body lengthens the creepage path between the live bar and the earthed mounting plate. The creepage you need depends on the system voltage, the pollution degree inside the enclosure, and the material group — a ribbed 660/1000 V insulator in a clean indoor panel is a very different proposition from a smooth one in a dusty, humid plant room. Stepped and ribbed profiles exist precisely because creepage, not just air clearance, is what fails in service.
Finally, the practical details: brass or steel inserts determine whether the fixing will hold torque over years of thermal cycling; the range of heights (commonly from around 16 mm up to 100 mm class standoffs) determines whether standard bar spacings can be met; and matching the support to the bar width and thickness avoids the point loads that crack moulded bodies at the fixing hole.
None of these checks requires exotic engineering — but skipping the short-circuit strength and creepage checks is how a support that looked fine on the shelf becomes the first thing to fail in a fault.