Key takeaways
- The termination converts a flexible conductor into a solid joint, and it is where most braid failures begin.
- Welding gives the lowest resistance and the most compact end, but it anneals the copper next to the joint.
- A crimp is fast and repeatable if the ferrule, die and crimp height are treated as one system.
- A bolted end adds a second joint. It is the right choice only when the connection has to be removable.
The termination is the joint
A braid carries current through hundreds of strands that slide over one another. At each end that freedom has to stop. The strands are gathered, compressed or fused into a solid block so the current can pass into a pad, a lug or a bar. The transition from free weave to solid block is the most heavily stressed part of the assembly.
Three routes are common: welding or sintering, crimping a ferrule, and bolting a terminated end to a bar. Each produces a different joint resistance, a different strength and a different inspection method. The choice is not a matter of preference. It follows from whether the joint must be permanent, how much current it carries and what the installer can reach.
In every case the joint resistance is dominated by the termination, not by the braid. A length of braid has a predictable resistance per metre. A pad that is half fused or a ferrule that missed a group of strands can add more resistance than several metres of conductor.
Welded and sintered terminations
Welding fuses the strands into a solid pad. Resistance welding, ultrasonic welding and sintering are all used. The common aim is to break the oxide on the copper, bring clean metal into contact and hold it long enough for a metallic bond across the full width.
A good weld gives a joint resistance close to the parent copper and a compact end that fits a tight enclosure. It also removes the need for a ferrule, which saves space and avoids a loose barrel that can trap strands. The pad can be drilled or punched after welding to suit the bolt pattern.
The cost is heat. Copper anneals at a relatively low temperature, so the metal beside the weld softens. That softened zone is where a fatigue crack will start if the link flexes close to the joint. The weld also needs a qualified procedure: current, pressure, time and electrode shape all affect the result.
Support the heat affected zone
Keep the first bend away from the weld and add a support or a sleeve over the transition. A weld that is strong in tension can still fail in flex if the softened copper is allowed to bend at the pad edge.
Crimped ferrules
A crimped termination compresses the braid inside a ferrule or a barrel. The die forms the barrel into a controlled shape, and the deformation cold works the copper into intimate contact. The result is a mechanical joint with a large contact area and no heat affected zone.
The crimp is repeatable only if the ferrule, the die and the crimp height are treated as one system. A ferrule that is too short will not capture the full braid, a die that is too narrow will cut strands, and a crimp height that is too tight will thin the barrel and crack it.
Braid is harder to crimp than a round cable because the strands are not compacted into a circular bundle. The ferrule has to gather the full width of a flat braid, and the crimp has to close the barrel without leaving a passage where strands can escape. A bellmouth at the braid entry helps, and a visual check for escaped strands belongs in the work instruction.
Bolted and clamped ends
A bolted end is used where the connection has to be opened for maintenance or where the braid has to be fitted on site. The braid is terminated into a lug or a pad first, by crimp or weld, and that pad is then bolted to the bar. The bolted interface is a second joint in series with the first.
The bolt joint depends on contact pressure and clean surfaces. Flatness, plating, washer type and torque all matter, and the torque has to be held after thermal cycling. A joint that is tight at assembly can relax as the copper and the fastener expand at different rates.
The advantage is serviceability. The disadvantage is that the installer now controls two variables, the crimp or weld and the bolt, and only one of them is usually inspected. If the link is removable, the drawing should state the torque, the contact preparation and the millivolt drop check that proves the joint after assembly.
Heat, plating and material effects
Welding and sintering change the metallurgy next to the joint. Crimping changes it through cold work. Bolting leaves the metal unchanged but depends on the surface. Those differences matter when the braid carries high current, because the joint runs hotter than the conductor.
Plating is chosen for the joint, not for the braid. Tin plating keeps a bolted contact stable but adds resistance and can creep under pressure. Silver plating gives a low resistance contact and resists oxide growth, at a cost. Nickel plating survives higher temperatures but raises the joint resistance.
Dissimilar metals are a separate risk. A copper braid bolted to an aluminium bar will corrode at the interface in the presence of moisture, and the corrosion products increase resistance. A bimetallic transition, a plated interface or a sealed joint is needed. The braid itself is rarely the problem; the joint between two metals is.
Inspection for each method
A welded end is inspected by visual check for full fusion across the width, by a peel or bend test on a coupon, and by a millivolt drop measurement across the joint. A cross-section on a sample shows voids and unfused strands that a visual check cannot see.
A crimped end is inspected by crimp height, by a pull test to a stated load and by a visual check for cracks, flash and escaped strands. The pull test should fail in the braid, not at the crimp, if the joint is stronger than the conductor. A joint that pulls out cleanly has not gripped.
A bolted end is inspected by torque and by millivolt drop after assembly and after thermal cycling. A witness mark on the fastener shows whether it has moved. For a critical joint, a thermographic check under load can reveal a hot spot that a resistance reading at low current misses.
| Method | Joint resistance | Strength | Main inspection | Best use |
|---|---|---|---|---|
| Welded or sintered | Low, close to parent copper | High, but heat affected | Visual, peel test, millivolt drop | Compact permanent joints |
| Crimped ferrule | Low to medium, set by crimp | High if die and height are right | Crimp height, pull test | Volume production, no heat |
| Bolted lug | Medium, set by contact | Set by bolt and pad | Torque, millivolt drop | Removable or site-fitted links |
| Crimp then bolt | Two joints in series | Set by the weaker joint | Both of the above | Serviceable high-current links |
The best method depends on whether the joint is permanent and who assembles it. A welded joint specified for a site-fitted link moves the risk to the installer.
Choosing a method
Start with serviceability. If the joint will never be opened, weld it and put the engineering effort into the procedure and the heat affected zone. If it will be opened, crimp or weld into a lug and design the bolted interface as a separate, inspected joint.
Then look at current and space. A welded pad gives the lowest resistance in the smallest envelope. A crimped ferrule needs a barrel length and a die access space. A bolted joint needs wrench access and a bolt circle, which often decides the layout more than the electrical design does.
Finally, match the inspection to the risk. A low-current earth bond can be checked visually and with a continuity meter. A high-current link in a switchgear assembly needs a millivolt drop record for every joint, because a single poor termination can become the hottest point in the cabinet.
Frequently asked questions
Is a welded braid termination stronger than a crimp?
Not necessarily. A sound weld is strong, but the heat softens the copper beside it, so the assembly can fail in the heat affected zone. A correct crimp can be stronger than the braid itself. Both need a qualified process and a pull test.
Can a braid be crimped without a ferrule?
It should not be. Braid strands are not compacted like a cable bundle, and a crimp directly onto the weave will miss strands and leave voids. A ferrule gathers the full width and gives the die a controlled barrel to form.
Why does a bolted braid joint loosen?
Contact pressure relaxes as copper and steel expand at different rates and as the joint thermal cycles. Plating creep and vibration add to it. A torque check after cycling and a millivolt drop record show whether the joint is still sound.
How is a braid termination resistance checked?
With a four-wire millivolt drop measurement across the joint at a stated current. The result is compared with a limit for that joint or with the resistance of an equal length of the same braid. A simple two-wire ohmmeter reading is not sensitive enough.
Sources and further reading
- IEC 61238-1, Compression and mechanical connectors for power cables
- IEC 60352-2, Solderless connections, Part 2: Crimped connections
- Qumek termination process and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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