Key takeaways
- A flat braid links a fixed bar to a moving or thermally expanding part, and its pad geometry is set by the bolt pattern, not by the braid.
- Stack height is a first-order constraint: the braid, the washers, the bolt head and the insulation all share the same space.
- The current rating has to suit the ambient inside the enclosure, not the test bench.
- Torque, contact preparation and loop support belong on the drawing and in the assembly instruction.
Where a flat braid fits in a cabinet
A flat braided connector sits between two points that have to stay electrically joined but cannot be rigidly tied. In a switchgear assembly it might link a main busbar to a draw-out unit, a fixed bar to a door-mounted device, or two sections of a bar that expand at different rates.
The flat form matters. A flat braid lies against a pad, so the joint can be bolted with a conventional fastener and a short stack. It can be folded into a shallow loop, routed behind a barrier or laid along a bar. A tubular braid in the same space would need a lug at each end and would sit higher.
The connector is usually supplied as an assembly: braid, two pads, plating and sometimes a sleeve. The pads may be welded or crimped onto the braid, or the braid may be consolidated into a solid block and drilled. Each route gives a different pad flatness, hole quality and stack height.
Pad geometry
The pad is designed around the bolt, not the braid. Start with the bolt diameter, the washer type and the torque, then set the hole size, the pad width and the edge distance. A pad that is too narrow for the washer will not spread the clamp load, and a hole that is too close to the edge will tear under a fault force.
Hole spacing is set by the bar it bolts to. Measure the existing pattern rather than scaling from a catalogue. A two-hole pad resists rotation better than a one-hole pad, which matters on a link that is handled during maintenance or exposed to vibration.
Pad thickness has to suit the current and the clamp load. A thin pad can dish under a bolt and lose contact area. A very thick pad raises the stack and may not sit flat against the bar. The pad face should be flat enough that the washer seats without rocking.
Measure the mating bar, not the drawing
Hole patterns on old switchgear rarely match the current catalogue. A pad designed from a datasheet can arrive with the right current and the wrong hole centres.
Stack height and space
Stack height is the sum of the braid thickness, the pad thickness, the washers and the fastener head. In a dense cabinet that stack competes with phase barriers, insulation sleeving and the clearance to the next phase. A connector that is correct electrically can still fail to fit.
The braid itself is thin, but a consolidated block is thicker than the free weave. If the block is welded, the pad may be 2 mm to 4 mm thick. If the braid is folded back on itself at the pad, the local thickness doubles. Both details have to be on the drawing, because they decide whether the bolt is long enough.
Leave room for the loop. A flat braid needs a free length to bend, and the loop sits in the plane of the braid. A loop pressed against a barrier will rub, and a loop pulled straight will load the pads. The loop space is part of the connector envelope, not an installation detail.
Current and temperature inside the enclosure
A braid rated in free air will run hotter inside a closed cabinet. The local ambient may be well above the room, and the braid may be surrounded by other heat sources. The rating has to be based on the temperature inside the enclosure, not on a bench test.
The assembly standard for low-voltage switchgear sets out how temperature rise is verified, by test or by calculation. A braided link is part of that verification. If the link is added after the type test, its effect on the local temperature and on the clearance has to be assessed.
Current sharing between the braid and a parallel bar is another variable. If the braid is bolted across a joint that also carries current through the bar, the split depends on the resistance of each path. A braid that is added as a flexible shunt may carry less current than its area suggests, or more, depending on the geometry.
Installation and torque
The contact faces have to be clean, flat and free of paint, oxide and burrs. A plated pad against a plated bar is the usual arrangement, but the plating on the bar is often tin over copper, and the pad may be silver or tin. The two have to be compatible and the joint has to be dry.
Torque is set by the fastener, the thread and the washer, not by the braid. Use a calibrated torque wrench and a stated sequence for a multi-bolt pad. A joint that is tightened by feel will be loose at one bolt and over-tight at another, and the loose side will run hot.
Do not pull the braid into place. A flat braid is often used to bridge a tolerance gap, and the installer may be tempted to stretch it to reach the hole. That preload is a permanent stress on the strands and the pads. The connector should reach both pads without tension, with the loop taking up the difference.
Insulation and creepage
A bare braid in a cabinet is a conductor at phase potential. It needs clearance to the next phase and to earth, and it may need a sleeve or a barrier. The sleeve has to suit the temperature and the movement, and it must not trap heat against the braid.
Creepage and clearance are set by the insulation coordination rules for the equipment, not by the braid. A sleeve adds a layer but also adds a surface path. If the sleeve stops short of the pad, the exposed braid at the transition becomes the shortest creepage path, and that is exactly where the design should be checked.
Heat shrink with an adhesive lining seals the transition and keeps moisture out. A loose sleeve can slide and expose the braid. A sleeve that is too tight can stiffen the loop and move the strain to the pad. The sleeve callout should state the material, the temperature rating, the coverage and whether it is sealed.
The data to send with an RFQ
Send the assembly drawing with the pad pattern, the hole size, the braid construction, the overall length and the loop envelope. Add the current, the enclosure ambient, the fault level and the movement duty. State the plating and the sleeve if there is one.
Add the installation data: bolt size, torque, washer type and the surface preparation. If the connector will be fitted on site, say so. A connector that is correct on the bench can be ruined by an installer who does not know the loop rule.
Finally, state the inspection. A flat braid for switchgear usually needs a dimensional check, a millivolt drop record across each pad and a visual check of the weave and the sleeve. If the link is part of a type-tested assembly, say which test it must support.
| Item | What to state | Why it matters |
|---|---|---|
| Pad pattern | Hole size, centres and edge distance | Fits the mating bar |
| Braid | Carriers, ends, wire diameter and width | Current and flexibility |
| Length | Pad to pad, plus loop envelope | Reaches without preload |
| Stack | Braid, pad, washer and bolt height | Fits the available space |
| Current | Rated current and enclosure ambient | Temperature rise |
| Plating | Pad and braid finish, thickness | Contact resistance and corrosion |
| Insulation | Sleeve material, coverage and seal | Creepage and protection |
| Inspection | Dimensions, millivolt drop, visual | Evidence for the shipment |
A connector drawing without the loop envelope and the stack height leaves the two most common fit problems to the installer.
Frequently asked questions
What is the difference between a flat braid and a laminated flexible bar?
A flat braid is woven from fine wire and flexes by letting strands slide. A laminated bar is a stack of copper foils consolidated at the ends. The braid tolerates more movement and torsion; the laminate packs more copper into a smaller space.
Can a flat braid be used as the main current path?
Yes, if it is rated for the current at the enclosure ambient and the terminations are sized for it. Many switchgear links are flexible shunts in parallel with a bar, and the current split depends on the resistance of each path.
How tight should the loop be?
Long enough to stay slack through the full movement, and short enough not to rub or sag into another part. The loop belongs in the plane of the braid, with the first bend kept away from the pad.
Does the braid need a sleeve?
It needs one where the clearance or creepage path requires it, where the braid could touch another conductor, or where the environment is damp or dirty. The sleeve has to suit the temperature and must not stiffen the loop.
Sources and further reading
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies, Part 1: General rules
- IEC 60664-1, Insulation coordination for equipment within low-voltage supply systems
- Qumek switchgear connector drawing review notes
Qumek Engineering Desk
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