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Selection Of Motor Circuit Breaker

Mar 24, 2017

1. circuit breaker the rated voltage and rated current
When used, must be taken into account: rated voltage shall not be less than the normal operating voltage of the line; The rated current shall be not less than the load (motor) operating current.
2. the thermal release setting current
Should be under the control of the motor rated current or load current.
3. the electromagnetic releaser instantaneous trip setting current
Instantaneous trip current setting should be greater than the peak current of the load circuit is working correctly. For the motor, and DZ-automatic switch electromagnetic releaser Iz instantaneous trip setting of current can be calculated according to the following formula
K is the factor in the formula, 1.7;
Iq-motor starting current.
For example: a pumping station with a 5.5kW motor, voltage 380V, current 11.25A, 7 times starting current the rated current, select automatic air circuit breaker the rated tripping current and electromagnetic releaser of instantaneous action set current.
Solution: (1) determine the rated tripping current according to the rated current of the motor 11.25A, heat release usually 15A rated current range.
(2) check the electromagnetic releaser of instantaneous action setting current electromagnetic releaser current instantaneous action setting for 10x15=150A. Iz ≥ kIq according to formula, it should satisfy the Iz ≥ 1.7x7x11.25=134A.
4. circuit breaker between selective coordination
Selective coordination between breaker principles are:
Shang, and subordinate circuit breaker device of protection characteristics cannot intersect, for before and after model same of circuit breaker device, General requirements Qian a level (route) circuit breaker device of off buckle current, should than Hou a level (extension) circuit breaker device of off buckle current big level above; if before and after circuit breaker device model different, is to ensure selective, Qian a level circuit breaker device should with delay action of off buckle device.
5. circuit breaker for use with fuse
Action at a distance and request high situation on selective protection, although the use of circuit breakers, but due to its breaking capacity is the capacity of fuses for low (especially small-capacity circuit breaker breaking capacity, more than fuses), protection did not do well. To this end, the circuit breakers and fuses in series. To do so, that can be automated and high breaking capacity, making protection more reliable. When using both, should be aware of the following:
(1) when small overload and short-circuit current is required, the circuit breaker; large short-circuit current, is protected by the fuse. To this end, the two characteristic of the junction current (lower limit upper limit equivalent to fuse and circuit breaker protection current value of the intersection point), should be less than circuit-breaker breaking capacity. When once you exceed the transfer of current of short circuit current values can be ensured before the fuse in the circuit breaker fuse.
(2) fuse installed before the circuit breaker as far as possible, in order to ensure safe use.
(3) the combination of current limiting circuit breaker protection, when used without fuses.