What is the optimal busbar joint overlap?
It should be noted that the experimentally derived optimum is for plain busbars and does not include a hole for the fixture.
Braunovic [3] shows the impact on resistance of the joint overlap.
The resistance ratio is the ratio of the resistance measured across the joint divided by the resistance of an equivalent length of plain busbar.
From this figure it appears that the streamline effect rapidly decreases until the overlap/thickness ratio reaches a value of 2 when its decrease is slowed down and practically stabilized upon passing a value of 6. In other words, upon reaching this value, the current distortion is minimized and the current lines become parallels.
Consequently, the optimal overlap length in a bolted joint should be 5–7 times the busbar thickness.
Milenko Braunovic, Effect of Connection Design on the Contact Resistance of High Power Overlapping Bolted Joints, IEEE Transactions on Components and Packaging Technologies,Vol. 25, No.4, Dec 2002
Braunovic [1] expands on this initial statement:
However, to meet all requirements, and for the sake of convenience, it is common practice to make overlaps equal to the width of the bus bar. Although this procedure is usually satisfactory for busbars of ordinary dimensions, it may not provide sufficient joint contact area on busbars that have a high thickness-to-width ratio. Hence, as a rule of thumb, the minimum overlap should be from 8 to 10 times the bar thickness.
Furthermore, since the actual area of contact is much less than the total area of overlap, the determination of overlap on the basis of the width alone is reasonably safe practice only if the current density in the contact surface does not exceed 1/3 to 1/4 of the current density in the busbar cross section.
This means that if we have a busbar that has a width of 30mm and thickness of 3mm the overlap with another busbar should be 24 to 30mm. Note that this is before we put a hole in the busbar for a bolted joint.
The Electrical Contact Resistance of the two busbars is really important. This determined by the busbar materials, surface roughness and the clamping pressure of the joint.
- Schneider Electric [2] suggest an overlap of 5x the busbar thickness in their FAQ.
- Greenwood [3] writes: usually an overlap of 2 to 3 times the thickness is all that is required. Any more doesn’t significantly improve the joint but does add cost and weight. For practical reasons we may specify the overlap (‘o’ above) as 5 to 10 times the thickness (‘t’ above) which also allows for holes/slots and bolts. The width of the busbars is not relevant assuming both conductors are identical.
This post has been built based on the support of and sponsorship from: Quarto Technical Services, Eatron Technologies and About:Energy
Schneider Electric [4] state that it has been proved that the contact resistance depends more on the total applied pressure than on the surface of contact. The more this pressure is high the less is the contact resistance. Moreover
a high pressure prevents any ingress of external contaminants.
In practice, the real surface of contact is limited to the areas on which the pressure is applied in an effective way. In a joint by overlap, such as a bolted joint, these areas consist of the zones bordering the bolts.
In this case, the “effective” section of contact corresponds to the surface of the contact washer
(less the hole for the passage of the screw).
Thus the effective surface of contact or pressure surface should be as close as possible to
the copper overlapping surface.
Rule of Thumb
What is the optimal busbar joint overlap?
For a bolted joint an overlap of 5 to 10 times the busbar thickness.
References
- Milenko Braunovic, Effect of Connection Design on the Contact Resistance of High Power Overlapping Bolted Joints, IEEE Transactions on Components and Packaging Technologies,Vol. 25, No.4, Dec 2002
- What is the recommended overlap distance in the Busbar joints of BlokSeT?, Schneider Electric
- A Primer on Busbar Bolted Joints, Roger Greenwood
- Bolted busbars connections by overlapping means, Schneider Electric