Flexible copper braids are electrical conductors composed of multiple thin copper wires (usually pure electrolytic copper) stranded or woven together, forming a highly flexible flat ribbon or cable.
This stranded design gives them excellent electrical conductivity, high flexibility and durability, essential qualities for absorbing vibration and movement without compromising the connection. In many cases, tinned (tin-plated) copper is used to improve corrosion resistance and longevity, especially in harsh environments.
On the other hand, copper sheets (or flexible strips) are flat connections consisting of several thin layers of copper joined (soldered) at their ends, also known as shunts or laminated connectors. These stacked sheets provide a flexible conductor of rectangular cross-section, maintaining a large contact surface and low resistance. The foils are typically very thin (between 0.05 mm and 0.3 mm each) and can be joined by pressure (diffusion) soldering or brazing, creating a robust, flexible connector customised to technical specifications.
Definition: In brief, flexible copper braids are meshes or tapes of stranded copper wires used as flexible electrical connection elements, while copper foil connections are flat assemblies of copper strips joined at their ends to achieve similar effects of flexibility and conductivity.
General Industrial Applications
In industrial environments, flexible copper braids and copper foil connections are used to ensure reliable electrical connections where systems are subject to vibration, movement or thermal expansion. Some common industrial uses include:
- Vibrating Industrial Machinery: Machine tools, large electric motors, generators and turbines. In such equipment, vibrations and cyclic movements can loosen rigid connections. Flexible braids absorb such vibrations, preserving the integrity of the connection. For example, they are used to connect bus bars to vibrating components, preventing the vibration from being transmitted throughout the system.
- Electrical Panels and Distribution Boards: Within industrial or control panels, flexible braids and flexible plates are used to interconnect board sections, bridge bus bars (copper bars) or connect metal cabinet doors to earth. Their flexibility facilitates assembly and maintenance, allowing doors to be opened/closed without forcing cables.
- Power Systems and Transformers: Connecting transformers or generators to rigid busbars can be problematic due to heat and vibration. Using flexible copper braids (also called flexbraids or shunts) at the output of a transformer attenuates vibrations and compensates for heat expansion, avoiding mechanical stress on the connections due to thermal expansion. This is crucial in electrical substations and power distribution centres.
- Moving or Misaligned Installations: Slightly moving equipment, electrified crane rails, machinery with moving parts (e.g. robotic arms). Flexible braids allow some freedom of movement (e.g. small oscillations or misalignments between two connection points) without cutting power. They are able to compensate for misalignments and relative movements due to their slack.
- High Current Applications: Because the braids consist of many strands, they can have a high effective cross-section. In fact, the addition of multiple strands increases the current carrying capacity, allowing them to handle high currents with effective heat dissipation. They are therefore used in stationary battery connections, UPS battery banks, power systems in data centres, etc., where large cross-section flexible conductors are required.
Key industry benefit: In all these cases, the main advantage is reliability under dynamic conditions. Unlike a rigid connector, a flexible braid resists vibration, expansion, contraction and repetitive motion without fracturing or loosening. They are also easier to install in confined spaces, as their flexible nature allows for bends and twists that rigid rods cannot tolerate. They also offer low electrical resistance and, being flat or woven, have greater heat dissipation surface area, avoiding hot spots.
Applications in the Railway Sector
In the railway industry, flexible copper braids are crucial components in multiple electrical systems in trains. Rail vehicles (passenger trains, locomotives, trams, metros) experience constant vibration, shock and movement and require secure electrical connections for both traction power and auxiliary and safety systems (such as earthing). Let’s look at some specific applications:
- Pantographs and Catenary: The pantograph (a device on the roof of electric trains that draws current from the catenary) uses flexible copper braids to ensure the transmission of current from the pantograph head (the contact with the wire) to the train’s electrical system. These braids must be very flexible in order to accommodate the continuous up and down of the pantograph and the high-speed beating of the catenary. For example, railway studies list pantograph connection braids of varying cross-sections (from 0.1 mm² to 120 mm²) that can conduct between 5 A and 420 A, depending on size. Their function is to maintain low resistance and continuity even when the pantograph vibrates or moves slightly.
