Why does the switch open the phase, not the zero?

Any switch that is responsible for turning on and off the light in a room, for example, must open the phase, and not zero. A phase in an AC network is one of the conductors on which an alternating voltage is constantly present with respect to the neutral conductor. The zero conductor, ideally, has zero potential with respect to earth, which always remains so in a healthy network, since the zero conductor is, by definition, grounded.

Whether the network is three-phase or single-phase, the neutral (neutral) conductor must be earthed, so it is, in principle, much safer than the phase conductor. In fact, generators and transformers have grounding, from which the electric network receives energy. If the neutral conductor is not grounded, then an accident has occurred in the network, neutral conductor break.

Usually in everyday life we ​​use single-pole switches, that is, those that open or close just one wire when a button is pressed. Let's say a chandelier hangs on the ceiling, powered by a 220-volt single-phase household network. Two wires go to the chandelier, one of them is phase, the second is zero. The switch is installed in the gap of one of these two wires.

Let the switch stand on the phase conductor, and it was put into the “off” state. Then both conductors, through which electricity is supplied to the chandelier, will be de-energized, their potentials will be zero, because the neutral conductor, which was not interrupted by the switch, by definition has zero potential, and the phase conductor is interrupted by the switch, that is, there is no phase voltage.

Both conductors are safe, you can change the light bulb, repair the ceiling, remove the chandelier, etc., without fear of getting under phase voltage and getting an electric shock. Although it is better in this case to turn off for reliability automatic machine in the electrical panel.

How not to do

But what if the switch is mistakenly installed in the gap of the neutral, and not the phase conductor? In this case, even if the switch is in the “off” position, one phase conductor still fits the chandelier. The second conductor is not connected to anything.

If in such a situation you start changing the light bulb, repairing the chandelier, working with the ceiling, then you can accidentally touch the phase wire, get an electric shock, especially if you are standing on a conductive step-ladder that accidentally comes into contact with something grounded or even stands on the ground.

Replacing a light bulb can end in tragedy with human casualties. Well, if you are standing on a wooden stool, in rubber boots, while working in protective gloves. Here, everything can end successfully. But in unfavorable circumstances, the switch on the neutral conductor can turn into mortal danger.

See also:Schematic and wiring diagrams of lighting in an apartment and a house