Our compatriots have long been accustomed to the fact that when cables for electrical installation are in their hands, each core in them will have a separate insulation. But in the United States of America, this is not the case. Often there you can find cables with several cores, one of which has no insulation at all. A logical question arises: why are they made at all? Are overseas manufacturers really trying to save money?
In the expanses of the former Soviet Union, when performing electrical installation and laying cables in walls, under ceiling or in screeds, as well as inside frame walls, it is prescribed to use a special non-flammable corrugation. Cables are placed in protective tubes in order to ensure an adequate level of fire safety. A similar approach is used in Western European countries. But in the United States of America and Canada, the approach to wiring is different.
In the mentioned countries, special cables of the category “NM” - “No Metall” are used in residential construction for internal wiring. They do not need to use a protective non-flammable corrugation or any other protective sleeves, since their design already has a special protective layer right from the factory. It is made of cardboard, which is impregnated with non-combustible substances.
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But what about the extra bare wire? Moreover, it is another important protective element of the No Metall cables. An additional wire without insulation in the US technical documentation is called "PE" - "Protective Earth". And as you might have guessed from the name, this is a grounding conductor, which, according to local rules, must be in every cable used for residential construction.
In continuation of the topic, read about what things you should not save on during the repairso that you don't regret it later.
Source: https://novate.ru/blogs/020122/61732/