- Bogies and Current Return: Bogies (wheeled undercarriage) usually incorporate earthing braids between the bogie frame and other components. In modern trains (e.g. manufactured by Alstom, CAF, Stadler, Siemens, Hitachi Rail, etc.), it is common to see copper braids connecting the frame to the axles or body to ensure efficient current return and electrical equipotentiality. For example, one technical specification states: “The earthing shall be done with copper wires of appropriate cross-section connecting the body to the bogie”. This means that in each car of the train, the body (carbody) is electrically connected to the bogies by means of flexible copper braids, ensuring that any leakage or return current flows smoothly to the wheels and from there to the rails. Furthermore, in light rail tenders it is explicitly required: ‘Sufficient conductive braids shall be installed between Passenger Lounge, engines and Bogie frame to ensure their earthing’.
- Wheel Current Return: In electric traction systems, the current return (negative pole) to the substation is via the wheels and rails. To guarantee this path, braids are installed from the wheels (e.g. the axle or the conductor ring) to the bogie mass. For example, in the specifications for metro trains: “The wheelset shall ensure the return of the negative current to the rectifier substation with braids mounted from the outside of the wheel and the ground, ensuring an electrical resistance of 0,1 Ohm or less”. These wheel braids provide a reliable low resistance path, even with the vibrations and movement of the suspensions.
- Bogie-mounted equipment connections: Traction motors, gearboxes, electric braking systems, sensors… many bogie-mounted equipment require flexible electrical connection. For example, traction motors are often connected to the power converter via high-current flexible cables; in some cases, these cables include braided flexible sections to absorb the relative movement between the bogie frame (where the motor is attached) and the body frame. Similarly, measuring devices on axles (such as tachometers to detect wheel slippage) are connected to electronic systems via braids or flexible cables that resist vibration and movement.
- General Train Grounding: Every train must have a grounding grid to ensure that the metal parts accessible to the passage are equipotential and grounded (the rails act as ground). Flexible copper braids are used at multiple points to interconnect parts of the structure (doors, panels, roofs, chassis) to the main earth. For example, braids are installed at: the joints between cars (on articulated trains), between the body and roof mounted systems (such as air conditioning or the pantograph itself, where there is an isolated high voltage phase but a structural earth is required), etc. This is critical for electrical safety, protecting against insulation faults and ensuring that, in the event of any shunt, the current goes to earth in a controlled manner.
Generic examples:
- A high-speed train from Alstom, Talgo or Hitachi Rail may have flexible braids linking pantographs to their converters, and linking boxes and bogies.
- A commuter train manufactured by Stadler or CAF will incorporate braids in the couplings between cars to ensure electrical continuity (in addition to the control cables).
- Siemens locomotives or CAF metro units use ground braids in their auxiliary equipment and chassis.
- All these applications share the same reasons: to absorb vibrations, allow relative movement and ensure low impedance connections in environments subject to shocks, temperature changes and dynamic constants.
Applications in Grounding Systems
Copper braids are very popular in grounding systems (also called earth grounding or equipotential bonding systems) due to several advantages:
- Grounding Doors and Panels: In metallic switchboards and cabinets, a flexible tinned copper braid is often used to connect the (metallic) door to the cabinet chassis, ensuring that the door is at ground potential when closed. These braids allow the door to be opened and closed without breaking the connection.
- Bonding of Structural Elements: In an extensive grounding system, such as in a substation or industrial building, tanks, pipes, metal structures, gratings, frames, etc., must be interconnected to the grounding grid. Many of these connections are made with flexible braids or braided straps because they can be easily adjusted to different geometries and do not introduce mechanical stresses. For example, to connect a railing or perimeter fence to earth, a braid with terminals is easily screwed on, adapts even if there is vibration (wind, light seismic activity) and ensures good contact.
- Electromagnetic Interference (EMI) Control: Copper braids are also used as grounding screens or bonding straps to shield equipment and minimise interference. Because of their mesh structure, some flat braids serve as shielding straps or bonding straps that connect the housings of different equipment to ground, ensuring a low-impedance path for high-frequency currents (e.g., in telecommunications booths, or between server racks, connecting them all to the technical floor).
