Since the alternating current flows from the live line to the zero line, and then from the zero line back to the live line, why can people touch the zero line without electric shock, but touch the live line with electric shock?

This is about to start from the power supply system. The figure below is our common TN-S power supply system.

TN-S power supply system

As we can see from the picture above,

In TN-S power supply system, the zero line at the low voltage side of the transformer is grounded. If the earth is taken as the reference point, the potential between the zero line and the earth is always zero. When people stand on the ground, the potential of people and zero line is always equal, so there is no potential difference (i.e. voltage), so there will be no electric shock. But the line of fire is different.

Let’s look at the waveform of alternating current.

AC waveform

If the earth is taken as the reference point, the potential between the zero line and the earth is always zero.

At 0-2, the potential of the live wire is + 220 V, so the current flows from the live wire to the zero wire;

At the moment of 2-4, the potential of the live line is – 220 V, so the current flows back from the zero line to the live line.

In any case, the potential of zero line and fire line is always kept at 220 v.

Editor in charge: LQ