電力保安安全大会2026（専門編）

Summary

The video is an electrical-safety training session for Metropolitan Expressway personnel. It covers the company’s power-supply arrangements, safe work practices, recent incidents, and general guidance on preventing electric shock, electrical fires, and cascading outages.

1. Electrical safety responsibilities and work planning

• A chief electrical engineer is responsible for the maintenance, management, and operation of electrical equipment. A designated deputy is available when needed.
• Each station has personnel responsible for daily electrical-safety measures.
• The conference is part of the organization’s recurring safety-management activities, intended to reinforce knowledge and prevent electrical accidents.

Before construction or maintenance near electrical equipment:

• Prepare and review a clear work plan and procedure.
• Identify electrical equipment and hazards in advance, using drawings and site checks.
• Assign appropriate roles, including a work supervisor.
• Confirm instructions with workers, and require mutual confirmation before switching or operating equipment.
• Use at least two workers where required, and have the supervisor perform a final check.
• Establish measures to prevent unauthorized entry into energized areas.
• Use qualified personnel for electrical work; do not undertake amateur electrical construction.

2. Power-supply systems and continuity

The Metropolitan Expressway engineer explains how substations and distribution systems are arranged to maintain service during faults or maintenance.

• Some substations use a two-source, two-transformer arrangement. Each source normally supplies part of the load, while the system is designed so one transformer can carry the full load if the other source fails.
• Transformer capacity must be selected by estimating loads and accounting for demand.
• A fault or outage at a power-receiving station can affect other equipment connected to the same supply system.
• Certain important loads—such as ETC and traffic-control equipment—need to remain powered during inspections. Their supply can be switched to a healthy line before the other line is taken out of service.
• General circuits may be disconnected by electromagnetic contactors during an outage. Even if a generator restores power to some station lighting, those disconnected loads may not automatically receive generator power.
• Protection coordination is intended to isolate only the faulty section and keep healthy sections operating. If an upstream breaker trips before the breaker nearest the fault, a larger area may lose power.

3. Voltage-drop calculations and design considerations

The presentation explains voltage drop as the reduction in voltage caused by current flowing through a cable’s resistance. Equipment may not function correctly if voltage falls outside its permitted range; the speaker gives design limits of 10% for high voltage and 6% for low voltage.

Key lessons from the calculation discussion include:

• Start from the basic relationship between resistance, current, and voltage drop.
• Account for the wiring arrangement, cable length and cross-sectional area, and the current actually flowing through each cable.
• In circuits with multiple loads, current is generally larger nearer the supply, so the current used in a calculation must correspond to the cable section being assessed.
• Some systems require vector treatment of voltage drops. Assumptions about neutral current being zero depend on loads being balanced.
• The connection arrangement can change how a circuit should be treated in a calculation. Designers should not assume that loads are balanced without confirming it.

4. Recent Metropolitan Expressway incidents and lessons

Short-circuit accident during a tunnel-equipment inspection — October 2025

During an inspection of ventilation-panel equipment, a worker tested the wrong side of a device after a voltage check was misunderstood as residual charge. The device was connected to 6,600 V, and contact with the energized side caused a short-circuit arc and facial burns. The subtitles say the worker’s burns took about a week to heal; the panel also required repairs.

The presentation identifies contributing factors including insufficient preparation using drawings, mistaken assumptions about which side was safe, work being performed alone, and inadequate restriction of access to energized parts.

Preventive measures emphasized:

• Create and follow written work procedures.
• Conduct hazard-prediction activities before starting.
• Clearly identify energized areas and prohibit access to them.
• Perform work with at least two people where required.
• Have the supervisor check the work and measurement point.
• Wear and regularly inspect the required insulating protective equipment.

Seasonal-lighting work near overhead power lines — around November 2019

During removal of seasonal lighting, a crane’s wire rope approached overhead power lines. An electrical arc and shocks occurred even though the line was not directly touched. A later restoration of power was followed by another arc. The rope broke, a lighting pole fell, and debris damaged passing vehicles. Workers had symptoms consistent with electric shock, though the subtitles say they were not seriously injured.

The incident revealed that power lines had not been adequately identified in the construction documents and were difficult to see at night.

Lessons and precautions:

• Identify all overhead power lines before work begins, including lines that may be difficult to see.
• Follow the utility’s required safe separation distances; approaching a line can be dangerous even without direct contact.
• Use records of line locations, voltages, required clearances, and utility contacts when planning construction.
• Mark work areas where overhead lines are present.

