Bimetallic lugs: aluminium cable to copper busbar

Why this joint is different

An aluminium conductor on a copper busbar is a galvanic couple. Aluminium sits lower in the galvanic series than copper, so in the presence of an electrolyte the aluminium becomes the anode and corrodes. The corrosion product is an oxide that is both voluminous and an insulator, so the joint resistance rises as the metal is consumed.

Aluminium also carries a native oxide layer that forms within seconds of exposure. That layer is hard, stable and electrically insulating. A clamp that relies on pressure alone has to break the oxide mechanically, and if the joint later moves, the oxide reforms in the gaps.

There is a third effect: creep. Aluminium flows under sustained clamp load more than copper, so a bolted aluminium joint can lose preload over months. The loss lets moisture in and the corrosion cycle starts. A bimetallic lug does not remove all three problems, but it moves the difficult transition away from the field.

How a bimetallic lug is made

A bimetallic lug joins an aluminium barrel to a copper palm. The joint is made in a factory under controlled conditions, usually by friction welding, roll bonding, explosive welding or a forged transition. The result is a metallurgical bond with a thin interlayer rather than a mechanical clamp.

The bond quality decides the performance. A poor bond has voids or an oxide film at the interface, which shows up as resistance and heat. A good bond is continuous across the interface and survives bending and thermal cycling without opening. That is why the purchase specification should name the joint method and ask for a bond test, not just a drawing with two materials.

Some designs use a copper barrel with an aluminium sleeve, or a compression joint with a bimetallic washer. These are not equivalent. A sleeve that is crimped over the aluminium does not create a metallurgical bond, and a washer only protects the contact face. Read the construction, not the product name.

Plating is part of the construction. A tin coating on both faces keeps the surface stable, but it does not stop a poor bond from failing. Ask whether the plating is applied before or after the bond, because a coating trapped in the interface is a defect, not a barrier.

The joint options compared

There are four common ways to land an aluminium cable on a copper terminal. They differ in where the bimetallic transition sits and how much of the work moves to the field.

Selecting the lug

Start with the aluminium conductor: size, class and strand count. Aluminium cable is often compacted or stranded in a way that changes the effective diameter, so a barrel sized on nominal area alone can be too loose. Use the cable manufacturer's data and the lug's conductor range.

Then match the palm to the copper terminal: stud size, hole spacing, palm angle and current. The palm is copper, so the copper side follows the normal rules for a copper lug. The barrel is aluminium, so the crimp die and the crimp height must be the ones specified for aluminium, not for copper of the same area.

Check the temperature rating of the joint against the cable insulation and the busbar. A bimetallic joint has its own resistance and will run slightly warmer than a plain copper joint of the same size. If the design is close to a limit, ask for the temperature rise data rather than assuming the lug is transparent.

Also check the marking. A bimetallic lug should identify the conductor range it accepts and the barrel material, because the two ends are not interchangeable in service. A lug that has been installed the wrong way round, with the aluminium barrel on a copper cable, is a joint with the wrong crimp geometry and the wrong transition.

Installing the joint

Strip the aluminium cable to the length stated for the barrel and remove the oxide with a wire brush or the abrasive tool specified by the lug maker. Apply the inhibitor if the design calls for one, and insert the conductor to the stop before crimping. Oxide removal is time sensitive: the layer reforms, so the crimp should follow the preparation without a long gap.

Crimp with the correct die and check the crimp height. On the copper side, clean the palm and the busbar, apply the specified joint compound if required, and torque to the value from the lug or stud supplier. Use a calibrated wrench and mark the fastener.

Support the cable so the joint does not carry the cable weight. Aluminium is lighter than copper for the same conductance, but a large cable still applies a moment to the palm, and a bimetallic joint is not a strain relief.

Where the joint is bolted, agree whether a re-torque check is required after the first load cycle. Aluminium creep is slow but real, and a joint that was correct at installation can be below the specified preload after a few weeks at temperature. A re-torque is not a sign of a bad lug; it is a known property of the conductor.

• Correct die for aluminium, not a copper die of the same size
• Oxide removed and inhibitor applied if specified
• Conductor inserted to the stop, with the correct strip length
• Crimp height checked and recorded
• Palm and busbar cleaned before assembly
• Torque value from the supplier, with a calibrated wrench
• Cable supported independently of the joint

Testing and inspection

The qualification test for a power connector is set out in IEC 61238-1. It includes mechanical tests, heat cycling and short-circuit withstand. For a bimetallic joint, the heat cycling part is the one that exposes a poor bond, because the interface sees repeated expansion and contraction of two metals with different coefficients.

At goods-in, ask for the bond test result and the plating thickness. A cross-section through the transition should show a continuous interface with no voids or oxide line. A pull test on the aluminium side confirms the crimp, and a resistance measurement across the transition confirms the bond.

In service, the useful check is temperature. A thermal image of the joint under load shows a hot spot at the transition or at the palm. Compare it with a baseline taken at commissioning, because an absolute temperature means little without the ambient and the load at the time.

Take that baseline seriously. Record the load current and the ambient temperature with the image, and repeat the check under similar conditions. A joint that has risen by a few degrees at the same load is telling you something that a single pass or fail limit will miss.

What to write on the drawing

State the conductor material, size and class, the barrel material, the palm material, the joint method and the plating on each side. Add the die reference, the crimp height, the torque and the standard the joint must meet. If the lug is tin plated, say so on both the aluminium and the copper side, because a plated palm against a bare busbar is a different joint from plated to plated.

Ask for a sample joint made with the actual cable and the actual die, plus the test records. A bimetallic lug is a purchased joint, and the evidence should travel with it.