- Lightning Arrester and Surge Protection Systems: In lightning arrester down conductors and surge protection systems, braided tapes are used to carry the lightning current to ground. This is because a braid has lower inductance than a solid wire (due to its larger surface area and interlaced pattern), which is beneficial for the rapid discharge of high frequency transients such as lightning.
- Mobile or Temporary Grounding: In applications where temporary grounding of a structure is required (e.g., when maintaining a high voltage line, grounding braids are connected from the line to grounding structures to avoid unintended discharges), flexible flat braids are preferred because they are easy to handle and offer contact reliability.
Grounding characteristics: These braids are usually tinned (tinned copper) to resist environmental corrosion, and often have integrated terminals at the ends (soldered or crimped) for direct bolting to grounding rods or spikes.
. A commercial example is the ready-to-install, pre-fabricated grounding braids with terminals (sometimes identified with green/yellow bi-colour insulation in industrial applications) offered by electrical suppliers.
. They are ubiquitous in switchboards: any factory distribution panel will have several of these braids connecting earth sections together.
Grounding braids, thanks to their mesh construction, counteract the Kelvin effect at high frequencies (by distributing currents superficially) and maintain low impedance even for high-frequency AC components. This is important in systems where pulsating leakage currents or harmonics may be present.
Advantages of Flexible Braids over Rigid Connections
Why opt for flexible copper braids or flat connections instead of rigid conductors (such as solid copper bars or thick single-wire cables)? Here are the main advantages:
- Flexibility and Vibration Absorption: The most obvious advantage is mechanical flexibility. They can bend, twist and move in any direction, allowing them to absorb vibrations and movements without rapid fatigue. In mechanically vibrating environments (engines, railways, machinery) this prolongs the life of the connection and avoids frequent maintenance.
- Thermal Expansion/Thermal Shrinkage Tolerance: In power systems, conductors are heated by the Joule effect and the environment. A rigid copper bar can expand and contract with temperature, generating stresses in the joints (bolts, flanges). A flexible braid accommodates these changes in length in a natural way. Therefore, in connections between transformers or substation busbars, braids are often used to compensate for this expansion.
- Correction of misalignments: In industrial assemblies, parts do not always fit perfectly aligned. Braids can adjust for small misalignments between terminals without forcing them. Also, if one device moves slightly relative to another (e.g. a motor on its mount), the braid allows this without transmitting forces.
- Ease of Installation in Tight Spaces: A flexible braid can snake through a narrow space where a rigid bar would not fit or would be too difficult to manoeuvre. Likewise, when installing multiple connectors in parallel, flat braids can be easily stacked or arranged without risk of short-circuiting (especially if they come with insulation).
- Less stress on terminals and screws: Thanks to its malleability, when screwing a braid between two surfaces, no high moment of force is introduced on the screw (as could occur with a misaligned rigid bar). This reduces mechanical stress points at splices.
- Better behaviour against high frequency transient currents: The flattened braided geometry of these conductors gives them lower inductance than an equivalent circular cable. This means that they respond better to short-pulse currents (e.g. surges, discharges, harmonics), offering a more direct path at high frequencies. They are therefore excellent for grounding sensitive electronic systems, minimising transient potential differences.
- Customisable Configurations: Manufacturers can supply custom braids and flexible connections: with the exact cross-section (combining number of wires or sheets), precise length, with terminals drilled as required (specific bolt diameters, multiple holes, etc.), with insulation (PVC, silicone, textile braid) or bare, tinned or nickel-plated depending on the environment (tinned for corrosion resistance, silver-plated for superior conductivity in special environments, nickel-plated for high temperature, etc.). This versatility of design makes them suitable for almost any need where a standard rigid conductor would not fit as well.
- Current Distribution and Redundancy: As they are composed of many wires or strands in parallel, if eventually one of the strands is damaged, the current is distributed among the remaining strands. This provides some internal redundancy. Not that they should be allowed to be damaged, but it indicates that there is no single point of failure as critical as a single solid conductor. In addition, the large contact surface of the filaments with the air improves thermal dissipation, allowing high currents to be handled without localised overheating.
