Key takeaways
- Epoxy powder coating adds insulation, touch protection and corrosion resistance, but only where the film is continuous and fully cured.
- Thickness is a range with a minimum on the finished part, because bends, edges and holes draw the film thinner than a flat face.
- Masking is a design decision. Every bare face needs a reason, a dimension and a note on the drawing.
- Specify pre-treatment, powder, thickness, masking, cure and inspection, or the coater will decide them for you.
What the coating is for
A busbar inside a panel is a live conductor with very little room around it. An epoxy coating does three jobs at once. It adds a dielectric layer between phases and to earth where the air gap is tight. It provides touch protection for a person working inside the enclosure. It also seals the copper or aluminium surface away from damp, dust and the pollutants that start corrosion on a bare bar.
It is not paint. Paint is thin, decorative and applied for appearance. A busbar coating is an insulation layer with a specified thickness, a cure schedule and an inspection routine. When it is treated as paint, the thickness is uncontrolled, the edges are thin, the joint faces get covered by accident and nobody can say what the part will withstand.
Why thickness is a range, not a number
A powder datasheet quotes dielectric strength in kilovolts per millimetre, measured on a flat laboratory plaque. A busbar is not a plaque. On a bend the film stretches on the outside and compresses on the inside. On a sheared edge it tries to cover a corner. Around a punched hole it pulls back. A coating that measures 0.30 mm on the flat face may be 0.18 mm at the outside of a bend, and that thin point is where a test will find a weakness.
The practical answer is to specify a minimum thickness on the finished part and to name the points where it must be measured. A nominal thickness on the drawing tells the coater what to aim for, not what the part has to achieve. The same drawing should set an upper limit, because a film that is too thick cracks at a tight bend, and a crack is worse than a thin area: it holds moisture against bare metal.
Measure the coating where it is thinnest
A reading on a flat face tells you what the process can do. Readings on a bend, a cut edge and around a hole tell you whether the part will pass.
The coating line, step by step
Powder coating is a sequence, and every step can fail on its own. The same five stages apply to a flat bar and to a formed bar, but a formed bar is harder at every stage after masking.
- 01
Pre-treat
Degrease, rinse and dry. A conversion coating helps adhesion on copper and aluminium. Oil left on the bar becomes a blister under the film.
- 02
Mask
Fit caps, plugs and tapes to the faces that must stay conductive. Masking is labour, so every masked area carries a cost and a handling risk.
- 03
Apply
Spray the powder electrostatically. Film build depends on the gun path, the part geometry and the way powder gathers in recesses and inside corners.
- 04
Cure
Follow the oven time and temperature on the powder datasheet. Under-cure gives poor adhesion, low dielectric strength and a film that softens in service.
- 05
Inspect
Check thickness at defined points, coverage, pinholes, adhesion, cure and the condition of the masked faces before packing.
What the drawing should state
A coating drawing does not need to be long, but it does need to close the decisions that the coater would otherwise make. The structure below is a specification skeleton. The values belong to the powder, the bar geometry, the voltage and the standard the assembly must meet.
| Callout | Typical statement | What it controls |
|---|---|---|
| Coating type | Epoxy powder, product family and colour | Dielectric behaviour and cure schedule |
| Pre-treatment | Degrease and conversion coat before application | Adhesion and corrosion resistance |
| Thickness | Minimum on the finished part, with measuring points | Dielectric strength and bend cracking |
| Masking | Dimensioned bare areas at joints, holes and earth points | Contact resistance and assembly fit |
| Edges | Deburr before coating, no sharp corner | Coverage at cut faces and holes |
| Cure | Oven time and temperature from the datasheet | Adhesion, hardness and dielectric strength |
| Inspection | Thickness, pinholes, adhesion and visual check | Evidence for the certificate |
This is a structure, not a set of values. The numbers come from the powder supplier, the design standard and the test that will be used to accept the part.
