The third-level distribution box needs to be connected to neutral
The neutral conductor is connected to earth at the distribution transformer, at each consumer's installation and at specified poles or underground pillars. With the neutral connection all lamps have the same voltage on them (assuming equal voltages P-N) despite there being only half the resistance due to the double load on L3. Though a 100% balanced load cannot be achieved most of the time, a higher percentage. (3) Power distribution from tertiary switch boxes to electrical equipment must follow the "one machine, one switch" principle, with no branching allowed.
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