Helen Podobreeva

         New residual-current device develops in Ukraine. Wide-ranging innovation in the power supply systems of apartment houses and public places possibly in the future [1].

         Short circuits (s.c.) and developing currents of leak through the isolation of the electric wirings are principal reason of fires in electricity generating plant (to 70 % from the general of electro-fires). The electric wirings are most the fire-hazardous type of electrotechnical wares.

         The low level of fire safety is explained of the rows factors:

- bad technical state;

- low quality of electrical appliances;

- absence of effective services of safe exploitation of electricity generating plant;

- non-observance rules of fire safety during exploitation of domestic technique;

- very low efficiency of the electric protecting from malfunctions [4].

         A fire can be caused the leakage current and to result in s.c., and also development of leakage current can result in a short circuit and subsequent fire [7].

         Safety devices and circuit breakers on the leakage current through an isolation are not respond. At the present time the very reliable mean of electric defence is a residual-current device (RCD). New principles of estimation of efficiency of electric defence are developed in Kievan State Technical University, which take into account time of its operation and fire-hazardous consequence of s.c. [2].

         By the instrumentality of a complex "ARIAS" the calculation of the followings indexes is produced:

- probabilities of ignition of isolation of cable wares at each of types of s.c. on every area of network;

- probabilities of fire on the object of power supply at each of types of s.c.;

- probabilities of fire at all types of s.c.;

- probabilities of fire at all types of k.z. without the stocktaking of k.z. on a corps.

         The danger of defeat is conditioned tension of touch or, otherwise speaking, by a current, passing through a man, As is generally known,

(1)

         where is resistance of body of man, Om.

         Principle of action is the rapid disconnecting of the damaged equipment from a network in case if a current, which passes through explorer, earthing the corps of this equipment, will exceed some limit at which tension of touch has most a legitimate value is protracted

         Principle chart is resulted on a picture. A sensor is a current relay of RT, which possesses small resistance and which is plugged directly in the section of earthing wire [7].

         Choice of setting. The condition of safe touch to the corps of equipment in the period of shorting on him of phase is expressed the following dependence:

(2)

         where is resistance of grounding of corps, Om; is complex resistance of relay coils, Om.

         Current of is a setpoint which comes through earthing explorer and determines the resulted terms of safety, i.e.:

(3)

         or

(4)

         On condition that will get

(5)

         Thereby,

(6)