In contrast, a rigid (busbar) copper bar is excellent for distributing current in a fixed frame, but it does not tolerate vibration or movement: if the frame vibrates, the bar transmits all the vibration to the bolted joints, which can eventually loosen. A single conductor (solid) cable is difficult to bend and prone to fatigue if flexed repeatedly. Even an insulated multi-wire cable (flexible power conductor type) has more stiffness than an equivalent flat braid and does not dissipate heat as well because its insulation traps heat.
In short, flexible copper braids bring reliability, safety and adaptability to electrical connections where mechanical or thermal conditions are demanding. This makes them virtually irreplaceable in many industrial and railway contexts.
Factores Técnicos Clave en las Trenzas y Conexiones Flexibles
Al seleccionar o diseñar una trenza de cobre flexible o un conector de láminas, se deben considerar varios aspectos técnicos para asegurar que cumpla su función correctamente:
- Sección Transversal (mm²): Determina la capacidad de corriente de la trenza. Debe elegirse según la corriente nominal y las sobrecargas posibles. Por ejemplo, trenzas pequeñas de 6 mm² pueden llevar unos pocos amperios, mientras que trenzas grandes de 120 mm² soportan cientos de amperios. Las tablas de fabricantes suelen dar la corriente admisible para cada sección. En las aplicaciones ferroviarias vistas, una trenza de 120 mm² manejaba hasta ~420 A. Siempre es importante sobredimensionar un poco la sección para evitar calentamiento excesivo.
- Longitud: La longitud física de la trenza debe ser adecuada para cubrir la distancia entre los puntos a unir, dejando holgura suficiente para movimiento. No conviene una trenza tirante; es mejor que tenga cierto juego. Por otro lado, una trenza excesivamente larga añade resistencia eléctrica innecesaria. Hay que equilibrar la necesidad mecánica con la eléctrica. Adicionalmente, las longitudes cortas en trenzas muy flexibles pueden manejar corrientes altísimas, pero una longitud excesiva puede latiguear con vibraciones; a veces se sujeta la trenza a mitad de camino para evitarlo.
- Terminales o Conexiones de Extremo: Pueden venir electrosoldados (los hilos fusionados en una pastilla rígida de cobre) o con terminales prensados (ojales, pletinas perforadas). La calidad de estas terminaciones es fundamental: deben ofrecer baja resistencia de contacto y su unión a la trenza ha de ser robusta para no romperse con flexiones. Muchos fabricantes optan por soldadura por forja o soldadura a tope para unir las láminas, obteniendo una pieza sin material de aporte, lo que mejora la conductividad. Los agujeros de los terminales suelen ser estándar (M5, M6, M8, etc., en métrica; o 1/4”, 1/2” en unidades imperiales), y deben coincidir con los pernos de las conexiones.
- Material y Recubrimiento: El cobre puede usarse desnudo o con recubrimientos. Cobre Estañado es sumamente común: la fina capa de estaño sobre los hilos de cobre previene la oxidación (el cobre tiende a oxidarse verde en exposición prolongada), mejora la soldabilidad, y en climas húmedos o marítimos es casi obligatorio. Otro recubrimiento menos común es el plateado, útil para altísimas frecuencias o temperaturas, pero costoso. Cobre Desnudo puede usarse en interiores secos o dentro de equipos donde no haya riesgo de corrosión.
Además del recubrimiento del metal, considerar si la aplicación requiere material especial: Por ejemplo, para entornos de alta temperatura, se podría usar cobre niquelado o incluso hilos de aleaciones especiales (pero en general, el cobre puro recocido es el estándar porque ofrece la mejor conductividad y flexibilidad). - Aislamiento: Muchas trenzas se suministran sin aislamiento (al desnudo), confiando en que se instalarán separadas de otros conductores. Sin embargo, existen versiones con aislamiento, típicamente una funda o extrusión de PVC o silicona. Ejemplo: la marca “Flexicobre” ofrece pletinas flexibles con aislamiento de PVC extruido libre de halógenos, lo que es útil en tableros de distribución para evitar cortocircuitos si hay contacto accidental con otra fase. El aislamiento añade grosor pero mejora la seguridad. En entornos ferroviarios, se prefiere a veces sin aislamiento para inspeccionar fácilmente la integridad de la trenza (y porque suelen estar en zonas protegidas o de difícil contacto).
