Isolated Phase Bus Maintenance: Critical During Outages

Although too often overlooked, properly maintaining and upgrading the isolated phase bus (isophase) is a crucial component to the overall health of a power plant's operations.

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By Cal Crader, P.E.

Although too often overlooked, properly maintaining and upgrading the isolated phase bus (isophase) is a crucial component to the overall health of a power plant’s operations. By ignoring the needs of the isophase, plant managers are running a higher risk of an unplanned outage, which could end up being a costly decision.

Because of inaccurate, preconceived notions about the isophase, maintenance and important upgrades to this system are often overlooked. Many operators incorrectly believe that because there are no moving parts to the isophase, as opposed to something such as a turbine generator, that it does not require the same kind of preventative maintenance during a planned outage. Operators tend to invest large sums of money to properly maintain the turbines, and rightfully so, but fail to give the isophase the same attention. In reality, a faulty isophase can just as easily lead to catastrophic failure, essentially wasting all of the time and effort maintenance workers put into the rest of the plant. In addition, emergency repairs on the isophase will often cost up to five times as much as preventative maintenance during a planned outage would have. Also, over time, plants have moved from having more planned outages to having longer ones, fewer and far between. Because of this, properly putting together a game plan for repairing and upgrading the isophase during these outages becomes even more essential.

Causes for failure inside of the isophase are diverse. Anything from condensation, dirt, dust and water intrusion to improper grounding, loose connections, cracked expansion welds and faulty, worn out gaskets and insulation can cause the isophase to malfunction and shut down. By doing a detailed inspection of the isophase leading up to a major outage, operators can be assured they are taking all the necessary precautions and planning any upgrades or repair work well in advance of the time-sensitive outage.

Leading up to a planned outage of a power generating station, it is crucial that a qualified isophase contractor is hired that can lay out a detailed scope of work. There are several key areas of the isophase that should be checked and documented.

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Some standard assessments that should take place at this time include checking the bolted joints to determine if the correct hardware and torque are present. In addition, the integrity of the contact surface should be inspected to ensure it is properly functioning. A key area of concern is in the insulators. Many times, these insulators are discovered to be cracked or chipped, needing repair or replacement. In addition, an active proper seal is critical so an inspection is needed for the seal-off bushings.

Leaks are a common issue throughout the isophase if not properly maintained, and covers for hardware gaskets are often a culprit. Both water and air leaks can cause problems. Conversely, in addition to leaks coming in, ensuring water is making its way out is just as important. Water build-up and clogs can often be observed in the filter drains. These should be noted during the assessment phase.

Temperature is another element that needs to be monitored to ensure proper functionality of the isophase. This includes both the enclosure/conductor and the support steel.

After the inspection concludes and a report is developed, maintenance and repair work should be planned and executed during the planned outage. This maintenance comes in the form of both minor and major repairs. Hand cleaning the insulators and vacuuming debris from the enclosures would fall under the category of minor maintenance. In contrast, tasks such as replacing insulators, laminates, rubber bellows and expansion joints are a little more involved. This is where proper planning is essential. If a major repair or replacement is necessary, operators will want to make sure they have all of the proper replacement parts and equipment to complete the work. Parts and equipment need to be on site at the time of the major outage. If not, there could be strong, unfortunate consequences and both time and money ultimately will be lost. Either an outage will need to be extended to finish the repairs outlined in the assessment, or the repairs simply do not get completed when the power generating station goes back online. The second option, of course, carries a tremendous amount of risk. As previously stated, unplanned outages can be extraordinarily costly.

To avoid costly, unplanned outages because of a failure inside the isophase, it is important for plant operators to align themselves with knowledgeable isophase specialists. This will ensure that thorough assessments and critical repairs and upgrades are executed in a timely and efficient manner.

About the Author: Cal Crader, P.E., is the CEO of SE Energy LLC, a nationwide specialty electrical construction, engineering and consulting firm serving clients in the utility and power generation, transmission and distribution, and heavy industrial markets. SE Energy specializes in the engineering, consulting, installation, repair and maintenance of distributed generation, modular substations, battery/energy storage, protective relay systems, medium voltage switchgear and bus, isolated phase bus duct, non-segregated bus duct, control system upgrades, excitation Systems, 24/7 emergency response and disaster recovery. For more information, visit

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