The mobile transformer method in Korea is used when a pole-mounted transformer is replaced or relocated, or the supply arrangement changes: a mobile transformer (vehicle-mounted) supplies low-voltage customers temporarily while the existing transformer is replaced dead. Customers are carried by the mobile unit and workers handle a dead transformer, so connecting the mobile unit and checking LV phases decide the outcome. This guide covers two stages — preparation and execution.
Contents
- Common Pre-Work Review
- 1. Preparation
- 2. Execution
- Mobile Transformer Work Sequence
- FAQ
- Reference Standards
- Summary
Common Pre-Work Review
This method applies to 22.9 kV-Y and below overhead distribution lines when work is done without de-energizing the line. Prepare an insulation resistance tester, HV ammeter, HV and LV phase meters and HV voltage detector, plus an insulated aerial device; and no-outage tools for overhead work — the mobile transformer unit, HV riser cables, intermediate cable, LV cables, cable clamps and a COS operating rod. Workers wear hard hats, safety vests, safety shoes, insulating gloves and harnesses, with line and insulator covers ready. Materials include sleeves, lugs and insulating tapes. Every worker must understand the full pole transformer sequence from transport and unloading to installation and testing.
1. Preparation
- Prepare the site: pedestrian route, traffic marshals, earthing of equipment and vehicles, wheel chocks and load current measurement.
- Hold a safety meeting covering task assignment, PPE, equipment and line checks and risk assessment.
- Confirm substation breaker and source-side recloser reclosing status, notify the distribution center, cover live parts and the neutral, and send site monitoring data.
On arrival, review traffic, pole-top configuration and buried services and carry out hazard prediction training; control traffic and pedestrians with barrier ropes, signs and marshals. Before no-outage work, notify the distribution center and confirm auto-reclosing is blocked; notify completion afterwards.
2. Execution
- Measure load current, cover the neutral and prepare to connect the mobile transformer to the load.
- Connect the mobile transformer as the procedure requires to supply LV customers temporarily, then, following the supervisor and watching live and dead sections, carry out dead work such as replacing or relocating the pole transformer.
- After the dead work, remove the mobile transformer connections in reverse order to restore normal service.
- Notify the distribution center of completion and tidy the site to avoid complaints.
The NCS text for this unit repeats the temporary switch method wording in its performance criteria; this guide follows the actual mobile transformer procedure given in the considerations.
Mobile Transformer Work Sequence
| Stage | Sequence |
|---|---|
| Temporary supply | Riser clamps → install and connect HV riser cables → install LV cables → close HV switch → phase check and close LV breaker |
| Transformer replacement | Disconnect pole transformer secondary leads → open COS → replace transformer → close COS → phase check and reconnect secondary leads |
| Restore | Open LV breaker and HV switch → disconnect, discharge and remove risers → remove live-part and neutral covers |
This sequence summarizes the NCS considerations; actual work follows KEPCO standard procedures and the site supervisor.
FAQ
What is dead-section work?
Work done after converting only the work section to a de-energized (dead) state, while the surrounding line and customers stay supplied, so the job needs no outage.
When is the mobile transformer method used?
When a pole transformer is replaced or relocated, or the supply arrangement changes, and LV customers must not lose supply.
Why check phases before closing the LV breaker?
If the mobile unit’s LV phases do not match the existing supply, customer equipment — especially three-phase motors — can be affected. Phases are checked again when reconnecting the new transformer’s secondary leads.
Reference Standards
- Electricity Business Act (전기사업법) — law.go.kr
- Electrical Safety Management Act (전기안전관리법) — law.go.kr
- Electric Technology Management Act (전력기술관리법) — law.go.kr
- Electrical Construction Business Act (전기공사업법) — law.go.kr
- Occupational Safety and Health Act (산업안전보건법) — law.go.kr
- Korea Electro-technical Code (KEC, 한국전기설비규정) — law.go.kr
- KEPCO design standards; field practice manuals for transmission and distribution work; Korean Industrial Standards (KS)
- NCS competency unit Mobile Transformer Method for No-Outage Work (이동용변압기공법 공사, 1901070241_22v3; NCS is Korea’s National Competency Standards) — ncs.go.kr
Summary
The most common mistake with the mobile transformer method is skipping the phase check during temporary supply or when reconnecting after replacement. Preparation depends on reclosing checks and covering; execution on temporary supply → dead replacement → phase-checked reconnection; and restoration on opening breakers and switches before discharging cables. Transformer installation standards are covered in Distribution Equipment Installation. For pre-construction preparation, see Distribution Line Construction Planning. A Korean version of this guide is available here.
This article restructures the Korean NCS competency unit ‘Mobile Transformer Method for No-Outage Work’ (이동용변압기공법 공사, code 1901070241_22v3) as a practical reference. Actual work must follow applicable standards and the judgment of qualified professionals; always confirm specific values, deadlines and criteria against the latest regulations and client standards.
Written/reviewed by VOLT · Last updated 2026-10-25