The first warning was not a trip, a flash or a complete loss of power. It was the sound of a motor struggling to finish a closing operation.
During a scheduled electrical safety inspection at a commercial building in South Korea, the AISS opened normally. When the automatic closing command was given, however, the motor strained, slowed and stopped before completing the operation.
Manual closing was still possible, but that did not make the equipment healthy. The failed automatic mechanism resulted in an unsuccessful inspection, and the AISS had to be replaced before the facility could pass reinspection.
- Equipment: Indoor 25.8 kV / 200 A AISS
- Failure: Automatic closing could not be completed
- Manual operation: Still available
- Decision: Replace the AISS and related operating components
- Result: Normal operation restored and reinspection passed
Replacing a heavy AISS inside a confined high-voltage cubicle required careful positioning as well as electrical skill.
The Failure Appeared During Automatic Closing
The AISS was tested as part of the building's periodic electrical safety inspection. Its opening operation was normal. The problem appeared only when the automatic closing command was issued.
The operating motor started with a strained whirring sound but could not transmit enough mechanical movement to complete the closing cycle. The abnormal condition was traced to the mechanical power-transmission cable connected to the operating mechanism. The cable had twisted and was interfering with the transfer of force from the motor.
This was an important distinction: the main switching assembly had not become completely immovable. It could still be closed manually, while the automatic mechanism had already failed.
Manual Operation Did Not Mean Normal Operation
A switch that remains manually operable should not automatically be considered serviceable. In this case, manual closing restored the system temporarily, but the failed automatic operation still required corrective work and reinspection.
Inspection and replacement of the AISS controller, from the existing unit and internal wiring to the completed new controller installation.
Why the Complete Set Was Replaced
The building owner delegated the practical replacement decision to me in my role as the facility's electrical safety manager. I coordinated the shutdown and replacement with an electrical contractor whose work I already knew.
I could have approved a smaller repair, and it would have cost less at that moment. However, I had encountered AISS operating failures several times during my years as an electrical safety manager. I did not want to restore power while knowing that another aging part could force the entire building through a second shutdown shortly afterward.
We selected an AISS from the same manufacturer with equivalent specifications. This reduced compatibility problems and allowed the protection and operating settings to be restored to the values already established for the building's receiving system.
Only part of the operating system had shown an obvious failure. Even so, replacing only the visibly damaged component would have left other aging components in service. Another failure could have required a second building-wide shutdown, another contractor mobilization and another inspection response.
The set was replaced to reduce the risk of another age-related failure shortly after the first repair. For this building, avoiding a second major shutdown was more valuable than retaining every component that had not yet visibly failed.
The replacement AISS delivered to the site, together with its operating cable and manufacturer instruction manual.
Preparing the Building for the Shutdown
This replacement could not begin with the opening of a cubicle door. A planned outage had to be coordinated in advance with the utility side and the building management office.
Occupants were notified of the outage. On the day of the work, a final announcement was made, elevators were taken out of service and the building loads were shut down in a controlled sequence.
Before the contractor began removing the AISS, the receiving system was isolated and the site's required voltage verification, discharge and grounding measures were completed. The work area was then released to the experienced replacement crew.
Korean field note: On Korean electrical sites, high-voltage receiving switchgear is commonly called sujeonban (수전반). After inspection or maintenance, workers use the term bokjeon (복전) for re-energizing the system and restoring power.
This is a field record, not a switching procedure. Work on 22.9 kV-class receiving equipment requires site-specific isolation, verification, grounding and supervision by appropriately qualified personnel.
Disconnecting and Removing the Existing AISS
The AISS was installed inside the high-voltage receiving cubicle. Its main conductors, control wiring and associated connections had to be identified, disconnected and later restored without leaving anything overlooked inside the panel.
Three experienced workers carried out the physical replacement. The photographs show the sequence clearly, but they do not fully communicate the difficulty of handling the equipment in such a restricted space.
The Hardest Part Was the Working Space
The work took place during summer. The electrical room was already hot, and the open cubicle offered very little room for three people to lift, guide and align a heavy switching assembly.
One worker had to climb into the structure while the others supported the equipment and controlled its position. The AISS could not simply be lifted vertically and dropped onto its mounting points. It had to be maneuvered around the existing frame, conductors and adjacent equipment.
The most difficult part was not disconnecting the conductors. It was moving the heavy AISS into position inside a cubicle where the workers had almost no room to stand.
Workers maneuvered the AISS through the cubicle frame and aligned it with the mounting position in a hot, confined workspace.
Watching the Whole Work Area
I was not one of the technicians physically replacing the AISS. My responsibility was to manage the facility's electrical safety and observe the work from a wider perspective.
Technicians naturally concentrate on the connection or fastener directly in front of them. The safety manager must keep checking the entire work area: critical bolts and conductors, lighting, tools, loose materials, clearances and anything that could be overlooked before energization.
Experience reduces mistakes, but it does not eliminate human error. In high-voltage equipment, a forgotten tool, a loose connection or an unchecked condition can become a serious problem after power is restored.
Installation, Operational Testing and Reinspection
After the failed unit was removed, the replacement AISS was positioned and secured. The main connections, control wiring, new controller and associated operating components were installed and checked.
With the utility-side supply still isolated, the contractor tested both automatic and manual opening and closing. The protection and operating settings were restored to the established values rather than changed arbitrarily.
After the work area and the receiving equipment had been checked, the utility supply was restored. I then performed the automatic closing operation as the building's electrical safety manager. This time, the AISS completed the closing operation normally.
The receiving system and building loads were returned to service in sequence. Photographs and video of the corrective work were retained as evidence for the follow-up inspection.
Before and after: the removed AISS on the floor and the replacement unit installed and connected inside the cubicle.
Automatic and manual operation were both confirmed after installation. A few days later, the facility underwent reinspection and passed.
What This Field Case Demonstrates
The first visible symptom was only an unusual motor sound during an automatic closing test. Behind that sound was a failure capable of preventing a critical high-voltage switch from completing its commanded operation.
Because this type of equipment is not casually operated every day, formal operational testing remains important. Manual operation alone also does not prove that the complete operating system is healthy.
This case also shows why an electrical safety manager's work extends beyond paperwork and switch operation. The role includes coordinating the outage, confirming safe conditions, watching the complete work area, checking restoration conditions and remaining responsible until the facility is safely returned to service.
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