breaking capacity) and Ics (rated service s.c. This rated value (Icu) for industrial CBs and (Icn) for domestic-type CBs is normally given in kA rms. the DC transient component (which is always present in the worst possible case of short-circuit) is assumed to be zero for calculating the standardized value. The value of current quoted in the standards is the rms value of the AC component of the fault current, i.e. The short-circuit current-breaking rating of a CB is the highest (prospective) value of current that the CB is capable of breaking without being damaged. Standard values for this relationship have been established in some standards The short-circuit current-breaking performance of a LV circuit-breaker is related (approximately) to the cos φ of the fault-current loop. Note: For circuit-breakers equipped with non-adjustable overcurrent-trip relays, Ir = In. The thermal-trip relays are generally adjustable from 0.7 to 1.0 times In, but when electronic devices are used for this duty, the adjustment range is greater typically 0.4 to 1 times In.Ī NSX630N circuit-breaker equipped with a 400 A Micrologic 6.3E overcurrent trip relay, set at 0.9, will have a trip-current setting: That value must be greater than the maximum load current IB, but less than the maximum current permitted in the circuit Iz (see chapter Sizing and protection of conductors ). It also represents the maximum current that the circuit-breaker can carry without tripping.
The trip-current setting Ir or Irth (both designations are in common use) is the current above which the circuit-breaker will trip.
What does it mean mccb circuit breaker install#
Moreover, in order to adapt a circuit-breaker to the requirements of the circuit it controls, and to avoid the need to install over-sized cables, the trip relays are generally adjustable. Overload relay trip-current setting (Irth or Ir)Īpart from small circuit-breakers which are very easily replaced, industrial circuit-breakers are equipped with removable, i.e. Note: In for circuit-breakers (in IEC 60947-2) is equal to Iu for switchgear generally, Iu being the rated uninterrupted current.Ī circuit breaker which can be fitted with overcurrent tripping units of different current level-setting ranges, is assigned a rating which corresponds to the highest current-level-setting tripping unit that can be fitted.Ī Compact NSX630N circuit-breaker can be equipped with 11 electronic trip units from 150 A to 630 A. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit-breakers (derated as described) to operate at 60☌ (or even at 70☌) ambient. Thus, the circuit-breaker in an ambient temperature of 50☌ could carry only 117 A indefinitely, or again, only 109 A at 60☌, while complying with the specified temperature limit.ĭerating a circuit-breaker is achieved therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The same circuit-breaker can be used at higher values of ambient temperature however, if suitably “derated”. This is the maximum value of current that a circuit-breaker, fitted with a specified overcurrent tripping relay, can carry indefinitely at an ambient temperature stated by the manufacturer, without exceeding the specified temperature limits of the current carrying parts.Ī circuit-breaker rated at In = 125 A for an ambient temperature of 40☌ will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). Other values of voltage are also assigned to the circuit-breaker, corresponding to disturbed conditions as noted in Other characteristics of a circuit-breaker. This is the voltage at which the circuit-breaker has been designed to operate, in normal (undisturbed) conditions.
The fundamental characteristics of a circuit-breaker are:
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