Sensitivity of the circuit breaker

In order to ensure that the instantaneous or short-delay overcurrent trip of the circuit breaker can operate reliably in the event of a slight short-circuit fault within its protection range under the small operation mode of the system. The sensitivity of circuit breaker protection must meet the requirements of the "Design Code for Low-voltage Power Distribution" (GB50054-2011), and its sensitivity should not be less than 1.3, that is, Sp=lk.min/Iop≥1.3. where lop is the action current of the instantaneous or short-delay overcurrent tripper, Ik.Min is the single-phase short-circuit current or two-phase short-circuit current at the end of the line protected by the circuit breaker in the system operation mode, and Sp is the sensitivity of the circuit breaker.

For selective circuit breakers with both short-delay and instantaneous overcurrent trippers, only the action sensitivity of the short-delay overcurrent tripper needs to be verified, and the sensitivity of the instantaneous overcurrent tripper is not required.

Selection and tuning of circuit breaker tripper:

(1) The setting of the action current of the instantaneous overcurrent tripper

Among the objects protected by the circuit breaker, there are certain electrical equipment, which will generate high peak current several times its rated current in a short period of time during the start-up process, so that the circuit breaker can withstand a large peak current in a short time. The operating current lop(o) of the instantaneous overcurrent tripper must avoid the peak current Ipk of the line, i.e. Iop(o) ≥Krel· Ipk。 where Krel is the reliability coefficient. When selecting the circuit breaker, attention should be paid to making the setting current of the instantaneous overcurrent tripper of the circuit breaker avoid the peak current, so as not to cause the malfunction of the circuit breaker.

(2) The setting of the action current and action time of the short-delay overcurrent tripper

The operating current lop(s) of the short-delay overcurrent tripper should also avoid the peak current Ipk of the line, i.e., Iop(s) ≥Krel· Ipk。 where Krel is the reliability coefficient. The action time of the short-delay overcurrent tripper is generally divided into 0.2s, 0.4s and 0.6s, which are determined according to the protection selectivity of the front and rear protection devices, and the action time of the previous protection should be one time difference longer than that of the later protection.

(3) The setting of the action current and action time of the long-delay overcurrent tripper

The long-delay overcurrent tripper is mainly used to protect against overload, so its action current Iop(l) only needs to avoid the load current of the line, that is, the current I30 is calculated, that is, Iop(l)≥Krel· I30。 where Krel is the reliability coefficient. The action time of the long-delay overcurrent tripper should avoid the duration of the allowable short-term overload, so as not to cause the malfunction of the low-voltage circuit breaker.

(4) The requirements for the action current of the overcurrent tripper and the protected line

In order to prevent the line from overheating or even fire caused by overload or short circuit, and the circuit breaker does not trip the accident, the action current lop of the circuit breaker overcurrent tripper should meet the requirements of the formula, lop ≤ Kol· Ial。 where Ial is the allowable ampacity of the insulated cable; Kol is the allowable short-term overload coefficient of insulated cables, and 4.5 is generally taken for instantaneous and short-delay overcurrent trippers; For long-delay over-current trippers, 1.1 is taken when short-circuit protection is done, and 1 is taken when overload protection is only done. If the above matching requirements are not met, the action current of the tripper should be selected instead, or the cross-sectional area of the wire or cable should be appropriately thickened.

Sensitivity of the circuit breaker

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