.png "50-Ton Crane Positioned for Rooftop MOF Replacement")
The existing 22.9 kV metering outfit had not failed, and the building was still operating normally. However, a periodic inspection by the Korea Electrical Safety Corporation (KESCO) recorded insulation resistance below 1,000 kΩ (1 MΩ) and recommended replacement before the condition could deteriorate further.
The MOF was installed outdoors on the rooftop, and immediate replacement was not presented as a mandatory order at this site. Even so, the inspector explained that insulation condition could worsen rapidly. The owner therefore chose planned preventive replacement rather than continuing operation until an unplanned failure forced an emergency outage.
The equipment was still energized and functioning, but the inspection result provided an early warning. A planned outage allowed the owner, contractor, crane team, and utility to prepare the replacement under controlled conditions.
Project at a Glance
Documentation note: The inspection result is reported here as recorded for this project. The test voltage and detailed measurement method were not preserved in the source material for this article, so no additional interpretation is claimed.
What the MOF Does
An MOF, or metering outfit, combines voltage-transforming and current-transforming elements in one assembly so that high-voltage system quantities can be reduced to values suitable for revenue metering. Its principal role in this installation was to provide the voltage and current signals required by the electricity meter; it was not the device supplying usable power to the building.
The building used a 500 kVA transformer. The replacement MOF nameplate identified a 22.9 kV primary, a 190 V/√3 secondary voltage, and a 15/5 A CT ratio. Transformer capacity is expressed in kVA, not kW, because the rating concerns apparent power.
.png "Replacement 22.9 kV MOF Nameplate Specifications")
Why a 50-Ton Crane Was Necessary
The building was seven stories high, but height alone did not determine the crane size. An overhead high-voltage distribution line ran beside the building, preventing the crane from taking the most direct lifting position. The mobile crane therefore had to stand beside the neighboring building and handle the load at a less favorable radius.
This was the practical variable that shaped the job. The larger crane was selected to obtain the required capacity at the actual working radius while maintaining clearance from the overhead line. Before the outage began, the replacement MOF was lifted close to the receiving area so the electrical work could start promptly after isolation.
A rooftop that appears reachable may still require a larger crane when the machine must stand farther away. Obstacles, boom angle, working radius, load, and overhead-line clearance must be considered together.
Coordinating the Outage
This replacement could not be treated like ordinary low-voltage maintenance. The owner, electrical safety manager, electrical contractor, crane team, and KEPCO had to work to the same schedule. Equipment, lifting access, personnel, tools, and the replacement unit were prepared before the incoming supply was disconnected because delays after shutdown would directly extend the building outage.
After the building-side shutdown steps were completed, KEPCO personnel operated the utility-side cut-out switch to isolate the incoming 22.9 kV supply. Opening a switch was not treated as proof that the work area was safe. The electrical team still had to verify the de-energized condition, address possible stored energy, and install the required temporary protective grounding before touching primary equipment.
.png "KEPCO Utility-Side Isolation for MOF Replacement")
The Isolation Sequence Used at This Site
The sequence below records this particular simplified receiving installation. It is not a universal switching instruction: equipment configuration, ownership boundaries, approved procedures, and operating authority can differ from one facility to another.
Switch position was only one part of the isolation process. Absence-of-voltage verification, discharge procedure, temporary grounding, controlled access, and coordination among authorized parties were required before work proceeded.
Removing the Existing MOF
Once the work area had been isolated and grounded, the primary conductors, secondary metering circuits, and grounding connection were identified and disconnected. The old unit was then removed, leaving the cubicle available for inspection and preparation of the mounting points and conductors.
.png "Receiving Cubicle After Existing MOF Removal")
No obvious external damage was required for the replacement decision. The reason for action came from the periodic inspection result, illustrating why visual appearance alone is not enough to judge the condition of high-voltage insulation.
.png "Existing MOF Removed After Inspection Recommendation")
Installing and Checking the Replacement
The replacement unit was positioned with the crane and aligned in the receiving equipment. Primary terminals, secondary metering circuits, and grounding connections were checked against their intended positions before final tightening. Because incorrect CT or voltage-circuit connections can affect revenue metering, conductor identification and terminal verification were treated as essential parts of the installation.
.png "Final Wiring Check After New MOF Installation")
Restoring the System
After the installation and connection checks were completed, temporary grounding was removed under the approved work sequence. The transformer-side power fuses, utility-side COS, ASS, main ACB, and selected MCCBs were then returned to service in the controlled sequence used for this site. Each stage was confirmed before the next device was operated.
Following energization, phase voltages and the metering circuits were checked. The receiving installation returned to normal operation, and no abnormal noise, heating, or other visible operating issue was observed during the recorded final check. Exact voltage values were not retained for this article, so none are claimed here.
What This Project Taught Me
The most important part of this project was not the mechanical act of replacing one piece of equipment. It was acting on a warning while the system was still operating, arranging a controlled outage, and preparing the lifting operation around a difficult rooftop location. Preventive work gave every party time to plan; a sudden failure would not have offered the same advantage.
The crane decision was another practical lesson. A seven-story building did not automatically mean a small crane would be adequate. The adjacent overhead distribution line changed the setup position and working radius, demonstrating that site geometry can determine equipment selection more strongly than building height.
Safety and Scope Note
This article records one field project carried out by qualified and authorized personnel under a coordinated maintenance plan. It is not a switching instruction or a universal procedure for other receiving systems. High-voltage equipment can cause fatal electric shock, arc-flash injury, fire, and equipment damage; site-specific approved procedures, protective equipment, operating authority, and utility coordination are required.
Image note: All images in this article originated from the actual field project. Identifying information was removed, and limited perspective correction was applied to some wide-angle images; the work itself was not reconstructed with AI.
Electric Korea
Korean Electrical Construction · Field Experience · Safety · Technical Education
Website: electrickorea.co.kr
.png)
.png)
.png)
