How to determine the parameters of the circuit breaker
1. Determination of rated voltage.
The rated voltage of the circuit breaker should be greater than or equal to the normal working voltage of the line and equipment.
2. Determination of rated current.
ib≤in≤iz
In the above equation: ib--line calculates the load current (a);
in--Main contact rated current(a);
iz – Conductor allows continuous ampacities (A).
Considering that there is a certain margin, it is generally selected: in=12ib~1.4ib。The rated current of the circuit breaker should neither be too small nor too large. If it is too small, it will cause frequent false trips;If it is too large, the load will not trip if it is overloaded, and it will lose its protective effect.
3. Determination of the setting current of the overcurrent tripper.
ilmax＜i2≤1.45ilmax
ismin≥1.25i2
In the above formula: i2 - the setting current of the overcurrent tripper (a);
ilmax - maximum load current (including motor starting current, charging current of power module input capacitor, etc.);
ISMIN - Minimum short-circuit current of the line (A) (short-circuit of the line away from the circuit breaker side).
The setting current of the overcurrent tripper is less than the minimum short-circuit current of the line (the line is far away from the short-circuit breaker end), and the circuit breaker can be tripped to ensure that a short circuit occurs anywhere in the circuit. The setting value of the operating current of the overcurrent tripper can be fixed or adjustable, and it is usually the lever to adjust the current tripper.
4. Determination of breaking capacity.
icu＞ics＞ismax
In the above formula: ismax - the maximum short-circuit current of the line (a) (the line is short-circuited near the circuit breaker end);
ICS - Rated operating short-circuit breaking capacity (A);
ICU – Rated limit short-circuit breaking capacity (A).
The rated operating short-circuit breaking capacity of the circuit breaker is greater than the maximum short-circuit current of the line (the line is short-circuited near the circuit breaker end), ensuring that the circuit can be reliably broken if a short circuit occurs anywhere in the circuit. When the circuit adopts hierarchical protection, the setting value of the overcurrent tripper of the circuit breaker at all levels should match.
5. Determination of overload protection characteristics.
The overload protection characteristics should match the allowable overload characteristics of the protected equipment. The protection characteristics of the circuit breaker must be below the allowable overload characteristics of the protected equipment, i.e. the circuit is broken before the protected object reaches damage. The current-time characteristic of a thermal tripper is an inverse time curve, and the larger the overload current, the shorter the time of time delay breaking.
6. Determination of the rated voltage of the undervoltage tripper.
The rated voltage of the undervoltage trip is equal to the rated voltage of the line. The undervoltage tripper is a protective accessory, when the power supply voltage is higher than 85% of the rated voltage of the undervoltage tripper, the undervoltage tripper can ensure the normal operation of the circuit breaker;When the power supply voltage drops to 35% to 70% of the rated voltage of the undervoltage tripper, the undervoltage tripper can trip the circuit breaker and protect the electrical equipment from the damage of the undervoltage. Such as: motor load, etc. Undervoltage trippers are available in a variety of operating voltage ratings and frequencies.
7. Determination of the rated voltage of the shunt tripper.
The rated voltage of the shunt trip is equal to the voltage of the control control power supply. The shunt tripper is an accessory that realizes the long-distance opening of the circuit breaker, and when the external voltage of the shunt tripper is 70% 110% of the rated control voltage of the shunt tripper, the circuit breaker can be reliably disconnected.
Generally, the shunt tripper is used to open the circuit breaker remotely in an emergency or as an actuator for the protection of electrical appliances such as leakage relays.
At present, it is mostly used in the power distribution cabinet door opening and power-off protection circuit.
Shunt trippers are available in a variety of operating voltage and frequency ratings.