Panels and Switchracks In Hazardous and Nonhazardous Areas

This article covers the minimum requirements for the design, fabrication, testing and inspection of outdoor low voltage (600 volts and under) rack-mounted combination motor starters and feeder breakers for installation in hazardous and non-hazardous locations. Main keywords for this article are Panels and Switchracks In Hazardous and Nonhazardous Areas, Panels and Switchracks References, Rack And Enclosures, Panels and Switchracks Inspection and Testing.

Panels and Switchracks In Hazardous and Nonhazardous Areas

Panels and Switchracks References

National Electrical Manufacturer’s Association (NEMA)
250 Enclosures for Electrical Equipment (1000 Volts Maximum)
ICS-1 Industrial Control and Systems General Requirements
ICS-2 Industrial Control and Systems Controllers, Contactors, and Overload Relays Rated Not
More Than 2000 Volts AC or 750 Volts DC
ICS-4 Terminal Blocks for Industrial Use
ICS-6 Industrial Control and Systems Enclosures
National Fire Protection Association (NFPA)
NFPA 70 National Electrical Code (NEC)
Underwriters Laboratories (UL)
67 Panel boards
248 (series) Low-Voltage Fuses
347 Medium-Voltage AC Contactors, Controllers, and Control Centers
489 Molded-Case Circuit Breakers and Circuit-Breaker Enclosures
886 Outlet Boxes and Fittings for use in Hazardous Areas

Panels and Switchracks In Hazardous and Nonhazardous Areas

  • The switchrack shall consist of a main circuit-breaker, combination motor  starters, circuit breakers for general purpose feeder applications, contactors for floodlighting, heaters, lighting transformers or other general switched loads, control relays, and other required components.
  • The area classification, type enclosure, type of components, number of poles, voltage, frequency, trip or current rating, and interrupting capacity of the equipment shall be specified on data sheets and one line diagram.  
  • The equipment shall be capable of proper operation for voltage deviations of +10% and frequency deviations of +5%. In addition, contactors, and relays shall be able to ride through short voltage dips of up to 80% of nominal such as those experienced during motor starting conditions.
  • The equipment shall be completely factory built, assembled, wired and tested. When it is necessary to partially disassemble the units for ease of transportation, adequate materials and instructions shall be provided for easy field assembly.  
  • Unless specified otherwise, the overall switchrack assembly shall be rated as follows:
    a. Rated Duty: Continuous
    b. Rated Current, main circuit phase bus (minimum): 600A
    c. Rated Current (minimum), main circuit neutral bus: 300A
    d. Rated Operational Voltage (nominal): 480V.
    e. Rated Control Supply Voltage (nominal): 120 VAC
  • Unless specified otherwise, the short-circuit rating shall be as follows:
    a. Rated Short-Circuit Withstand Current: 65 kA rms symmetrical for 3  cycles for power circuit elements.
    b. Rated short circuit withstand current for auxiliary circuits: 10 kA rms symmetrical
  • Interlocks and safety features shall comply with the provisions of UL 347.
  • All materials furnished by the Contractor shall be new.
  • Only certified/approved materials shall be used.
  • MCC shall be configured and wired as detailed in NEMA ICS-2.

Main keywords for this article are Panels and Switchracks In Hazardous and Nonhazardous Areas, Panels and Switchracks References, Rack And Enclosures, Panels and Switchracks Inspection and Testing.