Masking is a design decision
Every bare area on a coated bar is a decision: a joint face, an earth stud landing, a cable lug pad, a window for a temperature sensor, a strip where a support must bond to the bar. If those areas are not on the drawing, the coater will guess, and the usual guess is to mask the whole end, which may leave a live face exposed or remove insulation where the design needed it.
The boundary matters as much as the bare area. A coating of 0.3 mm creates a step, and a flat washer does not bridge a step cleanly. The film should stop far enough from the contact area that the washer sits on bare metal with full pressure, and the drawing should say how far.
- Give every masked area a dimension from a datum, not a note such as mask the ends.
- State whether the mask is a plug, a cap or a tape, because the finish at the boundary differs.
- Keep the coating edge clear of the contact area so the film does not sit under the washer.
- Allow for the coating step when two bars are bolted face to face.
- Identify the earth points that must stay bare for a protective conductor.
- Treat every coating boundary as a place where moisture can sit and corrosion can start.
Bends, edges and cut faces
A bend is the hardest place to coat. Outside the bend the film is in tension. Inside the bend it is in compression, and powder tends to build up in the corner. If the bend radius is small relative to the film thickness, the coating cracks when the bar is formed after coating, or when the bar flexes in service. The drawing should say whether the bar is coated before or after bending, because the two routes give different coverage and different masking.
Cut faces and punched holes are the other weak points. A sheared edge has a burr, and a burr is a spike that the film cannot cover evenly. Deburring before coating is not cosmetic; it decides whether the edge carries a continuous film. Holes should be deburred on both sides, and the coater should know whether the film may enter the hole or whether the hole is masked for a fastener.
A coated bar that is bent after coating will show bare metal at the bend, and the defect is easy to miss because the outside of the bend still looks green or grey. A bend test on a sample, or a visual check with a low-voltage pinhole detector, finds the damage before the part reaches the panel.
Specification and inspection
A useful coating specification has six lines: pre-treatment, powder product, thickness with measuring points, masking with dimensions, cure schedule and inspection method. Add the standard the assembly must meet and the pollution degree the coating will see, because both change the thickness and the test voltage.
Inspection should be proportionate to the duty. For each lot, check thickness at defined points including a bend and an edge, look for pinholes and bare patches, check adhesion by an agreed method, confirm the cure record, and confirm that the masked faces are clean and conductive. For a safety-related part, add a dielectric test on a sample at the voltage the standard requires and record the result against the lot.
Keep those records with the shipment. A certificate that states only the powder grade does not tell the buyer whether the film is continuous on the part. Thickness readings with locations, a cure record and a visual acceptance note are more useful than a product datasheet.
Frequently asked questions
How thick should epoxy coating be on a busbar?
There is no universal figure. It depends on the system voltage, the pollution degree, the clearance the design allows and the standard the assembly must meet. Many specifications land between 0.2 mm and 0.5 mm on the finished part, but the number should come from an insulation coordination check and be confirmed at the thinnest point on a bend or edge.
Can a busbar be powder coated after bending?
Yes, and coating after forming is usually the better route because the film is not damaged by the bend. The trade-off is that masking and coverage on a formed shape are harder, and powder builds up in inside corners. Confirm the bend radius and the film thickness with the coater before the drawing is released.
Does coating replace electrical clearance?
Not automatically. A coating counts as solid insulation only where the applicable standard allows it and the film is verified for thickness, continuity and adhesion. A bare bar with a thin coating in a polluted enclosure still needs a clearance check, and a damaged film can reduce the distance that the design assumed.
Why does the coating crack at a bend?
The usual causes are a film that is too thick for the bend radius, powder build-up in the inside corner, incomplete cure, or forming the bar after coating. A tight radius with a thick film puts the outside surface into tension beyond what the cured epoxy can take.
Sources and further reading
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies
- IEC 60664-1, Insulation coordination for equipment within low-voltage supply systems
- Qumek busbar coating and masking reviews
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Coating a busbar for a tight enclosure?
Send the drawing, the system voltage and the pollution degree. The engineering desk will review the masking, thickness and bend coverage before quoting.
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