Engineering art: ACB to busbar connections

Category: Engineering Category: Photo

This is the connection between a 3,200 amp air circuit breaker’s terminal and 4 off 60×10mm copper bus bars. The photo was taken from the back side of a switchboard, specifically a motor control centre (MCC) incomer tier.


Click image for 4000×3000 resolution JPG.

Each nut has two texta/sharpie marks, to indicate that the bolt torque has been double-checked. Tightening the bolts to the correct torque is important. The bolts need to be tight enough to clamp the metal parts into good electrical contact, but not so tight that the bolt is over-stretched and destroyed.

It’s hard to see in this photo, but I believe the washers are Belleville spring washers (conical spring washers). Spring washers are required to account for the thermal expansion of the metal parts as the load current varies. The spring washers allow a bit of ‘give’ in the joints. If there were no spring washers, the thermal expansion of the copper bars would over-stretch the steel bolts, loosening the connections over time. Loose connections have high electrical resistance, which leads to overheating at the joint - the ‘hot joint’ failure.

See the Copper Development Association’s Copper for Busbars publication, Section 6  Busbar Jointing, for more information about busbar jointing techniques.

Photo taken on a Canon S100 point-and-shoot, during a factory acceptance test. Camera put into aperture priority mode to achieve sharp depth-of-focus. Colouring done with The GIMP.