Technical FAQ
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  • Technical FAQ
    Circuit Beaker Type and Choose:
    Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.
    Low-voltage circuit breakers
    Low-voltage (less than 1,000 VAC) types are common in domestic, commercial and industrial application, and include: MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category. There are three main types of MCBs: 1. Type B - trips between 3 and 5 times full load current; 2. Type C - trips between 5 and 10 times full load current; 3. Type D - trips between 10 and 20 times full load current. In the UK all MCBs must be selected in accordance with BS 7671. MCCB (Molded Case Circuit Breaker)—rated current up to 2,500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings. Low-voltage power circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear cabinets.

    Low-voltage circuit breakers
    Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors. Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:
    It's mainly includes:
    1.Vacuum Circuit Breaker(With rated current up to 6,300 A, and higher for generator circuit breakers.)
    2.Air Circuit Breaker(Rated current up to 6,300 A and higher for generator circuit breakers.)
    3.SF6 Circuit Breaker(SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.)

    Other breakers
    1.Breakers for protections against earth faults too small to trip an over-current device:
    2.Residual-current device (RCD, formerly known as a residual current circuit breaker) — detects current imbalance, but does not provide over-current protection.
    3.Residual current breaker with over-current protection (RCBO) — combines the functions of an RCD and an MCB in one package. In theUnited States and Canada, panel-mounted devices that combine ground (earth) fault detection and over-current protection are called Ground Fault Interrupter (GFI) breakers; a wall mounted outlet device or separately enclosed plug-in device providing ground fault detection and interruption only (no overload protection) is called a Ground Fault Circuit Interrupter (GFCI).
    4.Earth leakage circuit breaker (ELCB)—This detects earth current directly rather than detecting imbalance. They are no longer seen in new installations for various reasons.
    5.Recloser—A type of circuit breaker that closes automatically after a delay. These are used on overhead electric power distributionsystems, to prevent short duration faults from causing sustained outages.
    6.Polyswitch (polyfuse)—A small device commonly described as an automatically resetting fuse rather than a circuit breaker.

    AC contactor selection:
    1. Triple pole contactor is usually selected for a three-phase AC system, but to control neutral wire simultaneously, a four pole AC contactor shall be selected.
    2. In a single phase AC system or DC system, it's common that two poles or three poles are connected in parallel.
    3. Generally, air-type electromagnetic contactor is selected, while explosion-proof type contactor or vacuum contactor shall be selected for flammable and combustible occasions.
    4. Rated working voltage, current, making & breaking capacity, and withstand overload current shall be greater than parameters of the main circuit.
    5. Coil voltage of a contactor shall be determined based upon the selected control-circuit voltage. Alternating current is usually adopted, but when the operations are frequent, direct current is usually selected.
    6. Type, quantity and combination form of auxiliary contact are generally determined as per system requirements, but attention is also required to ensure that making and bearing capacity, and other rated parameters of auxiliary contact meet requirements for control loop.

    Voltage stabilizer Selection:
    1. Pure resistive load(filament lamp, resistance wire, etc.): when selecting a voltage stabilizer, its capacity shall be greater than a value that is 1.1 to 1.3 times as large as the total load power.
    2. Active, capacitive load(motor, fluorescent lamp, air-conditioner, etc.): when selecting a voltage stabilizer, its capacity shall be greater than a value that is 2.5 to 3 times as large as the total load power.
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