- Flexibilidad (Clase de flejado): La flexibilidad viene dada por el número de hilos y el diámetro de cada hilo (en trenzas) o el espesor y número de láminas (en pletinas). Hilos más finos = trenza más flexible. Por ejemplo, una trenza hecha con hilos de 0,05 mm será extremadamente flexible (a veces llamadas trenzas extra-flexibles o super flexible, usadas en electrónica o instrumentos), mientras que una con hilos de 0,3 mm será algo más rígida pero aún flexible para aplicaciones de potencia. La norma Clase 5 o 6 de conductores (según IEC 60228) define flexibilidad de cables; las trenzas suelen superar incluso la clase 6. Para fines prácticos, verificar el radio de curvatura mínimo recomendado por el fabricante.
- Protección Mecánica: En ciertos casos, las trenzas pueden estar expuestas a roces o abrasión. Es posible incluir protectores (mangas corrugadas, por ejemplo) o comprar trenzas con funda. Un factor es la resistencia mecánica a la tracción: aunque no son elementos estructurales, uno debe asegurarse de que la trenza resista las fuerzas a las que será sometida (por vibración, inercia, etc.). En vehículos ferroviarios, durante décadas de servicio, la trenza sufre miles de ciclos de movimiento; los materiales deben ser de alta calidad (cobre recocido para flexibilidad).
- Resistencia Eléctrica y Pérdidas: Las trenzas tienen una resistencia eléctrica ligeramente mayor que una barra de cobre de la misma sección (debido al contacto entre hilos y a la geometría), pero sigue siendo muy baja. Aun así, es importante considerar la caída de tensión si la corriente es muy grande o la trenza muy larga. Los datos técnicos suelen indicar la resistencia en ohmios por metro. Para corrientes AC de alta frecuencia, la distribución en la trenza puede ser afectada por efecto pelicular, pero el trenzado ayuda a mitigar ese efecto.
- Temperatura de Operación: ¿Hasta qué temperatura ambiente y de conductor puede operar la trenza? Si está aislada con PVC, suele ser 70°C o 105°C (PVC especial). Si es silicona o sin halógenos, podría ser 125°C. Las trenzas desnudas en teoría aguantan la temperatura de fusión del cobre (~1085°C) en ausencia de oxígeno, pero en la práctica las limitan por los terminales y por la oxidación. Normalmente se clasifican para entornos de -40°C a +105°C, aunque las aplicaciones ferroviarias/extremo pueden requerir hasta +120°C. En puestos a tierra que pudieran conducir corrientes de falla importantes, deben soportar esos picos térmicos breves sin fundirse.
Con estos factores en mente, un ingeniero o técnico puede especificar correctamente la trenza o conector flexible que necesita para su proyecto. Por ejemplo, “trenza flexible de cobre estañado 50 mm², 300 mm de longitud, terminales M10, aislamiento de silicona” sería una descripción que aborda varios factores: material (estañado), sección, longitud, terminales y aislamiento.
Conclusion
Flexible copper braids, special flat connections and copper foil connections are essential engineering solutions in industrial environments, railways and grounding systems. Their ability to maintain electrical conductivity under conditions of vibration, movement or thermal expansion makes them irreplaceable in applications where rigid connections would fail or require extensive maintenance.
In general industry, they provide reliability in machinery and ease of assembly in switchboards; in the railway sector, they ensure the safety and operability of Renfe, Talgo, CAF, Alstom, Stadler, Siemens, Hitachi and other trains, from pantographs to bogies, ensuring both power supply and grounding of rolling stock; and in grounding systems, they protect people and equipment by ensuring a solid path for fault currents and attenuating interference.
When choosing a flexible copper braid it is important to consider the key technical factors (cross-section, length, surface protection, insulation, etc.) to match the product to the intended application. The advantages over rigid systems – flexibility, vibration resistance, accommodation of movement and expansion, ease of installation and high current capacity – justify their extensive use.
At Maghispan we are manufacturers of copper braid. You can contact us directly to request a quotation.