Rigid vs flexible busbars: selection guide

The difference that matters

A rigid busbar is a solid conductor, usually a flat bar, that holds its shape between supports. A flexible busbar is a stack of thin copper strips that can move relative to each other, so the conductor as a whole can bend, extend and absorb movement between two fixed points.

The distinction is mechanical, not electrical. Both carry current through copper. Both can be plated, insulated and bolted. The choice turns on whether the connection has to accommodate relative movement, and on how much fault force the installation has to survive.

Where movement comes from

Copper expands when it heats. The coefficient is about 16.5 x 10^-6 per kelvin, so a one metre copper bar at a 50 K rise grows by about 0.8 mm. That figure is illustrative, but the order of magnitude is real. On a long rigid run anchored at both ends, the expansion becomes a compressive stress in the bar and a load on the supports and terminals.

Movement also comes from assembly tolerance. Two enclosures built separately will not have their terminal pads in exactly the same place. A rigid bar can be made to fit one build and fight the next. A flexible bar absorbs the difference without preloading the joints.

Vibration, seismic movement and equipment that shifts on its mounts add another source. So does any connection between a fixed cabinet and a component on a door, a drawer or a moving carriage. If the two ends can move relative to each other by more than a millimetre or two, a rigid bar needs an expansion detail or a flexible link.

• Thermal expansion on long runs or high-temperature bars
• Tolerance stack-up between separately built assemblies
• Vibration from transformers, generators or vehicles
• Seismic or transport loads
• Doors, drawers and moving carriages
• Installation where the bar must be fitted after the equipment is positioned

When a rigid bar is the better choice

A rigid bar is the default for panel distribution because it is simple, compact and easy to support. It can be bent to a precise shape, drilled for several devices and braced against fault forces. It also gives the cleanest current path: a solid conductor has no interstitial gaps and no strip-to-strip contact resistance.

The limits appear when the run is long, the temperature swing is large or the two ends are not fixed. A long rigid run needs an expansion joint or a sliding support, and both add cost and a maintenance item. A rigid bar also transfers fault force directly to its supports, so the support spacing and the strength of the fixing have to be designed rather than assumed. On a short link between two devices on the same frame, none of these limits is likely to bind.

How a flexible bar is built

A flexible busbar is usually made from a stack of thin copper strips, each between roughly 0.1 mm and 0.5 mm thick. The ends are consolidated by welding, crimping or bolting so that the stack behaves as one conductor at the terminal. The middle stays free, which is what allows the movement.

The strip count sets the current capacity and the stiffness. More strips in the same stack height carry more current but make the bar harder to bend. Fewer, thinner strips bend more easily but need more space for the same current. The stack is often covered with a sleeve or a wrap, which protects the strips and limits how far the bar can move.

The weak points are the ends. The transition from a consolidated terminal to free strips concentrates stress, and a bend that starts too close to the weld will fatigue there. Good designs keep the first bend a defined distance from the terminal and use a radius that the strip thickness can survive.

Current path and thermal behaviour

At power frequency, a flexible stack and a solid bar of the same copper area have similar resistance, provided the strips are clean and the ends are properly consolidated. The stack has more surface area, which can help heat dissipation, but it also has more interfaces and more chance of a poor end connection.

At higher frequencies, the stack can be better. Skin effect pushes current to the surface of a conductor, and thin strips give more surface for the same copper area. That is one reason laminated and flexible links appear in inverter and drive connections where the current has a high-frequency component.

The derating is not automatic. A flexible bar with a sleeve has a different thermal path from a bare stack, and a stack clamped between two hot terminals will run hotter than the same stack in free air. The current rating has to match the mounting, the sleeve and the ambient.

Installation details that decide life

A flexible bar should be installed with a deliberate loop, not a straight pull. The loop gives the bar somewhere to move and keeps the strips in bending rather than tension. The loop should be in the plane of the strips, because twisting a stack separates the strips and concentrates stress on the outer edges.

The bend radius should be at least the value recommended for the strip thickness and the number of strips. A tight radius at the terminal is the most common cause of early failure. Keep the first bend clear of the welded end and leave enough free length for the expected movement.

Support the bar so that it does not hang on the terminals. The terminal should carry current, not the weight of the loop. On a long loop, add a support or a guide that controls the movement without clamping it.

1. 01

   Define the movement

   State the expected relative movement in each axis: thermal, tolerance, vibration and installation.

2. 02

   Set the loop

   Choose a loop length and shape that keeps the strips in bending and clear of sharp edges.

3. 03

   Check the radius

   Use a radius that suits the strip thickness and keeps the first bend away from the terminal.

4. 04

   Plan the support

   Support the loop so the terminals do not carry its weight or its fault load.

5. 05

   Protect the stack

   Use a sleeve or wrap where the bar passes near metal edges or moving parts.

A selection rule set

Use a rigid bar when both ends are fixed to the same structure, the run is short, the fault force is high and the layout will be repeated. Use a flexible bar when the ends can move relative to each other, when the assembly tolerance is wide, or when the connection must be fitted after the equipment is in place.

A useful check is to ask what happens when the bar is bolted at both ends and the equipment heats up. If the answer is that something has to move, the design needs a flexible link or a sliding joint. If nothing can move, the expansion becomes stress, and the terminals or the supports will find the weakest point.

For a long rigid run, do not simply add more supports. Add an expansion joint or a sliding support, because the expansion has to go somewhere. For a flexible link, do not simply add more strips. Check the loop, the radius and the terminal consolidation, because those decide the fatigue life.