Busbar thermal expansion and expansion joints

The movement is predictable

Metals expand when they get warmer. Copper expands by about 17 parts per million per kelvin, and aluminium by about 23. Those numbers are small, but a long run multiplies them. A two metre copper bar that rises by 60 kelvin grows by about two millimetres. The same bar in aluminium grows by about 2.8 millimetres.

Two millimetres does not sound like much until it is restrained. If both ends of the bar are bolted to fixed structures, the expansion has nowhere to go. The bar either bends, the joint slips or the bolts and the brackets take the load. The result is a bending stress in the bar, a shear load on the joint and a fatigue cycle every time the equipment heats and cools.

The first design step is to work out the movement at each end of the run. That means the length between fixed points, the temperature at installation and the maximum and minimum service temperatures. The temperature of the bar under load is not the same as the ambient air, so the current and the enclosure have to be considered as well.

What movement does to a fixed joint

A restrained bar does not simply push on the joint. It bends, and the bend puts a moment on the joint face. The contact pressure becomes uneven, one edge of the joint carries more current, and the contact resistance rises. The same movement can also open a gap at the other end of the run.

The joint bolts see shear and bending. A bolted joint that was designed for clamp force may not have been checked for lateral load. The bar can slide under the washer, fretting the plating and loosening the bolt. Over many cycles, the joint loses preload and the temperature rises, which adds more expansion.

The enclosure and the supports see the load too. A bracket that was sized for the weight of the bar may not be stiff enough to resist the expansion force. A long run can move a whole assembly if one part is fixed and the rest is not guided.

Fix one point and guide the rest

The usual arrangement is one fixed point per run and sliding supports elsewhere. The fixed point takes the locating load and sets the datum. The sliding supports carry the weight and keep the bar in line without restraining the movement. The bar then grows away from the fixed point in both directions, or in one direction if the other end is free.

A sliding support needs a low friction surface and a clear travel allowance. A bar resting on a painted steel bracket may not slide as freely as the design assumes, especially after dust or corrosion. A pad, a roller or a slotted bracket can make the movement more predictable. The support should also keep the bar from lifting or twisting.

The fixed point should be strong enough to take the expansion load of the whole run. If the run is long, more than one fixed point may be needed, with an expansion joint between the sections. That turns one long run into two shorter runs that each move a smaller distance.

Expansion joint options

A loop is the simplest expansion joint. The bar is formed into a U or an omega shape, and the loop bends as the run grows. The loop has to sit in the plane of the movement, so a loop that bends in the wrong plane does nothing. The leg length and the radius set the stress, and the loop needs space that the layout has to allow.

An offset leg works in a similar way with less space. Two bends put a step in the run, and the step flexes as the run moves. The offset is easier to fit into a crowded panel, but the movement it can absorb is smaller for the same leg length. The bends are also stress concentrations, so the radius and the burr condition matter.

A flexible section is a different approach. A laminated or braided copper section is bolted or welded between two rigid bars, and the flexible part absorbs the movement. It can take movement in more than one direction and needs less space than a loop. The trade-off is a higher resistance, a joint at each end and a bend life that has to be checked.

A slotted hole with a bolt is the cheapest option and the easiest to get wrong. The slot allows movement only if the bolt can slide. If the bolt is tightened to full clamp force, the friction can be higher than the expansion force and the joint behaves as fixed. A Belleville washer and a controlled torque can keep the clamp force while allowing slip, but the joint then needs a wear check.

Sizing a loop

A loop works by bending. For a given movement, the stress in the loop falls as the leg length rises. Doubling the leg length reduces the bending stress to roughly a quarter, because the bending stress for a given movement falls with the square of the length. That is why a small increase in loop size can make a large difference to the stress.

The loop should be formed with a generous radius and no sharp inside corner. The inside of the bend is where a crack starts, especially if the bar is sheared and the burr is on the tension side. Deburr the bend line, and keep the bend radius above the minimum for the bar thickness and temper.

The loop also has to carry current. A longer loop has more resistance and more surface area, so it runs cooler per unit length but adds voltage drop. The loop is part of the current path, not a mechanical add-on. Check the current density and the temperature rise on the loop as well as on the straight run.

Worked expansion figures

The table gives the free expansion of copper and aluminium runs for a 60 kelvin rise. The figures assume the bar is free to move and the temperature is uniform. They are illustrative, and they exclude the movement of the supports and the effect of a restrained joint.

Installation and inspection

The installation drawing should show the fixed point, the sliding points, the direction of movement and the travel allowance at each end. A slot should be set so the bolt sits in the middle of its travel at the installation temperature, not at one end. A loop should be installed without a preload that uses up part of its movement.

After the first thermal cycle, check the sliding points for movement and the joints for torque loss. A bar that has not moved may be restrained, and a bar that has moved further than expected may have a support that is not guiding it. Mark the initial position so the movement can be seen.

For a long run, a periodic check of the joint temperature is a practical way to catch a rising resistance before it becomes a failure. A joint that runs hotter than its neighbours is carrying more resistance, and expansion may be part of the cause.

• Show the fixed point and the sliding direction on the drawing.
• Set slots at mid travel at the installation temperature.
• Leave the loop free of preload and clear of other parts.
• Check joint torque after the first thermal cycle.
• Mark the bar position so movement can be checked.
• Compare joint temperatures across the run at commissioning.