Flexible connectors and copper braids: what they do

What a copper braid is

A copper braid is a conductor made from many fine wires woven or plaited into a tube or a flat strip. The strands are not insulated from each other, so the whole bundle conducts. The braid is flexible because each strand can move relative to its neighbours, and the weave distributes a bend over many small wires.

A flexible connector is the assembly: the braid plus the terminations at each end. The termination may be a crimped ferrule, a welded or sintered pad, a bolted lug or a pressed sleeve. The assembly is what carries the current and the movement, and the termination is where most failures start.

A braid is not the same as a laminated flexible busbar. A laminated bar stacks thin copper foils and bonds them at the ends. A braid is woven from wire. Both are flexible, but the construction, the current path and the inspection are different.

Where a braid earns its place

The first use is movement. A busbar expands when it heats and contracts when it cools. A braid between two fixed bars can take that movement without loading the joints. The same applies to a door, a drawer or a moving head that has to stay connected.

The second use is vibration. A braid isolates a vibrating component from a rigid bar, and it spreads the flex over many strands. A solid cable would concentrate the bending at the lug and fatigue the conductor at the crimp.

The third use is a current path where a rigid bar cannot go. A braid can be routed around a corner, folded into a tight space or used as an earth bond across a hinge. It is also used for high-frequency paths, where the many strands give a large surface area.

• Expansion joints between busbars and equipment
• Moving parts such as doors, drawers and plug-in units
• Vibration isolation between a device and a frame
• Earth bonding across a hinge or a removable panel
• A flexible current path in a dense enclosure

Construction choices that change behaviour

The strand diameter sets the fatigue behaviour. Fine strands bend more easily and last longer in flex, but they are more expensive to draw and to weave. Coarse strands carry more current per strand and resist abrasion better, but they fatigue sooner in a tight bend.

The total copper area sets the current capacity, but the weave matters too. A loose weave is more flexible and has more space between strands, which reduces the copper density for a given outside diameter. A tight weave packs more copper into the same space and carries more current, but it is stiffer.

The braid lay, the number of strands and the number of carriers decide the shape and the coverage. A flat braid is used where the connection has to sit against a surface. A tubular braid is used where the movement is in several directions. The end termination has to suit the braid construction, because a ferrule that crushes a loose weave may not grip every strand.

Terminations: the part that decides the joint

A crimped ferrule compresses the braid into a solid end. The ferrule must be long enough to capture the full braid and strong enough to hold the strands without cutting them. The crimp height and the die belong to the ferrule and the braid, not to the cable size alone.

A welded or sintered termination fuses the strands into a solid pad. It gives a low-resistance joint and a compact end, but the heat of the process can anneal the copper near the weld. The transition between the fused pad and the flexible braid is a stress point and should be supported.

A bolted lug is common where the braid has to be removable. The lug must be crimped or welded to the braid before it is bolted to the bar. The bolt joint then needs its own torque, contact surface and plating check. Two joints in series means two places to inspect.

Current capacity and derating

The current capacity of a braid is usually based on its total copper area, but the weave reduces the effective density and the heat transfer is different from a solid bar. A braid with the same copper area as a solid conductor will often run hotter, because the strands are not in intimate contact and the surface area is larger but the internal paths are longer.

Use the braid maker's current rating for the specific construction, and derate for the ambient temperature and the enclosure. If the braid is inside a cabinet with other heat sources, the local ambient may be well above the room temperature. A rating quoted for a free-air test does not apply inside a closed box.

At high frequency, the current crowds to the surface of each strand. A braid with many fine strands has more surface area than a solid conductor of the same area, which can reduce the high-frequency resistance. For power frequency and DC, the total copper area is the main factor.

Movement, bend radius and fatigue

A braid is flexible, not indestructible. It has a minimum bend radius, and it has a finite flex life. The two are related: a tighter bend concentrates more strain in the outer strands and shortens the life. The braid maker should state the radius for a given movement.

The shape of the loop matters. A braid that is pulled straight between two points has no slack for movement, so the strands take the load directly. A loop with slack allows the braid to flex without tension. A loop that is too large can rub against other parts or vibrate against the enclosure.

Distinguish flex from torsion. A braid that is twisted back and forth concentrates the movement at the termination, where the strands are clamped and cannot move. If the application has rotation, use a braid designed for torsion or add a mechanical joint that allows rotation without twisting the conductor.

What to inspect

Inspect the braid for broken strands, loose weave, abrasion and tarnish. A few broken strands at the surface may be cosmetic, but a cluster of broken strands at a bend or near a termination is a fatigue signal. Count and record the broken strands against the acceptance rule.

Inspect the termination for cracks, undercut, discolouration and incomplete fusion. A crimped ferrule should show a full crimp with no sharp edge cutting the strands. A welded pad should be smooth at the transition, with no void or crack. Check the plating on the pad and the lug hole.

Measure the resistance across the assembly with a four-wire method. A braid with a poor termination can have a resistance that is higher than the copper area suggests. Compare the reading with a sample from the same construction, and record it with the length and the termination type.

Specifying a braid assembly

State the current, the ambient, the conductor material and the plating. State the movement: direction, amplitude, bend radius and the number of cycles. State the length, the hole centres, the lug or ferrule type and the termination method. Add the inspection and resistance requirements.

A drawing that shows only the outline and the hole centres leaves the braid construction open. Two suppliers can make the same outline with different strand counts and different flex life. If the movement matters, put the construction and the cycle requirement on the drawing.

For a critical assembly, ask for a sample and a flex test. The test should use the actual termination and the actual bend radius. A braid that passes a static pull test can still fail after a few thousand cycles if the termination grips the strands too tightly or the loop is too tight.