Busbar insulation: creepage, clearance and coating thickness

Three distances, one decision

Insulation coordination on a busbar comes down to three distances. Clearance is the shortest path through air between two conductive parts. Creepage is the shortest path across the surface of an insulating material between two conductive parts. Solid insulation is the material itself: a coating, a barrier, a moulded shroud or a potting compound.

The three are related but not interchangeable. A bar can have generous clearance and still track across a dusty surface. A coated bar can have a thin film at a bend and fail where the drawing showed plenty of room. A barrier can add creepage without changing clearance, or the reverse, depending on where it sits. Each one has to be checked at the worst point, not at the point that is easiest to measure.

Clearance in air

Clearance is set by the impulse voltage the insulation has to withstand, adjusted for altitude and for the overvoltage category of the circuit. At sea level, air does most of the work. At 2,000 m the air is thinner, the breakdown voltage falls, and the same gap is no longer enough. A panel that is designed at sea level and installed at altitude needs a clearance review, not just a label change.

Clearance is also a mechanical dimension. A bar that moves under fault current, sags under its own weight or is bent a little too far reduces the gap. The measured clearance should be taken with the bar in its installed position and with the supports tightened, because a check on the bench with the bar resting on blocks can read several millimetres larger than the real assembly.

Creepage across a surface

Creepage is the path along the surface of the insulator between two conductive parts. It is longer than clearance whenever the surface is not a straight line, and it is the distance that decides tracking. Pollution degree and material group drive the requirement. A dirty, humid surface with conductive dust needs a longer creepage than a clean, dry one at the same voltage.

Material group comes from the comparative tracking index of the insulator. A material that tracks at a low voltage needs a longer creepage for the same duty. That is why the same panel layout can be acceptable with one support insulator and not with another, even though the two parts have the same shape. If the support material changes, the creepage check should be repeated.

Ribs, grooves and barriers increase creepage by lengthening the surface path. They do not increase clearance, and they can reduce it if the rib narrows an air gap. A rib also collects dust, so its benefit depends on the shape of the rib and the direction of the pollution.

What a coating changes

A coating can reduce the required clearance or creepage only where the standard allows it and the coating qualifies as solid insulation. That qualification is not automatic. The film has to be thick enough, continuous over the whole surface, adherent under thermal cycling and free of pinholes at the edges and bends.

In practice, a coated bar is often treated as having a smaller air gap because the coating fills the space. That is only defensible if the coating is specified and inspected as insulation. If the drawing says only epoxy coated and the coater applies a thin decorative film, the design has assumed a distance that the part does not provide.

A coating also changes the thermal behaviour. A film on a bar acts as a small thermal resistance and can raise the bar temperature slightly for the same current. The effect is usually small compared with the change in mounting and ambient, but it belongs in the thermal check rather than being ignored.

Adhesion is the part that a datasheet cannot promise for a finished part. The film has to stay attached through the coating cure, the assembly torque, the thermal cycles of service and any cleaning chemical used in the panel. A tape test or a bend test on a sample is a crude check, but it is better than assuming that a powder that passed a plaque test will hold on a bar with a sheared edge and a tight bend.

Pollution, condensation and altitude

Pollution degree is a description of the environment inside the enclosure, not of the factory around it. A sealed panel in a clean room and a ventilated panel in a foundry can carry the same bar and need different creepage. Condensation is the harder case, because a surface that is wet for a few hours a day behaves worse than one that is merely dirty.

Altitude affects clearance through air density, and it also affects cooling. A panel rated at sea level and installed at altitude needs both an insulation check and a thermal check. Treating altitude as a documentation issue rather than a design input is a common mistake.

Working through a compact panel

The sequence below keeps the three distances separate and makes the assumptions visible. It works for a new layout and for a change to an existing one.

1. 01

   Fix the system inputs

   Record the rated voltage, impulse withstand, overvoltage category, pollution degree, material group and maximum altitude.

2. 02

   Draw the air paths

   Mark the shortest clearance from every live part to earth and to the other phases, including bolts and supports.

3. 03

   Draw the surface paths

   Trace creepage over supports, barriers and coated surfaces, following the real contour rather than a straight line.

4. 04

   Decide what counts as solid insulation

   State the coating thickness, continuity and test that allow it to be treated as insulation rather than a cover.

5. 05

   Check the worst case

   Use the installed position, the hot and wet condition, the tolerance stack and the fault deflection of the bar.

6. 06

   Record the result

   Put the distances, the measurement points and the standard clause on the drawing so the assembly can be checked.

What to put on the drawing

A busbar drawing usually shows the shape, the holes and the plating. It rarely shows the insulation distances, which is why those distances get lost between the electrical design and the fabricator. Add a small table to the drawing with the required clearance, the required creepage, the coating thickness and the points where each is measured.

State the standard and the edition you are designing to, because the tables change between editions and a supplier may work to a different one. If the coating is credited as solid insulation, say so and give the test that proves it. If a barrier is part of the insulation, give its material, thickness and fixing method, not just its outline.

Finally, separate the dimensions that are safety-related from the ones that are mechanical. A clearance that must not be reduced by a bend tolerance is a critical characteristic. A coating thickness that must be met at a bend is a critical characteristic. Marking them makes the inspection plan obvious and stops a well-meant tolerance relaxation from removing the insulation margin.