# What are the reasons behind marking neutral and line terminals on a power supply AC input?

Even though AC has no polarity, almost all the power supply AC inputs or most AC powered device input terminals indicate line and neutral terminals as L and N just like in the following example:

Is there a fundamental reason to wire line to L and neutral to N but not the other way around? Could you give an example what might go wrong if one connects neural to L terminal and line to N terminal?

• N is fairly close to ground voltage and thus usually (i.e. in the absence of fault conditions) safer to touch than L. But don't taek that as a given, because fault conditions can exist (like swapping N and L).
– user16324
Commented Feb 27, 2020 at 10:46
• What do you mean by AC not having polarity? The neutral N wire has 0V potential that is connected to earth/ground, and the live L wire has a sine wave that goes between -325 and +325 volts in the case of 230VAC. Commented Feb 27, 2020 at 10:48

One reason is that typically the internal fuse is on the live wire. In the event of a catastrophic internal failure causing a short to the PSU output the circuit will be dead when the fuse blows. With reverse connection the fuse might not blow and even if it did the internal circuit would be live.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Left: correct. Right: incorrect.

• This is very reasonable! Commented Mar 11, 2020 at 8:47

One example where Live/Neutral does make a difference are some lamp fittings. For E27 and E14 fittings the "screw" part is more likely to be touched by the user when changing the lamp. That means that the screw part should connect to the neutral instead of live.

The other fittings are symmetrical so there is no difference.

Regarding electronic devices I have yet to see an example where the mains connection is not symmetrical. That makes sense because as remarked in Justme's answer, it depends in which country you are if the Neutral and Live are always connected in the same way (UK plug) or that they can be swapped (EU plug). To be on the safe side, the equipment is designed such that the live-neutral connection can be swapped.

In some circuits, especially permanently installed circuits, it might make a difference. Even if it does not make a difference, regulations can require clear markings how to connect equipment, so that there is consistent notation. But for normal equipment it does not matter much, as in some countries you can plug in the mains cable in any orientation so it will not be guaranteed which wire is live and neutral.

There are some good answers to this question at Why are some AC outlets and plugs polarized?

In particular https://electronics.stackexchange.com/a/15232

The currents through both pins are equal and opposite, but the voltages on each pin are not. The neutral is roughly 0 V relative to the Earth at all times. The hot alternates between positive and negative.

As Bimpelrekkie points out, depending on the appliance, there's a chance of coming into contact with one of the two sides of the circuit. You want that to be the neutral side not the hot side to lessen the chance of shock/electrocution.