Busbar for busway and busduct

What a busway run has to do

A busway, also called a busduct, is a prefabricated run of conductors inside a metallic or insulated enclosure. It carries power from a transformer or a switchboard along a building, down a riser or across a plant room, and it provides tap-off points along the route. The conductor is usually a flat copper or aluminium bar, sometimes several bars per phase, held in supports inside the housing.

The difference from a panel busbar is length. A panel busbar is a short, fixed shape inside one enclosure. A busway is a linear product that has to fit a building, cross a structural joint, turn a corner and connect to equipment at both ends. The design therefore has to deal with tolerance along the route, thermal movement and access for joints at height.

The run is also a system. The housing provides mechanical protection and, in some designs, the earth path. The supports hold the bars and set the clearance. The joints connect sections and carry the current across the gap. A weak joint or a missing expansion allowance can affect the whole run, not just one section.

Joints: the part that decides reliability

A busway joint has to do three things: carry the current with a low resistance, clamp the bars with a controlled force and keep the enclosure and the insulation closed. Most joints are bolted, with a torque value and a set of washers or a Belleville disc to maintain the clamp load. Some designs use a single bolt per phase, others use a multi-bolt plate.

The joint faces have to be flat, clean and plated. A joint that is assembled with a dirty face or a burr has a higher contact resistance, and the temperature rise at that point can be higher than the rest of the run. The joint is also where a fault force is concentrated, because the bars are not continuous through the connection.

The joint schedule is a design document. It lists every joint, its type, its torque and its inspection. A run with many short sections has many joints; a run with fewer, longer sections has fewer joints but harder handling and transport. The right balance depends on the route, the access and the transport limits, not on a default section length.

• Specify the joint type, the plating and the torque for each connection.
• Keep the joint faces covered until assembly to avoid contamination.
• Use a washer or disc that maintains the clamp load through thermal cycles.
• Mark the joint after tightening so a later check can see movement.
• Record the contact resistance on a sample joint at the first build.

Thermal expansion along a long run

Copper and aluminium expand when they heat. Over a long straight run the movement can be several millimetres, and if both ends are fixed the bar or the housing takes the strain. The result can be a bowed bar, a cracked support, a distorted joint or a housing that no longer fits its covers.

There are two common answers. One is an expansion joint that allows the conductor to slide while keeping the current path and the enclosure closed. The other is a sliding support that lets the bar move within the housing while the housing itself is fixed. Some designs use both, with the expansion joint at a defined point and sliding supports elsewhere.

The expansion allowance has to be calculated from the temperature range, the length and the material. A run that is installed cold and then carries current at a high ambient moves more than a run in a stable plant room. The building structure also moves, so a busway that crosses a structural joint needs its own allowance rather than being clamped across it.

Supports, clearance and fault bracing

Supports hold the bar weight, keep the phase spacing and resist the force during a fault. The span between supports is set by the allowable bending stress in the bar and by the reaction the building structure can take. A long span with a high fault level may need a deeper bar or a closer support, and the support is often the cheaper change.

The support also sets the clearance to the housing and to other conductors. A support that is slightly out of position can reduce the clearance at one point, and that point becomes the limiting dimension for the whole run. The support design should locate the bar positively rather than relying on the installer to centre it.

The housing has to take the support reaction and stay within its own temperature limit. A metal housing can carry the load but may need bonding. A non-metallic housing may need a metal frame or a separate earth conductor. The earth path should be continuous through every joint and every support.

Tap-offs, phase arrangement and balance

A tap-off unit connects a load to the run without cutting the main conductor. The outlet position has to be planned with the layout, because a tap-off that is added later may need a new section or a field modification. The tap-off also has to be protected and labelled, and the connection has to keep the enclosure rating.

The phase arrangement affects the inductance and the voltage drop. Keeping the phases close and repeating the same order along the run gives a more predictable result than a random arrangement. For a long run with a balanced load, the phase order should be maintained through every joint so the current path does not twist.

The neutral and earth conductors need the same attention as the phases. A neutral that is undersized for harmonic load runs hot. An earth path that depends on the housing alone may not have a defined resistance at every joint, so a separate earth bar or a bonded housing is often the clearer design.

Testing and commissioning

A busway should be tested before it is energised. The routine checks are continuity, insulation resistance, phase sequence and a visual check of the joints, supports and enclosure. A contact resistance check on sample joints gives a baseline for later maintenance.

At commissioning, the run should be checked for the specified torque at every joint, the correct expansion gap and the freedom of the sliding supports. A thermal image of the run under load shows a hot joint before it fails. The image is most useful when it is taken at a known load and compared with a later survey.

For a long run, the test plan should include the tap-offs and the end connections as well as the main sections. A joint that is correct in the factory can be disturbed during installation, so the commissioning check is not a repeat of the factory test, it is a check of the installed run.

What to send for a quote

Send the single line diagram, the route drawing, the building structure and the ambient along the route. Add the fault level, the protection curve, the tap-off positions and the enclosure rating. State the installation temperature and the access available for joints at height.

If the route crosses a structural joint or a fire barrier, say so. Those points often decide the expansion joint, the fire stop and the support arrangement, and they are easier to price when they are visible on the drawing.

• Single line diagram and route drawing
• Rated current, fault level and clearing time
• Ambient and installation temperature
• Support points and structural limits
• Tap-off positions and load per outlet
• Enclosure rating and earth requirement
• Barrier and structural joint locations