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Low Voltage Switchgear

asked May 30, 2016 in E&I by Eric Yakawa (205 points)
What are the required characteristics to select the Low Voltage Switchgear?

2 Answers

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answered May 31, 2016 by lulopez (2,220 points)

Here are some points to think about:

  • All low voltage switchgear shall be metal-enclosed low voltage power circuit-breaker switchgear type as defined in IEEE C37.20.1.
  • Electrically operated breakers shall be used for transformer secondary and bus tie breakers in secondary selective and spot network systems.  All other breakers shall be manually operated except where electrical operation is required to:
    • Permit remote operation such as for motor control.
    • Meet applicable codes, standards, or the breaker manufacturer's requirements for safe operation without instantaneous protective trip devices.
    • Conform to manufacturer's standard design.
  • Relays are preferred as the protective devices for incoming circuit breakers in spot network substations.  Relays plus circuit breakers with direct acting trip devices (with shunt trip capability) may be used.
  • If main bus neutral is specified without specifying its current rating, the rating shall be 50 percent of the phase bus current rating. Main bus neutrals, when specified, shall extend the length of the switchgear, and shall be insulated from ground.
  • A means for isolating neutral busses from the supply system source neutral, such as a bolted link, shall be provided.
  • Potential transformer fuses, control fuses, and control power switches shall be accessible for replacement or operation.
  • All breakers shall be electrically controlled, with either a stored energy (preferred) or a solenoid operating mechanism.
  • Potential transformers and their fuses shall be mounted in a separate compartment of draw-out construction or equivalent.  The primary connection shall be disconnected before the transformer or its primary fuses become accessible.  A similar arrangement shall be provided for the primary fuses of control power transformers.
  • Ammeters are required in outgoing circuits as follows:
    • In one phase for motor controllers.
    • In other circuits either a triplex meter or single meter and selector switch for three (3) phases.
  • Indoor switchgear installations and protected aisle outdoor switchgear shall be provided with a test cabinet for the breakers.  This will permit the operation and inspection of the removable breaker elements outside the switchgear structure.
  • Outdoor switchgear installations that are not of the protected aisle type shall be provided with test jumpers to permit operation of the removable breaker elements outside the switchgear structure.
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answered Jun 2, 2016 by redonetti (1,280 points)


  • ANSI LV switchgear should utilize current-limiting fuses to limit potential arc energy to minimum.
  • ANSI Low-Voltage switchgear should be designed to meet the followings:  Incoming line, tie and generator main breakers should be installed in individual vertical sections.  A maximum number of feeder breakers per vertical section should be determined per the ANSI standard but not more than three.
  • Switchgear to be provided with a high-resistance ground and detection system for transformers and emergency generator connections.
  • An interlock should be provided to prevent tie breaker from closing electrically or mechanically in an event of faults occurring on both buses.
  • The ground fault system should give an alarm to alert the operator whenever a ground fault occurs.  The system should have a provision to assist in locating the ground fault.
  • IEC Low-Voltage switchgear should be designed to meet the followings:  Stacking breakers or contactors vertically should be minimized and should only be allowed where suitable lifting aids have been designed both for the equipment and the room.
  • IEC Low-Voltage switchgear should be designed to comply with IEC 60439-1.