Busbar design for high humidity and polluted sites

What humidity and pollution actually do

Humidity alone is a mild condition. The problem starts when moisture condenses on a surface and dissolves whatever is on it. Dust, salt, sulphur compounds and industrial fallout form an electrolyte film, and that film turns an insulating surface into a weak conductor. The result is a leakage current, a surface discharge or a tracking path across the insulation.

The second effect is corrosion. A copper bar with a thin oxide film has a higher contact resistance at a joint. An aluminium bar forms an oxide quickly, and the oxide is both resistive and a barrier to a good joint. A plated bar resists this, but a scratch, a cut edge or a plating void becomes the starting point.

The third effect is thermal. A wet surface cools by evaporation, and a joint that is wet and then dry can cycle in temperature. The thermal cycling works on the joint and the coating, and the corrosion products can build up in the gap. Over time the joint resistance rises and the temperature rises with it.

Pollution degree, creepage and clearance

Insulation co-ordination uses a pollution degree to describe the environment. A clean, dry room is a low degree. An industrial site with dust and humidity is higher. A coastal or chemical site is higher again. The pollution degree sets the creepage distance for a given voltage, and the creepage is usually the dimension that decides the bar layout.

Clearance is the shortest path through air. Creepage is the path along the surface. In dry air the clearance may be the limit, but in a wet, polluted environment the surface path is the weak link, so the creepage governs. A bar that looks generous in air can be short on creepage at an edge or around a hole.

The shape of the surface matters. A rib or a groove lengthens the creepage path, but a groove that fills with dust can become a conductive bridge. A coating can increase the effective creepage if it is continuous and adheres well, but a coating with a void or a lifted edge can concentrate the field and make the problem worse.

Plating choices in humid air

Tin is the default for copper in humid air. It protects the surface, it is soft enough to conform at a joint and it is tolerant of small handling marks. A thin tin deposit can be displaced under a bolted joint, so the joint design should keep the contact pressure within the coating's ability.

Silver gives a lower contact resistance and is used for high current joints, but it tarnishes in a sulphur bearing atmosphere. The tarnish is a film, and a joint that is designed to break through it can still work. A joint that relies on a clean silver surface without a wiping or a high pressure contact may see a rise in resistance.

Nickel is harder and more resistant to wear and some chemicals, but it has a higher contact resistance than silver and it can be brittle. It is often used where the bar sees abrasion or a specific chemical rather than where the lowest resistance is needed. The choice should be made for the site, not for the catalogue.

• Plate the whole bar, including cut edges and hole walls.
• Match the plating thickness to the joint pressure and the environment.
• Avoid a thin deposit on a joint that will be re-torqued.
• Check the adhesion after bending, because a bend can crack the deposit.
• Seal the joint faces where moisture can be trapped.

Coatings, sleeving and sealed joints

A coating protects the bar body from corrosion and adds a layer between the conductor and the polluted air. Epoxy and powder coatings are common because they adhere well and can be applied in a controlled thickness. The coating has to be masked at the joint faces, the studs and any area that must make electrical contact.

The edge of the coating is the weak point. A coating that ends with a sharp step can lift, and moisture can creep under the edge. A rounded edge, a sealed edge or a overlap onto an insulating sleeve reduces the risk. The cut end of a bar should be coated or sealed, because a bare cut edge is where corrosion starts.

Heat shrink sleeving is used where the bar needs a thicker insulation or a flexible cover. It adds thickness and can hide a defect, so the bar should be inspected before the sleeve is fitted. The sleeve should be sealed at the ends and should not trap air that can condense.

Joints: the hardest place to protect

A joint cannot be fully coated, because the faces have to make electrical contact. That makes it the most exposed part of the bar. The plating protects the faces, but the gap between the bars and the edge of the joint can hold moisture. A joint that is tightened and then exposed to a wet atmosphere can draw water into the gap by capillary action.

The usual controls are a plated face, a sealed joint and a design that does not trap water. A sealant around the joint edge can help, but it has to be compatible with the plating and the temperature. A joint that is sealed before it is torqued can end up with a sealant film in the contact area, which raises the resistance.

The joint should be checked at the first build and at intervals in service. A millivolt drop reading gives a number that can be compared over time. A thermal image under load shows a joint that is running hotter than its neighbours. Both are more useful than a visual check alone.

Inspection and maintenance in a harsh site

The first article should check the plating thickness, the coating thickness, the adhesion and the coverage at the edges. A cross-section shows the deposit and the coating at a cut edge. A salt spray or a humidity test on a sample gives an indication of the corrosion resistance, but the result depends on the test conditions and should not be read as a service life.

In service, the checks should focus on the joints and the coating edges. A thermal image under load, a torque check and a millivolt drop reading are the practical tools. A coating that has lifted or cracked should be repaired before the corrosion spreads, and a joint that has risen in resistance should be re-made rather than re-torqued blindly.

The maintenance interval should match the site. A coastal or chemical plant may need a shorter interval than a clean industrial hall. The records should include the date, the load and the readings, so a trend can be seen before a failure.

What to send for a quote

Send the site description, the pollution degree, the humidity and the contaminants. Add the voltage, the current, the fault level and the ambient. State the enclosure rating, the cleaning method and the expected service life. If the site has a known problem, send a photograph of the corrosion or the tracking.

If the bar is a replacement, send the original drawing and the failed part. The location of the corrosion shows whether the problem is the plating, the coating edge or the joint, and that decides the fix.

• Site description and pollution degree
• Humidity, condensation and contaminants
• Voltage, current and fault level
• Ambient and enclosure rating
• Cleaning method and service life
• Photograph of any existing corrosion
• Original drawing for a replacement