Why does the voltage sag when turning on appliances in the house?

Dmitry asks:
House 15 kW 3 phases. The PEN conductor is split. Grounding done. Sockets and lighting are divorced. When you turn on the kettle or electric heater, part of the bulbs hanging on one branch is cut down and the voltage drops to 140 V. The voltage on the remaining phases is 220V and 230V. What needs to be done to prevent this from happening?
The answer to the question:
If it was a phase imbalance and a bad zero on the input, then on the other phases the voltage would increase, and you are almost normal. Check the connections, perhaps there is somewhere a bad connection in phase (or even at zero) on this line. Check all possible connection points, also check the machine. Maybe swap the wires at the poles, if the problem moves to another phase, then the problem is in the machine. But just in case, check the quality of the zero connection on the bus in the shield, and if the wire is bent away from the ground electrode.
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