General rules of electrical installation design Connection to the MV utility distribution network Connection to the LV utility distribution network MV and LV architecture selection guide for buildings Protection against electric shocks and electric fires Sizing and protection of conductors LV switchgear: functions and selection The basic functions of LV switchgear LV switchgear functions - Electrical protection LV switchgear functions - Isolation LV switchgear functions - Switchgear control Tabulated functional capabilities of LV switchgear Standards and description of circuit-breakers Fundamental characteristics of a circuit-breaker Other characteristics of a circuit-breaker Selection of a circuit-breaker Discrimination MV/LV in a consumer’s substation Circuit-breakers suitable for IT systems Maintenance of low voltage switchgear Energy Efficiency in electrical distribution Characteristics of particular sources and loads

Residential and other special locations The fundamental characteristics of a circuit-breaker are: This is the voltage at which the circuit-breaker has been designed to operate, in normal (undisturbed) conditions. Other values of voltage are also assigned to the circuit-breaker, corresponding to disturbed conditions. 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.how to make an old ac unit more efficient A circuit-breaker rated at In = 125 A for an ambient temperature of 40°C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). fix car ac cost

The same circuit-breaker can be used at higher values of ambient temperature however, if suitably “derated”. Thus, the circuit-breaker in an ambient temperature of 50°C could carry only 117 A indefinitely, or again, only 109 A at 60°C, while complying with the specified temperature limit. Derating a circuit-breaker is achieved therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit-breakers (derated as described) to operate at 60°C (or even at 70°C) ambient.portable ac reviews air conditioner Note: In for circuit-breakers (in IEC 60947-2) is equal to Iu for switchgear generally, Iu being the rated uninterrupted current. A 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.

A Compact NSX630N circuit-breaker can be equipped with 11 electronic trip units from 150 A to 630 A. The size of the circuit-breaker is 630A. Apart from small circuit-breakers which are very easily replaced, industrial circuit-breakers are equipped with removable, i.e. exchangeable, overcurrent-trip relays. 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. The trip-current setting Ir or Irth (both designations are in common use) is the current above which the circuit-breaker will trip. It also represents the maximum current that the circuit-breaker can carry without tripping. That value must be greater than the maximum load current IB, but less than the maximum current permitted in the circuit Iz (see chapter G).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. A NSX630N circuit-breaker equipped with a 400 A Micrologic 6.3E overcurrent trip relay, set at 0.9, will have a trip-current setting: Ir = 400 x 0.9 = 360 A Note: For circuit-breakers equipped with non-adjustable overcurrent-trip relays, Ir = In. Example: for iC60N 20 A circuit-breaker, Ir = In = 20 A.H30: Example of a NSX630N circuit-breaker equipped with a Micrologic 6.3E trip unit adjusted to 0.9, to give Ir = 360 A Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit-breaker rapidly on the occurrence of high values of fault current. Their tripping threshold Im is: For the latter circuit-breakers there exists a wide variety of tripping devices which allow a user to adapt the protective performance of the circuit-breaker to the particular requirements of a load (see Fig. (2) 50 In in IEC 60898, which is considered to be unrealistically high by most European manufacturers (Schneider Electric = 10 to 14 In).

(3) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.H31: Tripping-current ranges of overload and short-circuit protective devices for LV circuit-breakersH32: Performance curve of a circuit-breaker thermal-magnetic protective scheme Ir: Overload (thermal or long-delay) relay trip-current settingIm: Short-circuit (magnetic or short-delay) relay trip-current settingIi: Short-circuit instantaneous relay trip-current setting.H33: Performance curve of a circuit-breaker electronic protective scheme A circuit-breaker is suitable for isolating a circuit if it fulfills all the conditions prescribed for a disconnector (at its rated voltage) in the relevant standard. In such a case it is referred to as a circuit-breaker-disconnector and marked on its front face with the symbol All Acti 9, Compact NSX and Masterpact LV switchgear of Schneider Electric ranges are in this category. The short-circuit current-breaking performance of a LV circuit-breaker is related (approximately) to the cos φ of the fault-current loop.

Standard values for this relationship have been established in some standards 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. The value of current quoted in the standards is the rms value of the AC component of the fault current, i.e. 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. This rated value (Icu) for industrial CBs and (Icn) for domestic-type CBs is normally given in kA rms. Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c. breaking capacity) are defined in IEC 60947-2 together with a table relating Ics with Icu for different categories of utilization A (instantaneous tripping) and B (time-delayed tripping) as discussed in subclause 4.3. Tests for proving the rated s.c. breaking capacities of CBs are governed by standards, and include: