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Circuit Breaker Sizing

  1. Apr 27, 2010 #1
    I have Distribution board, Outgoing (D.B1)
    But I got confuse for MCB sizing:
    The busbar 400V, 125A, 50Hz, 4P, 10KA for 1Sec.
    The part of Outgoing Rates as the followings:
    Feeder-1: 2.5 KVA used 25A MCB 2P
    Feeder-2: 3.5 KVA used 25A MCB 2P
    Feeder-3: 2.0 KVA used 25A MCB 2P
    Feeder-4: 1.0 KVA used 16A MCB 2P
    Feeder-5: 1.5 KVA used 16A MCB 2P

    Onother one (D.B2):
    The busbar 230V, 100A, 1PH, 50Hz, 4P, 10KA for 1Sec.
    The part of Outgoing Rates as the followings:
    Feeder-1: 2 KW used 16A MCB 2P
    Feeder-2: 2 KW used 16A MCB 2P
    Feeder-3: 2 KW used 32A MCB 2P
    Feeder-4: 2 KW used 32A MCB 2P
    Feeder-5: 1 KW used 16A MCB 2P

    Is the above Rating acceptable, since they used different rate with same load in the D.B2
     
  2. jcsd
  3. Apr 27, 2010 #2

    berkeman

    User Avatar

    Staff: Mentor

    Welcome to the PF.

    What country are you in? What government regulating body sets the safety standards for these components? What publication has these safety standards and component sizing requirements in it?
     
  4. Apr 28, 2010 #3

    Dear berkeman,
    Thanks alot for your response,
    My Country : Sudan
    The government regulating body sets the safety standards for these components: The latest edition of the following codes, regulations and standards:
    • IEC 60146 Low voltage switchgear and control gear
    • IEC 60269 Low voltage fuses
    • IEC 60408 Low voltage air break switches, air break disconnectors, air break
    switch disconnectors and fuse combination units

    So these Outgoing Distribution board for 230V AC UPS, 20KVA for FPF Feild Production Facilities in oil feild.

    The part of Outgoing Rates as the followings:
    Feeder-1: 2.5 KVA used 25A MCB 2P For Building fire alarm system
    Feeder-2: 3.5 KVA used 25A MCB 2P For Emergency Shutdown System
    Feeder-3: 2.0 KVA used 25A MCB 2P For Operator Station
    Feeder-4: 1.0 KVA used 16A MCB 2P For Microwave system
    Feeder-5: 1.5 KVA used 16A MCB 2P For PAGA System

    Onother one (D.B2):
    The busbar 230V, 100A, 1PH, 50Hz, 4P, 10KA for 1Sec.
    The part of Outgoing Rates as the followings:
    Feeder-1: 2 KW used 16A MCB 2P For Building fire alarm system
    Feeder-2: 2 KW used 16A MCB 2P For Emergency Shutdown System
    Feeder-3: 2 KW used 32A MCB 2P For Operator Station
    Feeder-4: 2 KW used 32A MCB 2P For Microwave system
    Feeder-5: 1 KW used 16A MCB 2P For PAGA System
     
  5. Apr 28, 2010 #4
    There are three critical functions of a well designed UPS ac distribution system. First, short circuit protection of all underground conductors; second, isolation of panel pull-down during a branch fault; and third, isolation of the critical loads from electrical noise coupled through the static bypass. The power distribution system will usually consist of a panelboard, with circuit-breakers or fuses to protect the individual branch circuit conductors which will usually be run in conduits dedicated to these uninterruptible circuits. Indeed each distribution panel has related output cable circuit running which dictates its suitable feeder protection selection.
    Branch circuit breakers are selected based upon the voltage rating, the normal full load current of each branch circuit load (continuous and intermittent loads), the inrush current and its duration for each branch circuit load, the maximum time each connected load can tolerate a loss of voltage without failure (time for failure), and the available short circuit current at the uninterruptible ac main bus through the inverter or the bypass circuit when a static transfer switch is used (to be obtained from the UPS system vendor or calculated). When a fault (or short circuit) occurs on a branch circuit, the branch circuit breaker or fuse must isolate the fault before the UPS protective devices operate and trip the entire system. When this condition is fulfilled, the protection system is said to be coordinated. Fuses or circuit breakers of different ratings can be coordinated, so a main panelboard may supply a downstream panel if necessary, however, it rarely is necessary and should be avoided. Molded case circuit breakers (miniature circuit breakers) are instantaneous in action at currents above 8 to 10 times their continuous current rating. Since the UPS has limited output current, additional power for clearing fault currents rapidly may be provided from an alternate high power source by the use of a static switch. When a fault occurs on any branch circuit, the inverter reaches the current limit condition and the bus voltage falls almost to zero, thus depriving all branch circuit loads of power. The permissible duration of this loss of power must be longer than the clearing time of the breaker plus the transfer switching time in the UPS if the coordination is to be obtained.



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