A Brief Overview of Electricity in Japan

Although Japan is used as an example here, the general principles should be fairly universal across most countries! In Japan, there is a professional qualification exam called the “Electrician” (電気工事士, Denki Kōjishi). The voltage range one is permitted to work on varies depending on the certification level. General household appliance wiring and similar tasks fall under the scope of the Class 2 Electrician, while working on power lines on utility poles falls under Class 1.

Power Transmission

The electricity we use today is delivered from power plants. The fundamental principle of power generation is electromagnetic induction—when a magnetic field changes, an electric field is induced. In practice, power plants utilize energy from thermal, nuclear, or wind sources to drive a turbine/engine, which rotates magnets past stationary coils (or vice versa) placed at different positions to generate electric power.

Electrical transmission lines have inherent resistance, and longer distances mean greater resistance. To reduce transmission losses, power plants transmit electricity at high voltages, stepping it down via transformers to 100V only when it reaches commercial buildings or residences for consumption.

Single-Phase Three-Wire

One thing to note is that to increase the efficiency of power generation, three-phase power is generally used. While the number of phases could theoretically be increased indefinitely, research has shown that three phases offer the highest efficiency and economic viability. Adding more phases requires extra wiring and increases maintenance costs.

In Japan, regular households typically use a single-phase three-wire system (split-phase). The reason is that it provides two live (hot) wires, which can form a 200V supply for high-wattage equipment such as air conditioners.

Distribution Board

This section is based on my own observations compared with articles online. If any readers living in Japan spot any mistakes, please feel free to correct me.

In my experience living here, Japanese homes usually place the distribution board near the washroom/dressing area. Each household receives three incoming service wires (two live wires and one neutral wire). They first connect to the “main earth leakage circuit breaker” (主幹用漏電ブレーカー) before power is distributed to individual branch circuits and outlets.

It primarily serves a few functions:

1. Tripping when an overcurrent occurs due to a short circuit or the simultaneous operation of multiple high-wattage appliances (e.g., using a microwave and a rice cooker at the same time).
2. Tripping when the leakage current exceeds a certain threshold.
3. Open-phase (neutral loss) protection.

Open-Phase Protection

In a single-phase three-wire system, if the neutral wire accidentally breaks or an incident causes an open-phase condition, appliances that were originally connected in parallel will effectively end up in series across the two live wires, as illustrated below. Because they are now connected across both live wires, the total voltage becomes 200V. If the divided voltage exceeds the rated voltage of an appliance, it can damage the equipment or cause an accident. In this scenario, open-phase protection helps by detecting whether the voltage on the load side exceeds the rated voltage; if it does, the breaker will trip.

Grounding

Earlier, we mentioned leakage current, but whether leakage current can be detected is closely tied to whether proper grounding is present.

In Japan, grounding is primarily categorized into four types: Class A, Class B, Class C, and Class D grounding. Here, I’ll introduce the two most common ones: Class B and Class D.

Class B grounding is connected to the secondary side of the distribution transformer and tied to the neutral wire. Its primary purpose is to protect against internal faults between the high-voltage and low-voltage windings of the transformer. The two sides of the transformer operate at entirely different voltage levels (high voltage vs. low voltage). If an accidental contact or breakdown occurs between them, the voltage on the low-voltage side will spike. In Japan, distribution power lines run at 6600V; such a surge could destroy all household appliances. Class B grounding creates a closed circuit if the high-voltage and low-voltage sides make contact, allowing protective devices to detect the fault and cut off power.

Class D grounding is the type of grounding work used for lower voltage applications (typically low-voltage equipment). In practice, it involves driving a copper rod into the ground and ensuring the grounding resistance remains below 500 ohms (or 100 ohms depending on the breaker setup).

This equipment ground is completely separate from the Class B grounding mentioned earlier. Even though both are called “grounding,” their purposes are different. Therefore, saying that “the neutral wire is the ground” is not entirely accurate, nor does grounding simply mean connecting to the neutral wire.

Electric Leakage

If, for some reason, an appliance’s metal chassis comes into contact with a live wire and causes a ground fault, touching the appliance without proper grounding will complete a circuit through your body: Chassis → Human Body → Floor/Ground.

However, if the appliance is properly grounded, the ground path offers significantly lower resistance than the human body. Even if a person touches it, most of the current will flow through the ground conductor. Furthermore, homes in Japan typically feature Earth Leakage Circuit Breakers (ELB). Under a leakage condition, the breaker should trip even before anyone touches the appliance.

Comparison with Taiwan

Looking at the situation in Taiwan, discussions frequently mention bonding equipment grounding with system grounding (inside the main electrical panel). However, I haven’t seen similar discussions in Japan. I arrived at the following conclusions, though I am not completely sure if they are entirely accurate:

1. When equipment ground and system ground are not bonded, grounding reduces the shock current during a fault, but the standard overcurrent circuit breaker won’t trip if the fault current threshold isn’t met. Consequently, you could still experience a shock even with grounding, though most current is shunted away through the ground.
2. When bonded, an insulation fault directly causes a short circuit back to the source, resulting in a large current that immediately trips standard circuit breakers.

The reason why Japan might not bond equipment ground to the neutral at residential panels (or perhaps they do in some systems and I haven’t confirmed it) is that virtually all modern Japanese residential distribution boards come equipped with Earth Leakage Circuit Breakers (ELBs). As long as there is leakage, the breaker will trip immediately regardless of bonding. In contrast, perhaps not all homes in Taiwan are equipped with residual current breakers on their main panels?

Conclusion

After living in Japan for a while, I naturally grew curious about the differences between both regions. I haven’t fully mastered all the practical electrical distribution details and field practices of both places yet, so this article is based on my current understanding and online research. Hopefully, this post serves as a stepping stone and invites electrical experts to chime in!