Rack And Enclosures

  • Starters shall have metal enclosures suited for the specified locations. Unless  otherwise specified, enclosures in unclassified areas shall be NEMA 4X. Enclosures in classified areas.
  • Combination starters should be centralized as much as possible and the group of combination starters shall be enclosed or covered by suitable method recommended by the manufacturer to protect from sunlight. A sunshield canopy shall be provided as follows:
    a. Shall extend a minimum of 1 meter (3.28 feet) horizontally from the front and back of switchrack structural frame.
    b. With the top of the structure as the horizontal plane, the gable canopy shall be sloped a minimum of 15 degrees downwards at the front and the back.
    c. Lifting attachments/connectors shall be provided for lifting the entire switchrack, with canopy attached, without permanent distortion to any part of the structure.
    d. Lighting with a minimum initial horizontal illumination level of 90 lux.
  • Outdoor-type enclosures shall be made of cast aluminum for hazardous areas  and NEMA type 4X for non-hazardous areas. Enclosure covers shall be hinged. Enclosures shall be fitted with breather-drains for exclusion of moisture. The breather-drain used in a Class I area shall be an explosion-proof type.
  • In division 2 locations, all arcing devices in power and control circuits must be housed in enclosures approved for the Areas, (explosion proof, hermetically sealed). Sealing fittings shall be supplied as required by NFPA 70 National Electrical Code (NEC).
  • The combination starters shall be grouped together and mounted on switchracks. Vertical and horizontal clearances between enclosures shall be sufficient to permit installation and removal of all auxiliary devices such as breathers, drains and so forth.
  • Switchracks shall be self-supporting and fabricated from standard structural shapes of adequate size and weight to assure a rigid structure. Racks shall be of steel that is hot-dip galvanized after completion of welding and drilling.
  • The bus portion of switchracks shall have bolted covers with handles with maximum length per cover section of 1.83 meters (6 feet). Enclosures shall be either #5052 aluminum alloy with a 0.4 percent copper content or steel that is hot-dip galvanized. Enclosures shall be provided with stainless steel drains. The enclosures shall be assembled with hot-dip galvanized hardware. Nuts for cover bolts shall be captive and corrosion resistant. Aluminum alloy vent caps are acceptable.
  • Equipment shall be connected to bus enclosures using rigid conduit. Two-part, screw type grounding hubs may be used at the bus enclosure.
  • Conduits used shall be rigid steel conduits conforming to ANSI C80.1 and with NPT threads.
  • Enclosures and fitting in hazardous areas shall comply with UL-886.
  • Initial installation of controllers on switchracks shall not occupy more than 75% of the available area.

Space Heaters

  • Space heaters shall be provided to eliminate condensation within the  enclosures. The space heater circuit shall be thermostatically controlled with a manual bypass switch. A circuit breaker shall be provided on each heater circuit for overload protection and means of disconnection.
  • A blue light shall be provided on the enclosure door separately for each space heater to indicate that the respective space heater power is available.
  • Heaters shall be rated 240 VAC, and operate at 120V single phase. The load on each heater circuit shall be limited to 1500 watts.
  • When specified on the data sheet, the power supply for each space heater circuit shall be provided to the enclosure separately, otherwise, the power shall be obtained from a 120 V AC power bus to be provided within the switchrack assembly.

Wiring And Cabling

  • All wiring shall be continuous from terminal to terminal with no splicing. Wiring  shall be flame retardant, 90°C and 600V insulated, stranded copper.
  • Extra flexible-type cable shall be used between stationary components and doors or hinged panels.
  • Minimum wire size for power shall be 4mm² and for control circuit shall be  2.5mm².
  • Wiring connections shall be made with compression terminal lugs.  
  • Where control or secondary wiring is run through a metal sheet or barrier,  bushings, grommets or other mechanical protection shall be provided.
    All wiring shall be marked on each end with permanently embossed wire markers of the heat shrinkable or slip-on types. Wrap-around adhesive or rigid Snap-On type markers are NOT acceptable.
  • Terminal blocks shall be provided for connection of external control, relaying, and metering wiring, and shall be conveniently located, clearly numbered, and identified. Terminal blocks shall be of the heavy duty, screw-type, designed to accommodate pre-insulated, crimp-type, ring-tongue, wire lugs.
  • A minimum of 20% spare control terminal blocks shall be provided.
  • All terminal blocks for current transformer secondary must be of the shorting  type.
    5.4.10 All terminal blocks shall comply with NEMA ICS 4.  

Bus Elements

  • All primary bus elements and ground buses shall be copper with all joints  silver-plated. Tapered bus shall not be used. All joints shall be made up with sufficient bolts, flat and spring-type washers to maintain the initial contact pressure. A single bolt connection is not acceptable.  
  • Main bus bars shall run the entire length of the switchrack and have provision for future expansion.
  • Buses shall be braced to withstand without damage, the dynamic and thermal stresses imposed by the maximum available fault current. The short circuit withstand duration shall be two seconds.  
  • Phasing arrangement of bus and primary circuit connectors shall be A-B-C, 12-3, top-to bottom, left-to-right, or front-to-back, as viewed from the front of the switchrack.

Incoming Line

The incoming line section shall include the provisions for connecting the switchrack .

A main circuit breaker shall be provided and shall function to isolate the  switchrack from the main incoming power supply. The circuit breaker shall have a continuous current rating not less than the rating of the switchrack main buses as described on Section 5.1. Interlock between the main breaker and the power supply source circuit breaker shall be provided.

