LV Motor Protection - LS"I" Recommendations

Instantaneous Release setting recommendations:

Instantaneous Release setting in the MCCB is set to a value higher than the highest anticipated magnetizing inrush current, while switching-on the motor.

Lets Have a Generalized approach to evaluate best setting,

Staring Current (Ist) 

Motor rated current (IrM)

Tolerance on Starting current:

Ratio Ist/IrM is normally allowed to be 20 % higher than the rated value given by the manufacturer, this needs to be checked in datasheet furnished by manufacturer (The ratio Ist/IrM takes care of the saturation of the reactances of asynchronous motors).

Hence,

Maximum anticipated starting current:           = 1.2 x Ist

Transient inrush current (peak):                     = √2 x 1.2 Ist   = 1.697 x Ist

As per IEC 60947-2, there is a tolerance of +/- 20% in the values of current for tripping of the circuit breaker by short-circuit releases.

Hence,

Effective setting of the instantaneous release setting in the MCCB should be above this tolerance band

Suggesting Setting: 1.697 / 0.8 = 2.1213 Ist (peak value)

We should note that Instantaneous setting in the MCCB is in RMS value

Hence,

Suggesting Setting: 2.1213 / √2 = 1.5 Ist

Thus, the instantaneous setting in the MCCB has to be above 1.5 Ist

i.e; if Ist/IrM = 6 then Instantaneous stage setting should be set ≥ 9 x IrM

Reference: Paper by KSK on MCCB for LV Motor feeders