A typical 120-volt electrical circuit has a hot wire and a neutral wire. The hot wire has 120 volts to ground and so it can easily shock you. But the neutral wire is physically connected to the ground, so it shouldn’t have any voltage present. This makes it a lot safer — but there are plenty of things that can go wrong, so please don’t ever touch any wire.
Electrical devices are designed to take advantage of this difference between the hot and neutral wires. Any appliance that has a switch is going to be safer if, when the switch is off, there’s no voltage inside the appliance.
The way to be sure that there’s no voltage inside the appliance when the switch is off is to put the switch on the hot wire. So when the switch is off only the neutral wire is still connected back to the electrical panel, and since the neutral wire is connected to the ground there’s no voltage difference to push electrical current.
Here’s a simple schematic of an appliance showing the issue. The switch is on the hot side of the appliance, so that when it’s switched off, the load is only connected to the neutral wire, which is connected to the ground and so it should have zero electrical potential. The appliance is much safer this way.
A typical appliance plugged into a receptacle with correct polarity. When the switch is off there are no live electrical connections in the appliance.
To make sure this happens, we use polarized plugs and polarized receptacles. Plugs on devices that need to be polarized have one blade wider than the other. And all receptacles have one slot wider than the other, so that the plug can only physically be inserted in one way. This makes sure that the appliance sees the proper polarity.
One blade of the plug and one slot of the receptacle are wider (this is the neutral side), so that the plug can only be be inserted with one orientation. This ensures proper polarization — as long as the receptacle is wired correctly.
Of course the only way this works is if the wires feeding into the electrical receptacle are connected in the proper way, with the hot wire attached to its proper location on the receptacle (the narrow slot), and the neutral wire connected to its proper location (the wide slot). If the receptacle is wired backwards, then we have a condition known as reversed polarity.
Here’s our appliance schematic showing what happens if it’s plugged into a receptacle with reversed polarity. Now even when the switch is off the load inside the appliance is connected to the hot side of the wiring and all the electrical components are live all the time. This dramatically increases the chance of electrical shock.
An appliance plugged into a receptacle with reversed polarity. Even when the switch is off all of the components of the appliance are energized.
One very important example of this situation is a lamp. Consider the lamp holder — where you screw in the light bulb. You want the threads of the shell (green arrow in picture below) to always be on the neutral side, because it’s fairly easy to touch this accidentally. You want the button at the bottom (red arrow) to always be on the hot side, because it’s hard to touch it. If a lamp is plugged into a receptacle with reversed polarity then the shell of the lamp holder is always going to be live, even when the lamp is switched off, and so it’s easy to be shocked when you change the bulb.
If a receptacle has reversed polarity it’s probably just because the wires at the receptacle are reversed. This is an easy fix. But it might be the case that the reversal was done somewhere farther back upstream. An electrician needs to find the source of this problem.