One of the most popular ways to connect suburban individual housing construction to the electrical network is air input. Recently, self-supporting self-supporting insulated wires in various versions of their design are often used for this, Figure 1. They compare favorably with the previously widespread bare wires by the presence of insulation. The latter not only provides convenience in work and subsequent operation, but also reliably protects the wire from the effects of precipitation and other atmospheric influencing factors.
Features of insulation of wires SIP
Insulation of wires SIP belongs to the hose type and is formed from cable polyethylene (PE). This material, as you know, differs from other compounds used for this purpose (PVC, polyamide, polyurethane) a unique combination of very low water absorption coefficient and high chemical neutrality to acids and alkalis. The operating temperature range of PE fully complies with the requirements for external gasket products. The necessary resistance to UV sunlight is achieved by adding gas soot to the PE, the presence of which gives the insulation its characteristic black color.
The main disadvantage of PE is its high fire hazard. The cause of a fire is an external source of flame or a short circuit in the network, which is accompanied by the release of a significant amount of heat. Moreover, PE-insulated cables themselves become sources of secondary ignition.
According to the degree of fire hazard, cable products are divided into groups, each of which corresponds to its own type of test, Figure 2. PE-insulated cable is at the bottom of this hierarchy without special measures. This is due to the fact that the oxygen index of the PE is approximately 18, i.e. the insulation made from it burns perfectly in the open air.
Another feature of cable PE is poor thermal conductivity. When the line is overloaded, this is accompanied by a rapid increase in the temperature of the wire, which leads to softening of the insulation and is accompanied by an increase in the likelihood of fire.
Compliance with fire regulations when organizing cable entry
The high fire hazard of polyethylene has become the reason for the direct prohibition of PUE on the laying of cables with these materials in the structure along wooden walls. To connect the internal wiring part of the house, the only effective way to comply with the regulations is to switch to a certified indoor cable even before entering the building. Figure 3 shows the main options for organizing such a transition.
Other methods are known for solving the problem of achieving the required level of fire safety for external cables. The greatest successes have been achieved in suppressing the flammability of the PE itself by adding the so-called. fire retardants. Unfortunately, this is accompanied by a deterioration in the mechanical parameters of insulation and an increase in the price of products, as a result of which this technique has not become widespread.