Gotanda power outage — February 2026

A ground fault in the supply lines from Tokyo Electric Power Company disabled two of three incoming lines. With only one transformer available, the remaining capacity was insufficient for peak demand, resulting in a loss of power and tunnel-lighting disruption.

The incident also revealed that a tunnel ventilation fan was not connected to a direct power circuit, which caused a malfunction alarm. The presentation describes subsequent improvements to automatic supply arrangements and procedures for prolonged outages. It also notes that a three-line network is highly reliable but can still fail when multiple lines are lost, particularly if the supply routes share a vulnerable route.

5. General electric-shock risks and prevention

A representative from the Kanto Electric Safety Association explains that electric shock occurs when current passes through the body. The danger depends on factors such as the amount of current, the duration of contact, and the path through the body. A path through the chest or heart is especially hazardous.

Common situations and precautions include:

• Downed power lines: Do not touch or move a fallen line. Keep away and contact the power company.
• Faulty or improperly grounded equipment: Ground equipment as required so a fault can be detected and protective devices can disconnect power.
• Energized equipment during maintenance: Isolate power before work whenever possible, verify the equipment’s state, and use suitable protective equipment when energized work cannot be avoided.
• Unusual plugs or wiring: Confirm that plugs and outlets are correctly wired. A plug that unexpectedly carries power can expose live contacts.
• Electric-shock reporting: The presentation says the chief electrical engineer must be notified immediately after an electric-shock accident and refers to reporting requirements to the Ministry of Economy, Trade and Industry, including an initial report within 24 hours and a further report within 30 days.

The speaker cites workplace statistics to underline the severity of electric shock, reporting 94 cases involving more than four days away from work and 10 deaths in 2025.

6. Electrical-fire causes and prevention

The Kanto Electric Safety Association speaker discusses electrical fires and the importance of equipment care. Causes described include loose or overheated connections, damaged or broken wires, excessive current, leakage, short circuits, tracking, and improper use.

Practical precautions include:

• Inspect cords for damage. Conductors can break internally if cords are pulled, crushed, tightly fixed, sharply bent, or caught.
• Prevent short circuits, which can produce extremely high currents, intense heat, and dangerous arcs.
• Use lithium-ion batteries carefully: avoid impacts and high temperatures, charge them in a safe location, and check for product recalls.
• The speaker recommends purchasing products bearing the PSE mark and following transport-specific battery rules.
• If a battery catches fire, the speaker advises using a large amount of water and checking for recall information.

7. Cascading outages and communication failures

A second Kanto Electric Safety Association speaker, Kimura, discusses cascading electrical accidents and an example involving incorrect operation during maintenance.

In the case described, a worker switched a molded-case circuit breaker while a short-circuit grounding device was still installed. Confusion arose because the supervisor changed the plan, the workers’ expectations differed, and instructions were not clearly confirmed. The speaker also notes that language comprehension can affect whether instructions are understood.

Lessons for preventing similar incidents:

• Do not change a work plan informally; clearly communicate and confirm any change.
• Use explicit instructions for switching operations, including who may operate a device and when.
• Require workers to repeat back or otherwise confirm critical instructions.
• Ensure all workers can understand the instructions; provide additional support where language may be a barrier.
• Confirm that grounding and protective equipment have been removed or correctly positioned before restoring power.

Kimura also describes an electric-shock incident during solar-facility inspection. A worker attempted to remove a locked terminal cover and contacted a live 400 V AC part. The guidance is to de-energize the relevant equipment where possible, check the cover and its locking mechanism carefully, and use suitable low-voltage rubber gloves if energized work is unavoidable.

8. Overall safety requests

The speakers’ recurring messages are to plan work carefully, use qualified people, verify the actual electrical condition rather than relying on assumptions, communicate and confirm instructions, maintain equipment, and use protective equipment correctly. After a power outage, equipment should be switched off as appropriate before power is restored, to avoid multiple machines starting unexpectedly. Any abnormality should be reported to the person responsible.

Speakers and sources featured

• Unnamed Metropolitan Expressway Company electrical engineer: Explains the company’s electrical-safety system, power distribution, voltage-drop design, protection coordination, and recent incidents.
• Ko, Kanto Electric Safety Association: Discusses electric-shock hazards, electrical fires, battery safety, and practical precautions.
• Kimura, Kanto Electric Safety Association: Discusses cascading accidents, work communication, and electric-shock case studies.
• Sources and authorities mentioned: Tokyo Electric Power Company; the Ministry of Economy, Trade and Industry; the Tokyo Fire Department; electrical-safety and labor-safety regulations; and the Kanto Electric Safety Association.