During electrical installation, we are used to holding a cable in our hands, in which each core has its own insulation, and all the conductors are bundled into one common rubberized sheath, usually white or black colors.
We use these power cables for laying in walls, under the ceiling and in screeds. They can also be used for laying inside the frame walls of wooden houses, but with the obligatory condition - electrical cables must be placed in a non-flammable corrugation, which is done for fire protection.
But, this is not the case in all countries! For example, the most common cable used for electrical wiring in private homes in the United States and Canada is the NM cable, which stands for "No Metall" or "cable with non-metallic shell ". It is made in a yellow or orange sheath, the color depends on the cross-section of the cores.
The NM cable is used in US and Canadian residential applications and is used exclusively for internal wiring. It is immediately striking that the installation of power cables inside the frame flammable walls does not assumes any kind of protection in the form of corrugation or other protective sleeves, in contrast to our requirements.
The fact is that under the rubber sheath there is also an insert made of cardboard, which is impregnated with a non-combustible substance, so the cable is considered non-combustible.
This is what the NM cable looks like:
The next cable is UF-B cable
This cable looks like this:
It has a gray shell and is used in residential construction for underground laying. Additional protection in the form of sleeves and corrugations is not required for underground installation.
Both of these cables have been in use in the United States for over 5 decades and according to statistics, not a single case of wiring fire has been recorded, provided that the cable was laid correctly (according to the Code). Like ours, power lines are powered through automatic machines and RCDs, which protect circuits from overloads and short-circuits, so how circuit breakers open electrical circuits much faster than the temperature rises in veins.
US cables have three or more conductors, and as you have already noticed, one of the conductors, which is in the center, has no insulation. This conductive core is called Protective Earth (PE), or in our opinion - grounding conductor.
In installation, such a cable is extremely inconvenient, electricians, when installing sockets, often close this core with neighboring ones and the RCD is constantly triggered. Many people wrap the bare core with electrical tape, leaving the tip for connection to the socket terminals ...
NM cable:
Have you really saved? But no!
The National Electrical Code (NEC), by which electrical installations are performed in the United States, must take into account the likelihood of a fire in the wiring and, in order to increase the degree of protection against fire, the grounding conductor is performed without isolation. Here the logic contradicts ours, we do our best to protect the conductors from their contact with each other, while US specialists, on the contrary, show interest in the fact that if there is still damage or fire, then it is necessary for the protection to work faster without wasting time for the excess layer to burn out isolation.
A number of studies were carried out and the results showed that if the insulation of any of the conductors is damaged, the probability of their closure with a core grounding increases when it is without insulation, and therefore the likelihood of protection operation in the shield also increases!