Busbars are simple in principle, complicated in practice: part 2

These substantial conductors serve a vital role and must meet many conflicting constraints. This part explores additional bus-bar considerations. Part 1 can be found here.

Laminated bus bars

While some busbars are single conductors and carry only one power rail, there’s an increasing trend towards using laminated busbars, seen in Figure 1. These multilayer units carry two, three, or more independent power rails in parallel.

Although they seem like simple mechanical extensions of the single-conductor busbar, their manufacturing process involves multiple, complicated steps including adding interlayer insulation materials such as epoxy-glass or silicone rubber, or using fused powder (powder coated), positioning conductors precisely within insulation layers, and bonding these layers using heat, pressure, or adhesive.

Constructing a high-voltage/current multilayer laminated bus bar requires that the layers be fully insulated yet function as a single mechanical component. This requires attention to through-holes, connecting and interlayer bolting, and more. Special tests are also required to ensure the integrity of the bus bar and electrical safety. Even the edge of the bar is critical: should it be chamfered or rounded, and with what radius?

Bus bar coatings

While it is possible to install a bare bus bar using only air spacing for insulation, they generally require insulation to protect users and other components from the voltage and current. This protection works both ways, as the outer coating also protects the bus bar itself from potential corrosion, normal abrasion, and even abuse, while also improving its appearance. Among the options for single bus board or a multilayer laminated bus bar are heat-shrink tubing, insulators such as Mylar (PET), and epoxy powder coatings.

Epoxy powder coatings (also called dielectric epoxy powder coating) are often the preferred option for insulating and protecting the bus bar. Although there are different formulations, these coatings are based on polymer resins combined with curatives, pigments, leveling agents, flow modifiers, and other additives

Powder coating is not just a “paint” with a fancier name – it is an enhanced conformal coating technique providing chemical, mechanical, and electrical benefits compared to paint, even though it is applied similarly to how paint is, by spraying or dipping the bus bar in a vat. The powdered-coated bus bar is placed in an oven to be cured, and once it has passed its transition temperature, the powder chemically reacts by fusing to produce long molecular chains.

These chains, which also fuse to the bus bar copper or aluminum substrate, have high cross-link density and molecular weight, which makes them very resistant to material breakdown. When applied properly, powder coating offers multiple advantages, including smooth and continuous, crack-free coverage, high dielectric strength, and long-lasting insulating properties. It is also resistant to heat, flame, impact, abrasion, moisture, corrosion, various chemicals, ultraviolet light, and weather extremes. In short, it provides and maintains a coating integrity as it forms a “skin-tight” covering around the bus bar surface.

Jointing bus bars

There are many situations where it is necessary to join two busbars to create a single, unified unit. This process, called “jointing,” may be needed to create a longer busbar from shorter, more manageable pieces or to create a T-shaped tap-off connection from the main busbar.

The electrical objective of any jointing is to minimize contact resistance and maintain that low value. Two factors affect the joint contact resistance: the “spreading resistance,” also known as the “streamline effect,” due to the distortion of the current flow path as the current is diverted through the joint; and the contact resistance itself. These must be factored into the design, and the analysis is more complicated for laminated bus bars.

The result of jointing must simultaneously meet multiple objectives. It must be mechanically strong, be resistant to environmental stress, maintain performance over time and load cycles, and, of course, ensure good electrical performance in the form of low resistance. As with all technical processes, there are options for meeting the objective and subtleties in doing so. For busbars, the common options are bolted, clamped, riveted, soldered, brazed, or welded, as shown in Figure 2.

Figure 2. There are many ways to joint bus bars; (top to bottom) bolted, clamped, riveted, soldered or brazed, and welded are the most common. (Image: Copper Development Association)

Other design considerations include surface preparation, oxidation buildup, creep (cold flow), torque, and washers for bolted joints, as well as the effects of thermal stress and cycling on the joint and any coatings. Even tiny surface-to-surface motion due to thermal expansion and contraction can cause corrosion and fretting, resulting in increased joint resistance.

There are also installation issues. For example, it’s often difficult to properly weld bus bars in the cramped settings of many installations, so some sort of bolted connection is more practical.

The final part of this article examines smaller and even PC board-sized bus bars, which also address voltage-drop issues and provide additional benefits.