This is the post. Took me long enough to find it...lol. Don't know if it helps much.
Nice info.
That link is about clamping copper-clad aluminum busbars.
They show two bolts (which keeps the busbars from moving and loosening the screws.
The 4.5 Nm torque of 6mm is with lubricant (big difference in friction and clamping force vs. dry)
Contact only taking place at about 1% of the overlap area!
"6.3.2.1 Condition of Contact Surfaces
In practice, an electrical contact between the solids is formed only at discrete areas within the contact interface and these areas (known as ‘a-spots’) are the only current conducting paths. The a-spots typically occupy an area of the order of 1% of the overlap area.
Obviously, the larger the number of a-spots, the more uniform the current distribution across the joint area will be. This can be encouraged by ensuring that the surfaces of the conductors are flat and roughened (which removes the oxide layer and produces a large number of asperities) immediately before assembly. As the contact pressure is increased, the higher peaks make contact, disrupt any remaining surface oxide and form metal to metal contact.
In some areas an oxide film may remain. Copper oxide films on copper form relatively slowly and are semiconducting because copper ions diffuse into the oxide layer. When copper oxide films are compressed between two copper surfaces, diffusion can take place in both directions so conduction takes place in both directions. This is very different from aluminium, where the oxide is a very good insulator and forms within microseconds of exposure to air.
Since the area of each a-spot contact is small, the current density is high, leading to higher voltage drop and local heating. In a well-made joint this heat is quickly dissipated into the mass of the conductor and the temperature of the interface will be only slightly above that of the bulk material. However, if the contact pressure is too low and the joint has deteriorated, local over-heating may be enough to induce basic metallurgical changes including softening and melting of the material at the a-spot. At first sight this may appear to be advantageous, however, as the joint cools the material contracts and fractures and is subsequently liable to oxidise.
Since elevated temperature is the first symptom of joint failure, maintenance procedures should be established to monitor the temperature of joints with respect to that of nearby bar using thermal imaging. If, under similar load conditions, the differential temperature increases, it may be a sign of early joint degradation. As a first step, more intensive monitoring should be undertaken and, if the trend continues, remedial action taken.
It is not normally recommended that the surfaces of copper-to-copper joints are plated unless required by environmental considerations. In fact, plating may reduce the stability of the joint because, as soft materials, the plating may flow at elevated temperatures leading to reduced contact pressure.
However, to ensure a long service life, a contact aid compound is recommended to fill the voids in the contact area and prevent oxidation or corrosion. Many proprietary compounds are available or, if none are available, petroleum jelly or, for higher temperatures, silicone vacuum grease may be used.
6.3.2.2 Effect of Pressure on Contact Resistance
Joint resistance normally decreases with an increase in the size and number of bolts used. Bolt sizes usually vary from M6 to M20 with either four or six bolts being used. The appropriate torque for each bolt size depends on the bolt material and the maximum operating temperature expected.
Contact resistance falls rapidly with increasing pressure, as shown in Figure 69, but above a pressure of about 30 N/mm² there is little further improvement. In most cases it is not advisable to use contact pressures of less than 7 N/mm², with pressures above 10 N/mm² being preferred. The contact resistance for a joint of a particular overlap area is obtained from Figure 69 by dividing the contact resistance for 1 mm2 by the overlap area in mm2 ."