Sub-Circuit protection

Sub Circuit Protection

H R C fuses

When a motor is first connected to the line, the power consumed is very much higher then after the motor comes up to speed. The starting current of a motor may be 5 to 8 times the full-load current. For example, an ordinary single phase 1H.P,230V motor consuming only 8A while running at full speed and full load, will require as much as 48 A, when motor is started. When motor is first connected to the power line, it is not turning, so that locked-rotor condition does exists, until the motor starts to turn. The wire to the motor must then be big enough to carry easily its starting current momentarily, its running current continuously and its normal current plus a considerable overload current for short periods. There fore it is advised to use circuit breaker or fuses. The fuses or circuit breakers which are installed in branch circuit of motor, protect the branch circuit against overload condition. The rating of these fuses or circuit breakers always depends on the size of the wire in branch circuit. These fuses or circuit breaker have amperage rating sufficient to carry the starting current of the motor and rarely will protect the motor against damage from continued overloads.
In sub-circuit, if we use fuses for protection the H.R.C fuses are mostly preferred, because it's fusing factor (The ratio between minimum fusing current to rating current is called a fusing factor) is 2. In some circuits, the starting current of the motor is large, in these circuits circuit breakers are used for protection. The small motors are feeding from the multi-way distribution boards, always use H.R.C fuses on these distribution boards.
When we choose H.R.C fuses for a 3-phase motor circuit than be carefully select the rating of fuses. Because, if we choose the low value of the H.R.C fuse, then the fuses protecting the circuit are not large enough to carry the starting current for a sufficient time it is possible that one may operate, thus causing the motor to run on two phases. This could serious damage to the motor.