Examples of selection of fuses and circuit breakers

Example 1. The main line of the electrical network of an industrial enterprise with a voltage of 380/220 V supplies a group of electric motors. The line is laid indoors with an armored three-core cable with aluminum cores and paper insulation at an ambient temperature of 25 ° C. The long-term rated current of the line is 100 A, and the short-term current when starting the motors is 500 A. The start is easy.

It is necessary to determine the rated current of PN2 type fuses protecting the line and to select the cable cross-section for the following conditions:

a) the production area is non-explosive and non-flammable, the line must be protected from overload;

b) the room is a fire hazard, the line must be protected from overload;

c) the line must be protected only against short-circuit currents.

Answer. Determine the value of the rated current of the fuses of the fuses protecting the line, for the continuous current: AzVT = 100 A, for the short-term current: Azvt = 500 / 2.5 = 200 A. Fuse type PN2-250 with fuse 200 A.

1.For a cable with paper insulation, protected from overload and passing into a non-explosive and non-flammable room, the value of the protection factor ks = 1. In this case, the continuous current load of the cable Azadd = ksAzh = 1×200 = 200 A.

We choose a three-wire cable for a voltage of up to 3 kV with aluminum conductors with a cross section of 120 mm2 for laying in air, for which the permissible load Azadd = 220 A.

2. For a cable operating in a fire-hazardous room and protected against overload k2 = 1.25, then Iadd = 1.25, I3 = 1.25x 200 = 250 A. In this case, the cable section is taken equal to 150 mm2, Iadd = 255 A.

3. For a cable protected only against short-circuit currents, we get at ks = 0.33 permissible current Azaddition = 0.33Azvt = 0.33 x 200 = 66 A, which corresponds to a cable cross-section of 50 mm and Azaddition = 120.

Example 2. A switchboard with circuit breakers receives power from the busbars of the main switchboard to which six squirrel-cage rotor induction motors are connected. Electric motors 3 and 4 are installed in a class B1a explosion room, the remaining electric motors, distribution points and starting equipment are installed in a room with a normal environment. The technical data for the electric motors are given in a table. 1.

Section. 1. Technical data of electric motors

The mode of operation of engines excludes the possibility of prolonged overloads, starting conditions are light, self-starting of large engines is excluded. One of the engines (1 or 2) is in reserve, the other engines can work simultaneously.

Rice. 2. Scheme for example 2

It is necessary to determine the rated currents of the breaker releases and to select the cross-sections of the wire and cable based on the heating conditions and compliance with the currents of the trip unit.

Answer. Since the air temperature in the premises is 25 ° C, the correction factor kn = 1, which is taken into account when choosing the cross-sections of wires and cables.

Line to motor 1 (or 2). Selection of combined release (breaker type A3710B for 160 A for continuous linear current Azd = 73.1 A, equal in this case to the rated current of the electric motors (table 1).

A thermal correction factor of 0.85 must be taken into account when choosing the magnetic release current rating of the circuit breaker built into the cabinet. Therefore, Aznom el =73.1 / 0.85 = 86 A.

We choose a release with a rated current of 100 A and an instantaneous current of 1600 A.

We establish the impossibility of working with the machine at startup: Azaverage el = 1.25x 437 = 550 A, 1600 A> 550 A.

We choose a single-core wire with aluminum wires of the APRTO brand with a cross section of 25 mm2, for which the permissible current load is 80 A. We check the selected cross section according to the protection factor of the device. Since the setting current is not regulated in the A3700 series breakers, the multiple of the permissible line current must be determined in terms of the rated current of the splitter, in this case equal to 100 A. Find the value of kz for networks that do not require protection from overload for the rated release current of a circuit-breaker with unregulated inverse current-dependent characteristic ks = 1.

Substituting numerical values ​​into the ratio kzАзs = 1×100 A>Азadd = 80 A, we find that the necessary condition is not fulfilled.

Therefore, we finally choose the cross-section of the wire equal to 50 mm2/AAdd = 130 A, for which the condition AAdd>xAz is fulfilled, since 130 A> 1 x 100 A.

Line to motor 3. Motor 3 is installed in a Class B1a explosion room, in connection with which:

1) the rated current of the motor, increased by 1.25 times, is taken as the rated current when choosing the cross section of the line;

2) the use of wires and cables with aluminum wires is not allowed; therefore, the line from the magnetic starter to the electric motor must be made with a wire with copper conductors (PRTO brand).

Line to the electric motor 4. The cross-section of the PRTO wire from the magnetic starter to the motor is taken as 2.5 mm2, since a smaller cross-section for power networks in potentially explosive areas is not permitted by the PUE.

Lines to electric motors 5 and b. The rated current of the line is determined by the sum of the currents of motors 5 and 6.

Main line. The calculated long-term permissible current load of the line is determined by the sum of the currents of all electric motors, except for the current of one of the electric motors (1 or 2): Azdl = 73.1 + 69 + 10.5 + 2 x 7.7 = 168 A. The short-term current load is determined by the starting conditions of motor 3, which has the largest starting current: Azcr = 448 + 73.1 + 10.5 + 2 x 7.7 = 547 A.

We choose the electromagnetic release of the circuit breaker AVM-4C for 400 A for continuous line current from the condition Az nom = 400 A>Azdl = 168 A.

The short-term current load is determined by the starting conditions of motor 3, which has the largest starting current:

Azcr = 448 + 73.1 + 10.5 + 2-7.7 = 547 A.

We select the operating current on a scale that depends on the current characteristic, 250 A, and on a scale that does not depend on the current characteristic (interruption with a time delay) 1600 A.

We establish the impossibility of turning off the circuit breaker when starting the engine 3Isral = 1.25Azcr, 1600> 1.25×547 = 682 A.

Continuous line current Azdl = 168 A, we choose a three-core cable with aluminum conductors for a voltage of up to 3 kV with a section of 95 mm2, with a permissible load of 190 A.

For networks that do not require overload protection, at the tripping current of the release of a circuit breaker with an adjustable, inverse current-dependent characteristic Azaverage el = 250 A and k2 = 0.66, Azadd> k3Is = 190> 0.66 x 250 = 165 A.

Therefore, the necessary condition is satisfied. The calculated data from the example is tabulated. 2.