Secondary Bus System Maintenance

Secondary bus systems, or Delta Closures/Deltas, tend to be a low maintenance section of most mills. Secondary bus systems provide a pathway for power to transfer from the secondary side of the transformer, through the vault wall, and to the terminals of water cooled cables. Although low maintenance, checking your bus system is necessary to avoid high-priced repairs. This secondary bus system module will mention basic checks on your secondary bus system, vault wall, and bus tube (for traditional arms).

Knowing Your Secondary Bus System

Secondary bus systems are either air cooled or water cooled depending on their application. Air cooled systems use highly conductive copper to carry roughly 900 to 1,000 AMPS per square inch while water cooled systems typically sustain 3,000 to 3,500 AMPS per inchA water cooled Delta system can typically carry 3 times the power delivered by a similar air cooled system. Overall, you can identify if your system’s air cooled or water cooled by checking if it utilizes copper tubing or copper bar; tubing always carries fluid while bar uses rising air to cool itself.

Note: Similar to most materials, copper with a higher temperature will have a higher resistance in comparison to copper at a lower temperature. Therefore conductive materials with inadequate cooling will continue to increase in temperature until a failure occurs. Most electricity will flow within the outside parameters of conductive material making water cooled bus tube a good candidate for power distribution.

Image 1 - Air Cooled and Water Cooled Bus Heat Extraction

Bus System Inspection

A) Cracking
Cracks result in the poor flow of electricity, severely reduce structural integrity, and may cause leaks. Excessive weight on a secondary system combined with vibrations from cyclic loading causes fatigue within joints, bends, and weld seems. Check your tubes for cracks in high stress areas or have them sent in for inspection. We’ve repaired numerous bus tube cracks to prevent catastrophic failures!

Image 2 - Discovered Fatigue Crack From Overloaded Bus Connection

B) Bending and Deflection
Fastening bus connections should not bend the bar or tubing. Bus tube and bar should not visibly bend with load. Bending may be caused by a change in cable construction, a modification in furnace power delivery, or improper manufacturing. Do NOT lift your arms by the bus tube. Bus should be free of significant load since copper is not typically designed as a structural material.

Timbers and insulative braces are commonly used in secondary bus systems to support weight and reduce fatigue. Most system designers will choose optimal placement for supports, and the placements should remain as instructed.

Image 3 - Insulative Timbers for Bus System Support

C) Flow Restriction and Overheating
Unclean water can cause buildup within pipes, hosing, and tubes. Build up within bus tube restricts coolant flow and creates an insulative barrier of sediment. A barrier around the inner wall of a copper bus tube reduces the heat transfer from the heating copper to the cool water. A reduction in heat transfer could cause overheating of a bus system. Erie Copper Works performs inspections, repairs, and cleanings of buildup within secondary power systems. Preventative maintenance should include ensuring a mill’s water source is filtered and routinely inspected. Clean cooling water may help the longevity of many furnace components. 

Image 4 - Water Sediment Buildup in Bus Tube