Characteristics of structures of the cable penetrations are considered, and their effect on heat exchange and cables heating at continuous permissible load current is studied. Experimental technique is proposed that allows to carry out assessment of fire hazard and to investigate the processes of heat distribution in the structures of cable penetrations at their operation.
To study the effect of structures of cable penetrations on cable heating processes at the permissible load current, three most widely-spread types of penetrations were used: penetration № 1 made of mineral wool with fire-retardant coating and air gap inside the structure; penetration № 2 made of mineral wool with fire-retardant coating and with filling the internal space with fire-fighting foam; penetration № 3 with filling the entire internal space of the structure with fire-fighting foam.
The experimental data are obtained and the regularities of the temperature change of the cable elements over time with continuous permissible load current are revealed. For all types of cable penetrations at the initial moments of time (up to 25–29 min) the linear increase in the temperature of the cable elements is typical. Further, the steady-state thermal regime is observed, while the critical heating temperature of the cable is exceeded (70 °C).
The most dangerous type of cable penetration with filling of internal space with fire foam is defined. When the permissible load current is flowing, the cable elements in such a cable penetration reach an overload temperature of 90 °C after 50 minutes with a further increase in temperature values and the development of the process of thermal decomposition of the insulation and the cable sheath. Heating dynamics of the cable elements allowed to estimate the time of complete decomposition of the insulation and cable sheath (9 hours) and the occurrence of a short circuit (23 hours).
The calculations of continuous permissible load currents for the considered cable and cable penetrations structures were carried out, which showed that in order to eliminate emergency heating of cable products during their operation, the reduction in the normative values of continuous load currents is required.