Fires of electrical reasons, make up around 20-25 % of total number of fires in the world. The most flammable (more than 60 % of the total number of electrical fires) are cable products. In the operation of the power plant possible an emergency mode is parallel or serial arc fault. It causes a fire due to the high temperature of the arc up to 7000 °C. It was found that fire arises primarily as a result of the electric arc. An arc fault is often a consequence of damage of electrical insulation. AFDD/AFCI is fairly new relay device for fire safety. Standard of Russian Federation for these devices is identical to the International standard IEC 62606:2013 “General requirements for arc fault detection devices”. The standard requires limits of response time of device depending of arc fault current. A time-current characteristic is one of the most important indicators of AFDD/AFCI performance. Late trip of emergency circuit with arc fault are most likely to cause a fire of insulation. Thus, to ensure the fire safety of cable it is necessary to know and use in design of electrical networks the maximum allowable time-current characteristics of the arc fault protection device. The case of a serial arc fault by fracture of cable conductor is considered. In practice, the most frequent reasons of this are rupture of cable wire due to metal fatigue or excessive tension and damage by external object. In the damaged conductor arises the small gap which punches by operating voltage. Thus, the current on this cable continues, and remains close to the nominal value. The gap arises the arc discharge accompanied by intense heating causes further degradation of the cable insulation and arises a fire. Determination of fire risk parameters of ignition energy of an arc fault is practical and is related to the fire safety of wire and electric cable networks. By the heat balance equation in the proposed model we obtained the expressions relating the general thermophysical parameters of the process are obtained. In particular, the formula of maximum allowable time-current characteristics of AFDD/AFCI is obtained. Furthermore we done the evaluation of termophysical parameters and time-current characteristics. Then given the comparison of these results with the values of the standard GOST R IEC 62606. Conformity of the standard values and the calculated data is found. Thus, we identified the model by which the standard values are calculated.