information on a 3phase breaker

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Answer by Bob-Bear
Submitted on 2/18/2005
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An electrical three phase circuit breaker is used in AC applications. Resistive as with 277 lighting or inductive such as motors and other inductive loads. Commercial and Industrial three phase breakers are manufactured to protect conductors by controlling the amount of current. A three phase circuit breaker's sole purpose is to protect the conductors insulation. There are many three phase circuit breakers with different accessories (such as a shunt trip... which can remotely trip the breaker or thermal trip units and motor protectors} With any three phase motor, by reversing any two leads you reverse the rotation. Also keep in mind that there are different configurations for three phase applications. I refer to Wye and Delta distribution. By using a three phase power application, you have a diverse range verses single phase applications. When a circuit is designed, the qualified/ licensed Master Electrician shall size the applicable circuit breaker according to the requirements of the National Electrical Code in effect at that time. Circuit breakers are normally manufactured with a thermo-plastic housing and rated to handle different voltages safely (smaller circuit breakers are rated at 250 volt AC and 600 volt's AC and the rating go up from there). The higher the voltage and current, the higher the dielectric rating will have to be to ensure the housing material and internal switch and contact wipe face(refers to the internal contact face) rating will not breakdown and allow heat damage to occur. Even the huge power substations have circuit breakers, although sometimes very large. Their purpose is to protect the conductors on the load side. These breakers will have remote metering that monitors many aspects of the load being served... many times a community or possibly the miles of lights along the highway may be three phase or single phase. When the circuit breaker is used with a inductive three phase load (ie: motor, etc) The load would have a higher starting load but would drop to a running load after starting quickly. The qualified electrician shall size and set the unit to trip at a percentage beyond the load units full load amperage and protect the conductor being used. This is to allow the unit to be used for motor starting and not false trip. The Electrician can set the adjustment at 150%, 200% or even 300% dependent on the applicable National Electrical Code and Local Code as well. Again... Please keep in mind that the three phase breaker's purpose is to PROTECT THE CONDUCTOR (wire) and NOT the motor. Other devices, such as motor starters and contactors with thermal trip devices, etc are designed and sized to protect the actual load being controlled.

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Question by andres
Submitted on 3/1/2004
Related FAQ: Electrical Wiring FAQ (Part 1 of 2)
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information on a 3phase breaker

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