Key takeaways
- Contact resistance comes from the small real area where two surfaces touch, not from the nominal overlap.
- Surface films, plating and oxide layers add resistance that a torque wrench cannot fix.
- Preparation should be a defined sequence: degrease, flatten, deburr, abrade, plate and protect.
- Millivolt drop at a stated current is the evidence that the joint is sound; torque alone is not.
What contact resistance is
When two metal faces are pressed together, they do not touch everywhere. They touch at a set of small contact points where the highest asperities meet. The current crowds through those points, and the crowding adds resistance. This is the constriction resistance, and it is the largest part of the resistance of a good bolted joint.
The rest comes from what sits between the metals. Oxide, sulphide, oil, dust, plating and joint compound all add a film resistance. A clean tin-plated copper joint can have a contact resistance in the micro-ohm range, but the same joint with an oxidised face, a scratch through the plating or a dry assembly can be several times higher. That difference shows up as heat at the joint, not as a fault on a drawing.
The resistances in series
It helps to think of a joint as several resistances in series: the bulk metal of each bar, the plating on each face, the film at the interface and the constriction resistance where the asperities touch. The bulk metal is fixed by the bar size. The other three are decided by preparation and assembly.
The shares in the infographic are illustrative for a tin-plated copper bolted joint at normal torque. They are not measured values and they will change with material, plating, flatness and pressure. Their purpose is to show where the effort belongs: most of the resistance sits at the interface, which is exactly the part that a drawing usually does not describe.
Surface preparation, step by step
Preparation is a sequence, not a single wipe. Skipping a step usually moves the contamination rather than removing it, and a solvent that is not allowed to flash off becomes a film under the joint.
- 01
Degrease
Remove oil, cutting fluid and handling grease with a solvent or an alkaline cleaner, then dry fully. A residue becomes a film resistance.
- 02
Check flatness
Put a straight edge across the contact face. A bowed bar touches at one end and leaves a gap at the other, whatever the torque.
- 03
Deburr
Remove burrs at holes and edges. A burr holds the faces apart and creates a point contact that overheats.
- 04
Abrade
Use a mild abrasive or a dedicated pad to break the oxide, then clean off the debris. Do not polish the face to a mirror; a light tooth helps a compound or plating key.
- 05
Protect
Plate, or apply the specified joint compound, within the working window. A cleaned copper face oxidises again in hours in a damp shop.
- 06
Assemble and torque
Fit the washers and bolts, tighten in a cross pattern in two passes, and mark the fasteners so movement can be seen later.
Plating and the interface
Tin plating is the usual choice for copper busbars. It is soft, it conforms to the asperities under pressure, and it keeps the copper from oxidising. The tin oxide that forms on the surface is thin and breaks down under pressure, so a tin joint can still make good contact after storage if the faces are clean. Thickness and adhesion matter: a thin, porous film does not protect the copper, and a poorly adhered film can flake under the washer.
Silver plating gives lower contact resistance and is used for high-current or high-temperature joints. It tarnishes in sulphur-bearing air, and silver sulphide is a poor conductor, so the environment decides whether the benefit survives. Nickel plating is hard and corrosion resistant but has a higher contact resistance than tin or silver, so it is chosen for wear or temperature rather than for the lowest drop.
Bare copper is acceptable in a dry, clean enclosure, but it oxidises quickly and the oxide must be removed immediately before assembly. Aluminium is different again: its oxide is an insulator and reforms at once, so an aluminium joint needs a brush, a compound and usually a bimetallic transition. The preparation method has to match the metal, not just the drawing note.
Clean is not the same as conductive
A degreased face can still carry an oxide film. The preparation has to remove the film and then keep it from coming back before the joint closes.
Pressure, flatness and torque
Contact pressure is what turns a touching pair of faces into a conducting joint. It comes from bolt tension, and bolt tension comes from torque, thread friction and the washer. A lubricated thread gives more tension for the same torque than a dry one, which is why the torque value must be tied to the assembly condition. A torque figure copied from another supplier, with a different lubricant, may give a very different pressure.
