Selection of MCCB on main line and branch circuits

Selection of MCCB on main line

(1) When the loads are motors, etc.

When motors, etc. are connected to the main line, MCCB rated current shall be less than the value obtained by multiplying the sum of rated currents of the motors by 3 and adding the sum of the rated currents of other loads to the tripled sum. However, when the sum total exceeds 2.5 times the allowable current of the main line, the rated current shall be less than the value obtained by multiplying the allowable current by 2.5. If the allowable current of the main line exceeds 100A and the value does not conform to the standard rating of any MCCB, it is allowed to select the rating just above the value. Actually, select MCCB in accordance with the following procedures. Divide the loads on the branch circuits into groups of motors which will start simultaneously. Regard each motor group as one motor (hereinafter, referred to as a synthesized motor) which has the total full-load current of the full-load currents of the motors in the group, and the synthesized motors will start successively. Determine the rated current of the circuit breaker for the branch circuit of each synthesized motor. The maximum rated current is IB max. When the full-load currents of other synthesized motors is I1, I2 ···· In-1, the rated current IB of main line circuit breaker can be obtained by the following formula.

IB = IB max + (I1 + I2 + ······· + In-1) 3 D

D is the demand factor, and, if it is unknown, it is regarded as 1.

(2)When loads are only lamp and heater circuits

MCCB rated current shall be less than the allowable current of the main line and determined by multiplying the sum total of the rated currents of MCCB on each branch circuit by the demand factor.

Selection of MCCB for lamp or heater branch circuit

The lamp and heater circuits refer to circuits on which the starting current and starting time are not so significant that the operation of MCCB is affected. For lamp circuits for mercury lamps, etc. which have rather large starting current 3 long starting time, select MCCB in accordance with the procedures for motor circuits. It is better to allow a margin between the load current of lamp or heater circuit and the rated current of MCCB for the following reasons.

1)     MCCB are designed to protect wires on the outside of panels according to the temperatures in the panels. Generally, MCCB are adjusted based on an ambient temperature of 40°C. If the estimated maximum temperature in a panel is higher than 40°C, it is better to reduce the load at a rate of 1% per difference of 1K.

2)     In addition, it is better to allow a margin of 10 to 15% separately from the margin stated (in the above point) in consideration of difference between nominal value and actual value of full-load current of load device, increase in full load current due to deterioration of load device and fluctuation of supply voltage and frequency.

Selection of MCCB for motor branch circuit

When selecting the rated current of MCCB for a motor branch circuit, it is necessary to take into consideration that considerably larger transient currents, such as starting current and starting inrush current, than the full-load current will flow. Select MCCB rated current to prevent operation of MCCB due to these starting transient characteristics. The relationship between them is shown in Fig. 5. 2.

(1) Starting inrush current

The starting inrush current reaches the maximum value in 1/2 cycle after power is applied and then rapidly attenuates. The starting inrush current was explained in detail in “Protection Coordination” of 4. 5. 3 and 4. 5. 4. If the starting inrush current enters the operating range of the instantaneous tripping element of MCCB, MCCB will trip. Select MCCB which has an instantaneous tripping current higher than the starting inrush current depending on the starting method.

     a. In the case of full voltage starting (direct-line starting)

Starting inrush current owing to superposition of transient current caused by low power factor of starting current and reduction of motor impedance caused by saturation of magnetic path may lead to incorrect operation of MCCB. To prevent the incorrect operation, the instantaneous tripping current of MCCB must be approx. 12 times higher than the full-load current.

     b. In the case of star-delta starting

If the phase of residual voltage is reversed to the phase of supply voltage on an open transition system, starting at the supply voltage corresponds to starting at overvoltage, and the power factor of current upon switching is improved compared to that upon starting in the stopped state. However, to prevent incorrect operation caused by starting inrush current owing to superposition of transient current, the instantaneous tripping current of MCCB must be approx. 17 times higher than the full-load current. On a closed transition system, the instantaneous tripping current of MCCB is allowed to be almost equal to that in the case of direct-line starting.

     c. In the case of instantaneous restarting

As in the case of star-delta starting, if the phase of residual voltage is reversed to the phase of supply voltage, starting at the supply voltage corresponds to starting at overvoltage, and inrush current flows. The instantaneous tripping current of MCCB must be approx. 19 times higher than the full-load current.

     d. Plugging

Since there is a phase shift of 120° between residual voltage and supply voltage, starting at the supply voltage corresponds to starting at overvoltage, the power factor reduces considerably, and large starting current flows. The instantaneous tripping current of MCCB must be approx. 29 times higher than the full-load current.

(2) Starting current and starting time

The multiplying factors stated in a, b and c apply in the case where the starting current is 8 times the full-load current. The duration of starting current is affected by the inertia moment of load. Generally, for standard motors, if the starting time is less than 15 s, they are considered to be improper when the safety time exceeds 15 s.

(3) Selection

Concretely, select MCCB in accordance with the following procedures.

      a. When the starting time is relatively short

When the starting current is 600% and the starting time is within 2 to 3 s, a motor breaker can be used. However, the scope of protection is limited depending on the device to be protected as shown in Table 5. 2.

      b. When the starting time is relatively long

When the starting current is 600% and the starting time is within 10 s, apply a combination starter method with MCCB and magnetic switch.