Motor Controllers and Feeder Units

  • Each motor controller shall be a rated 600 Volts, three-pole combination starter  consisting of the following:
    a. a magnetic contactor,  
    b. an overload relay
    c. motor circuit protector (MCP), with (i.e. a circuit breaker with a self-contained instantaneous overcurrent trip device).
  • Unless otherwise specified, the starter shall be suitable for across-the-line  starting of a squirrel-cage induction motor. Controllers shall have a seal-in auxiliary contact for three-wire control.
  • Magnetic contactors shall be air-break type suitable for making and breaking the locked-rotor current of the motor to be controlled.  Magnetic contactors shall be suitable for control by field mounted pushbutton stations with momentary contacts, and shall be provided with normally open auxiliary holding contacts.
  • Overload relays shall be NEMA class 20 and of 3-element ambient temperature compensated thermal type and shall be provided with manual reset operable from the front of the enclosure without opening doors and drawers. 
  • Motor circuit protectors shall be of the molded case, air-break type with adjustable magnetic protection. Suitable provisions shall be made to limit the  adjustment to 1300% of motor full load current, in accordance with NFPA 70, NEC.
  • Control transformers shall be provided in each motor starter. Cartridge type fuses or fuses in cartridge type holders shall be provided in one leg of the secondary of all control transformers. The other leg of the secondary shall be grounded to the motor control center ground bus. Control voltage shall be 230 Vac. Control transformers shall be sized to ensure proper operation on  voltages 25 percent below normal.

Fuses shall comply with UL 248 (series)

  • In addition to the seal-in contact, each controller shall have two normally- closed and two normally-open contacts, as spares wired to a terminal block.
    Provisions shall be made for the future addition of additional auxiliary contacts.
  •  Indicating lights shall be provided for each motor controller and feeder unit. The color of indicating lamps shall be:
    a. Red for motor running or feeder breaker closed
    b. Green for motor stopped or feeder breaker open
    c. Orange for thermal relay tripped.
  • Controllers for motors rated 15 kW and larger shall be provided with 230 Vac motor space heater circuit complete with its circuit breaker and blue indicating lamp.
  •  All indication lamps shall be cluster-type LED of the reduced voltage type. The design of the indication lamp circuit shall be such that a short circuit fault at the lamp-holder should not cause the loss of control power to the other control circuit components.

Feeder Circuit Breakers

  • Circuit breakers shall be molded-case, air-break type, three-pole, 600 volt. Circuit breaker for motor load shall be motor circuit protectors with adjustable magnetic trip only. Circuit breaker for feeders and non-motor loads shall be thermal-magnetic trip, ambient-compensated to 50°C allowing the breaker to carry full rated current with uniform tripping characteristics throughout the temperature range from 25 °C to 50 °C.
  • The circuit breakers shall be designed and tested to meet the requirements of UL 489, and applicable ANSI standards.
  • Each circuit breaker installed shall be manually operable by a trip-free handle on the enclosure. Each handle shall give visual indication of the “On”, “Off”, or “Trip” positions and shall be capable of being padlocked in the “Off” and “Test” position. Each handle shall interlock the compartment door so that the door cannot be opened unless the breaker is open. Padlocks shall be provided for each motor controller and feeder units.
  • Power and control fuses shall be accessible for replacement without removing the control device assembly from the rack or disconnecting cables.


  •  All enclosures shall be grounded to a common copper ground bus running the  full horizontal length of the switchrack assembly. Switchracks shall have a ground bus horizontally mounted below the main bus bars.
  • Compression lugs shall be provided on each end for terminating 120mm² copper grounding conductors.
  • Lugs shall be provided at diagonally opposite ends of the switchrack structural support for connection to 70mm2 copper grounding conductors.

Panels and Switchracks Testing

The routine tests identified below shall be performed in addition to the Vendor‟s standard tests:
a. Visual inspection of appearance, construction, dimensions, workmanship, internal wiring, etc.
b. Relay test
c. Composite performance test
d. Insulation resistance measurement
e. Dielectric Strength test
The correctness of all control wiring, including interlocks, shall be verified by actual electrical operation of the component control devices under all modes of operation.

Main keywords for this article are Panels and Switchracks In Hazardous and Nonhazardous Areas, Panels and Switchracks References, Rack And Enclosures, Panels and Switchracks Inspection and Testing.

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