MCCB Short-circuit range

Thermal Capacity 

When a large current flows on a wire for a short time (JIS C 0364-4-43: 5s or less), the following formula can be established assuming that all of the generated heat is accumulated in the conductor. (When conductor is copper)

I  : Short-circuit current active value S : Wire cross-section area (mm2)

t : Short-circuit current passage time (s)

T : Conductor temperature at short-circuit (°C)

T₀ : Conductor temperature before short-circuit (°C)

The relation of this formula is shown in Fig. 4. 23.

It is assumed that the short-circuit occurred when the wire was passing the tolerable current (T₀ = 60°C). If the temperature that can be tolerated as the short-circuit conductor temperature T is 150°C, then based on Fig. 4. 23

I2 t = 14000S2

The tolerable I2 t calculated with the above formula is shown in Table 4. 8.

Notes (1) Tolerable I2·t is calculated with hot start from 60°C, assuming that all generated heat is accumulated in the conductor, and that the conductor tolerable maximum temperature is 150°C.

(1)     Fa calculates the tolerable compressive strength when the insulator thickness drops to 60%.

(2)     ia is the instantaneous current value at which a suction force equal to Fa is generated, but in a normal circuit, the 

          current flows in the opposite direction and ia will be the reaction force equal to Fa.

(3)     Ia indicates the symmetrical active current value when the reaction force relative to Fa or the suction force 

          equal to 1/3Fa is generated in the 3-phase circuit.

Is is the tolerable short-circuit current symmetrical value limited by the tolerance I2·t when a half-cycle (10ms) interruption in respect to 14mm2 or less, and one cycle (20ms) interruption in respect to 22m2 is considered.

Electromagnetic mechanical strength

            When currents flow in the same direction to a parallel wire, the currents will mutually attract. If flowing in the opposite directions, they will repulse. The size of this force is expressed with the following formula:

F : Force applied on conductor (N/cm) D : Conductor pitch (cm)

i : Current instantaneous value (A)

Geometric mean when currents of two conductors are different

Where, the above formula applies when the length of the section where the parallel conductors run parallel is longer than the pitch D (5-times or more).

The conductor’s compression strength and the support’s strength must be considered so that the insulated wires do not compress each other during a short-circuit and cause an insulation breakdown.

If the wire’s effective compression area is 20(mm2/cm), then the wire’s tolerable compression strength Fa (MPa) will be as shown in Table 4. 7. In the above formula, C is 40% (mm) of the conductor thickness and d is the conductor’s outer diameter (mm).

The conductor pitch D during a short-circuit shall be the value (cm) obtained by subtracting the conductor’s compression amount from the wire’s outer diameter.

When the tolerable instantaneous short-circuit current ia is calculated with the above formula, the results are as shown in Table 4. 7.

In the event of a 3-phase short-circuit, each phase’s maxium instantaneous value is not attained simultaneously, so the tolerable instantaneous current can be larger than the above ia.

If the active value of the sine wave current distanced by 

is I , then the maximum sum of the instantaneous values in the same direction will be, 

 for opposing directions. However, when considering the transient direct current element when the switch is turned ON, then, each will be as follows:

= is the circuit resistance or reactance ratio.

If the Ia equal to the square of the above ia is obtained, the following will apply for the currents in the same direction (attraction force)

When this tolerable short-circuit current Ia is obtained using the Table 4. 7 tolerable instantaneous current ia, the results are as shown in Table 4. 7.

Generally with a 3-phase electrical circuit, the force of repulsion is larger than the attraction force so the tolerable current from the force of repulsion is smaller. Once the current is repelled and the distance between wires increases, both the attraction force and force of repulsion decrease and try to find a balancing point.

As explained above, if the distance between wires is small, the wires must be mutually be supported strongly taking the above force of repulsion into consideration. Special caution must be taken to prevent excessive force from being applied on the connections and terminals.

Table 4. 9 shows the electromagnetic force when the distance between wires is 10cm and 20cm

Table 4. 9 Electromagnetic force applied per1m of conductor (for 3-phase short-circuit) N