A forensic site inspection reveals melted aluminum bus bars. But what was the source of heat, and who was at fault?
As we opened the motor control center (MCC) door, we were surprised to see such damage. (See Photo 1, original article.) The partially melted bus bars revealed a serious problem in the MCC. Interested to learn more, we began to look more closely at its construction. From here, we began our forensic investigation of a complete shutdown of a major hotel.
Not unusual, the horizontal feeder bus bars were made of aluminum. What was somewhat unusual is these bus bars connected to vertical branch section bus bars made of copper. The damaged aluminum bus bars resulted in the open circuiting of the last two vertical distribution sections.
The vertical bus bar sections fed only electrical resistance duct heaters, which are prone to cyclical loading. We made an extensive on-site investigation of the entire system; studied Polaroid photographs, taken by others immediately after the electrical mishap; and interviewed the building superintendent. We also closely examined physical evidence, such as spacers, bus bars, bolts, nuts, etc. So what caused these bus bars to melt?
Here's what we saw. While examining the joints connecting the horizontal feeder bus bars to the to vertical branch section bus bars, we noticed the lock-down nuts were backed off to where they engaged only a very few threads of the bolts. As a result, the Belleville washers weren't snugged down tight. They had little or no pressure on them to flatten them out. As such, there was no compressive pressure at the bus bar joints. Due to these conditions, the aluminum bus bars experienced high heat, because of the substantial resistance placed in the circuit-resulting in the partial melt down. You can see the burn marks on the red micarta/phenolic insulation and bracing backboard in Photo 2 (original article). But, who (or what) backed off the lock-down nuts?
Typical effects of load cycling and different bus bar materials. Aluminum is lighter and weaker than copper; it also has a lower tensile strength. This means it can't sustain the same electromagnetic forces a bolted short circuit imposes. That's why aluminum bus bars require additional bracing-to protect them from deformation/damage due to high fault currents.
Thermal cycling is another potential problem. Electrical load operation tends to be periodic: There are times of low electric energy requirements and periods of high electric energy demand (and higher current density draw). With cyclic loading being present in almost all electrical systems, the periodic heavy- and light-current draw cause electrical bus bars to heat up and cool down. Because the coefficient of expansion is greater for aluminum than copper, the heat produced during peak periods of energy use causes aluminum bus bars to expand much more than copper.
This periodic bus bar heating and cooling can loosen connections at bus and cable lug joints, reducing current transfer from lug to bus and bus to bus. If this condition exists, it increases electrical resistance. This, in turn, increases energy loss, a higher resistance (higher I2R heat losses), and heat buildup at these connection points.
Applying load cycling to this incident. The conditions described above reduced the cross-sectional area of contact between the horizontal feeder and vertical branch bus bars, thus reducing the area of conductivity for current transfer between these two surfaces.
This reduction of the current carrying capacity, at the point of contact, created higher resistance and increased current density at the junction of the aluminum and copper bus work. In other words, this acted like a miniature heater, placing greater stress on the already weakened connection.
The additional resistance at the point of contact also caused an increased voltage drop. This caused the electric duct heaters (fed from the involved vertical sections of the MCC) to try to compensate for this condition, by drawing even more current to maintain constant wattage. The result: a further increase in overheating of the aluminum bus bars and a partial meltdown of the involved area.
Neither the contractor nor MCC manufacturer was at fault. We included the following conclusions in our report:
The end-user did not verify the contractor correctly torqued the bolts connecting the bus bars upon completing the MCC installation.
The end-user did no periodic maintenance, such as checking torques to ensure they met manufacturer's specifications. (This would have identified incorrect initial torquing.)
There were no identifiable maintenance procedures or scheduling the hotel staff could follow.
The lack of accessibility to some of the electrical equipment in the MCC made it difficult for maintenance personnel to effectively check the condition of the bus bars. (The contractor installed the MCC against a wall.)
The periodic cycling of electric resistance duct heater loads caused thermal cycling of the aluminum bus bars (a metal with high-coefficient expansion), placing extra strain on bolted aluminum-to-copper bus connections.