Flatness decides how that pressure is distributed. A bar with a hollow in the middle touches at the edges, and the bolt load bends it until the middle closes, if the bar is flexible enough. A thick bar may never close the gap. A straight edge and a feeler gauge on the contact face are quick checks that catch most of these problems before assembly.
Thermal cycling changes the pressure. The joint expands and contracts, the plating creeps and the bolt relaxes. A joint that is correct at assembly can lose tension after a few hundred cycles. Belleville washers store more travel than a plain washer and help the joint keep its pressure, but they do not replace a proper re-torque plan.
Measuring contact resistance
The usual production check is a millivolt drop test. A known current is passed through the joint, and the voltage is measured across it with probes placed at a fixed distance either side. The result is compared with the voltage across an equal length of solid bar at the same current, or with a limit stated in the specification.
The test only means something if the current, the probe positions and the surface condition are controlled. Probes on painted or oxidised surfaces read the surface, not the joint. Probes too close to the bolt read the bolt rather than the contact face. A four-wire measurement avoids lead resistance, and a record of the current and the probe spacing makes the result comparable between lots.
Acceptance and records
Acceptance should be written before the first joint is made. State the current, the probe positions, the limit and the action if the joint fails. If the limit comes from a standard, quote the clause. If it comes from a comparison with an equal length of bar, say so and give the bar length used.
Records should travel with the lot: the material certificate, the plating certificate, the preparation method, the torque values, the operator and the millivolt drop results. If the joint is reworked, record the rework. A joint that was opened and re-tightened has a different history from one that was assembled once, and the difference matters when a hot joint appears in service.
| Step | What good looks like | What to record |
|---|---|---|
| Degrease | No oil or residue, surface dry | Cleaner and drying method |
| Flatness | Straight edge touches across the face | Gap and checking method |
| Deburr | No raised metal at holes or edges | Visual acceptance |
| Abrade | Uniform dull surface, oxide removed | Method and time before assembly |
| Plate or compound | Specified thickness or coverage | Certificate or batch |
| Torque | Cross pattern, two passes, marked | Torque value and wrench check |
| Millivolt drop | Within the stated limit at the stated current | Reading, current and probe spacing |
The order matters. Abrading after plating removes the protection, and torquing before the faces are flat leaves a gap that no later check will close.
Frequently asked questions
What is a normal contact resistance for a busbar joint?
There is no single figure. It depends on the bar size, the overlap, the plating, the pressure and the measurement length. The useful approach is to compare the joint with an equal length of solid bar at the same current, or to apply a limit from the specification that was set for that joint design.
Should I use a joint compound on a tin-plated busbar?
Follow the drawing. A compound can exclude moisture and oxygen, which helps in a damp or polluted enclosure, but an incompatible compound can soften the plating or add film resistance. If a compound is used, specify the product and the thinness of the film.
Why does my millivolt drop reading change between operators?
Probe position, probe pressure, surface condition and current are the usual causes. Fix the probe spacing with a jig, clean the measurement points, use a four-wire meter and record the current. Without those controls the reading is not comparable between lots.
Can I clean a joint with a wire brush?
A wire brush removes oxide but can also embed foreign metal and leave deep scratches. Use a brush or pad that suits the bar material, clean off the debris, and do not use the same brush on aluminium and copper. For plated faces, a mild abrasion that does not cut through the plating is safer.
Sources and further reading
- IEC 61439-1, Low-voltage switchgear and controlgear assemblies
- ASTM B545, Standard specification for electrodeposited coatings of tin
- Qumek busbar joint preparation and millivolt drop reviews
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.
Chasing a joint that runs hot?
Send the joint detail, plating, torque and millivolt drop readings. The engineering desk will review the preparation and the contact path before proposing a change.
Send the joint data