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How Do Seismic Breakers Cut Electricity to Prevent Fires After Earthquakes?

Category: Technology

After a big earthquake, the power might go out while your family is away. When the electricity comes back, things that fell over could touch hot appliances or damaged wires. This can start a fire when no one is home to put it out. A device called a seismic breaker helps prevent this by cutting the power automatically when it detects shaking. There are three main types, and one type waits 3 minutes before switching off. We can learn about these devices from government reports.

1. Why returning power to an empty house after an earthquake can be dangerous

During a large earthquake, items on shelves can fall, and furniture can tip over. If the power goes out and your family evacuates, the house is empty. Later, the electricity might be restored. At that moment, fallen objects could touch electric heaters, or sparks could come from damaged wires. Because no one is home, the fire cannot be put out quickly. This type of fire, which starts when power comes back on, is called a "power-on fire" [3]. It has been pointed out that in the Great Hanshin-Awaji Earthquake, many electrical fires happened when power was restored to empty homes, and they were not put out early [2]. The main causes of fires after earthquakes have changed over time. In the Great Kanto Earthquake (1923), it was cooking stoves and charcoal braziers. After the Niigata Earthquake (1964), it was gas and oil machines. In recent years, electricity has become more prominent [3]. As our tools change, our safety measures must change too.

2. Electrical causes made up about 60% of known fire causes in the Great Hanshin-Awaji Earthquake

According to a Cabinet Office report, in the Great Hanshin-Awaji Earthquake (1995), there were 139 fires where the cause was confirmed. Of these, 85 were electrical fires, which is about 60% [1]. In the Great East Japan Earthquake (2011), there were 110 fires with known causes, and 71 were electrical, which is about 65% [1]. These numbers only include fires where the cause was identified. Fires caused by tsunamis are counted separately. Still, electricity is a major cause. Right after an earthquake, it is hard to check for safety because of the shaking and darkness from power outages. Sometimes, people are not even home. So, the idea came up to cut the power automatically if a certain level of shaking is felt, even if people cannot move [1]. This is intended to help reduce fires when safety checks are impossible due to shaking, darkness, or absence.

3. The three types of seismic breakers: panel, outlet, and simple

The Cabinet Office divides seismic breakers into three main types [1]. The first is the panel type. It is attached to the main distribution panel, where all the home’s electricity gathers. There are built-in and add-on versions. One add-on version attaches a seismic relay to the existing panel. When shaking is felt, it sends a signal that imitates an electrical leak, which makes the earth leakage breaker (a safety switch that cuts power when electricity leaks) trip. Installing this requires electrical work [4]. The second is the outlet type. It cuts power only to that outlet. You can keep power on for medical devices or emergency lights but cut it for heaters. However, it does not protect the wiring inside the walls [4]. The third is the simple type. It uses a weight or pendulum that moves with gravity or a spring to trip the breaker. No electrical work is needed. When the report was written, the price was about 3,000 to 4,000 yen. We cannot tell the current price from this document [4].

4. Why panel-type seismic breakers wait 3 minutes before cutting power

The panel type usually cuts the power 3 minutes after feeling the shaking. This gives people inside the building time to evacuate or turn off appliances [3]. It is often easier to move if there is a little time before it goes completely dark. The simple and outlet types cut the power at the same time as the shaking. Because of this, you need to have separate lighting for evacuation [4]. It is recommended to have minimum light sources ready, like footlights or flashlights in the bedroom [3]. Imagine a pencil rolling off a desk and knocking over dominoes. The simple type works similarly: it uses gravity or a spring as a weight or pendulum moves and pushes the switch down. However, real devices are designed to move based on the strength of the shaking. When power is restored, users must check that the inside of the building is safe before turning it back on.

5. How strong shaking must be to trigger seismic breakers, and what their limits are

Performance guidelines test these devices with shaking equivalent to seismic intensity 5-strong. They also test that they do not activate at seismic intensity 4 (intensity is Japan's scale for how strongly the ground shakes) [3]. Seismic intensity 5-strong is when many dishes and books fall, and unsecured furniture may tip over [3]. This level is chosen because heaters can fall from shelves, or furniture can tip over so that flammable items touch a heater. However, seismic intensity 5-strong is a test condition. It does not mean the device will always activate in a real earthquake. The actual movement depends on how the building and installation location shake. It might activate even if the reported intensity is lower than set, or not activate even if it is higher [3]. Also, it is thought that these devices do not prevent fires that start after electricity has already been used [3]. Seismic breakers are just one part of safety; they do not remove all fire risks.

6. How to find your home's distribution panel with your family

Try to find your home’s distribution panel with your family. It is often a gray or white box on the wall near the entrance or hallway. Just look at it. Leave opening the lid or touching switches to adults. Check visually if a seismic breaker is attached to the panel or wall outlets. Ask your family which type could be installed. Some types need electrical work, and some do not. Also, check where the flashlight is in the bedroom. If you want to know more, the Cabinet Office has published a summary report and performance guidelines with diagrams comparing the three types [1][3].

Sources

  1. Cabinet Office, "Summary Report of the Study Group on Prevention of Electrical Fires during Large Earthquakes" https://www.bousai.go.jp/jishin/syuto/denkikasaitaisaku/pdf/gaiyou_houkoku.pdf (Supports data on electrical fire counts, the three types, and the concept of automatic cutoff.)
  2. Cabinet Office, "Great Hanshin-Awaji Earthquake Lessons Information Collection [04] Fire Occurrence and Spread" https://www.bousai.go.jp/kyoiku/kyokun/hanshin_awaji/data/detail/1-1-4.html (Supports the point that fires occurred when power was restored during evacuation.)
  3. Cabinet Office, "Performance Evaluation Guidelines for Seismic Breakers, etc." https://www.bousai.go.jp/jishin/syuto/denkikasaitaisaku/pdf/kanden_guideline.pdf (Supports the scenario of power-on fires, the 3-minute wait time, test intensities, and limits of effectiveness.)
  4. Cabinet Office, "Types and Features of Seismic Breakers, etc. (Document 4)" https://www.bousai.go.jp/jishin/syuto/denkikasaitaisaku/1/pdf/4.pdf (Supports the features of each type, price estimates, and lighting preparations.)