People have been using household electrical appliances for so long that they have already forgotten that they are actually not as safe as they seem at first glance. This is especially true for devices without a solid plastic case, which is an excellent dielectric. In all other cases, there is by no means a zero probability of getting an electric shock. Especially if the equipment is old and not in the best condition.
In order to prevent electric shocks when connecting electrical appliances, a variety of technical solutions are used. Today, first of all, they try to make plastic contacts and exclude the possibility of human interaction with the metal parts of the structure. However, the most reliable method of protection, proven over the years and used in technology to this day, has been and remains the grounding of an electrical appliance.
Grounding is the intentional connection of any point on the electrical network or electrical appliance to a grounding device. With its help, protection of a person from the dangerous effects of electric current is achieved by reducing the voltage to safe values. Different devices use different grounding methods. And as you might have guessed, the mysterious holes in the euro sockets are needed just for this. True, in our realities, they are most often useless. But do not despair, because not everything is so simple.
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So let's start with holes. In European-style plugs, they are made so that a special grounding pin is inserted into them, which is installed in the outlet. Most often, such sockets can be seen in the countries of the European Union. Although in practice a variety of plug-sockets are used in European countries. It's just that this one is universal. There are dozens of varieties of plug sockets. The plug socket is a "French type".
On many euro plugs you can see two metal strips in the recess, while in euro sockets used, including in Russia (for example, in network splitters), everyone must have seen a pair of metal staples on the sides. This is another grounding mechanism. It is called the "German type". At the same time, very often modern "European" plugs combine the tools of the French and German grounding methods.
In continuation of the topic, read about why in Russia in sockets 220 voltsif the US has 110 volts.
Source: https://novate.ru/blogs/040622/63185/