Skip Navigation LinksDEP > Businesses > Land > Mining > Bureau of Mine Safety > Services > Equipment Approvals > Load Center

Load Center

  1. Input will be through a coupler.
  2. Input coupler to meet requirements of Section 331(c) of the Act.
  3. A positive visible disconnect can be:
    1. Air disconnect load break switch (LBS)
    2. Vacuum disconnect LBS
    3. A circuit breaker with a cut-out.
  4. If an air disconnect LBS (see 3a) is chosen, it can be designed to disconnect power on the outby feeding circuit breaker (see section 332(2) (ii) of the Act); or as an alternative, it can be designed to open the load center’s main secondary breaker. This alternative is detailed in the Dilworth Mine Section 334 Commission, titled “Alternative Circuitry for Section 332 Load Centers” dated November 9, 1987 (attached).
  5. If a vacuum LBS (see 3b) is chosen, the design shall include the following:
    1. A vacuum bottle and non-load break cut-out switch.
    2. A set of electrical interlocks to remove the load as stated in Item 4.
    3. A mechanical interlock to trip the vacuum contacts before the visible contacts open.
  6. If a circuit breaker (see 3c) is chosen, no other requirements are necessary.
  7. Protection:
    1. If a circuit breaker is used, it shall be equipped with instantaneous and inverse time limit phase overcurrent and undervoltage relaying protection (see section 332(2) (i) of the Act).
    2. If a fuse is used, it shall be sized to provide for adequate short circuit and overload protection.
    3. If the load center has high-voltage feel-thru, circuit breakers and suitable load break switches used as visual positive disconnects shall conform to the agreement on section 331(h).
  8. An emergency stop switch which requires manual reset will be included.
  9. Barriers and interlocks are to work together to provide protection from inadvertent access. Barriers and interlock switches will be so arranged to prevent inadvertent contact with energized components of a voltage of greater than 1,000 volts nominal. These provisions apply to both the incoming and outgoing circuits. For multiple voltages, barriers and interlock switches must be used to isolate circuits of greater than 1,000 volts nominal from utilization circuits.
    1. Cover switches for top and side covers will be used and arranged which, when removed, expose energized components of greater than 1,000 volts nominal.
    2. Switches will be arranged so that the cover cannot be opened to allow access to energized components of greater than 1,000 volts nominal.
    3. Barriers are required when choosing the design alternative discussed in Item 4 (see section 334 Commission for Dilworth Mine). No other application requires the use of barriers. When used, a barrier should be rigidly mounted and substantial enough to prevent inadvertent access. Ventilation holes can be used provided these holes are sized to prevent a person’s finger from passing through them.
  10. There will be no wye-wye connections unless there is a tertiary winding. At least one of these windings must be a delta connection.
  11. The load center shall be of essentially fireproof construction. Viewing windows may be made out of glass or polycarbonate, such as Lexan 940 TM or equivalent. [1] Acrylics, such as plexiglass are prohibited.
  12. Ground limiting resistor.
    1. Use Section 332(4) as amended for language structure.
  13. Main breaker tripping devices, which must include:
    1. Overload and short circuit.
    2. Ground fault by current protector.
    3. Ground fault by potential protector in the event of an open ground resistor.
    4. Continuous monitoring of the ground resistor is acceptable in lieu of potential protector.
    5. Undervoltage
    6. Time delay tripping for coordination of tripping is allowed.
  14. Individual machine breakers (secondary)
    1. Each output circuit shall have the following protective features.
      1. Phase fault (with lockout).
      2. Overload (with lockout). The primary overload protection may be an integral part of the load center circuit breaker, external to the load center circuit breaker but installed in the load center, installed in the equipment or may be provided by the equipment design and approval. MSHA approved permissible equipment is an example of primary overload protection provided by equipment design and approval. When the primary overload protection is not installed in the load center, a supplemental overload device must be installed in the load center. The supplemental overload device must operate at an unspecified value less than or equal to the breaker frame size. The breaker frame size must be reasonable and is generally determined by the trip range necessary for proper phase fault protection.
      3. Ground fault (with lockout). Ground fault protective devices shall be adjusted to operate at not more than 50 percent of the current rating of the neutral grounding resistor.
      4. Undervoltage. Three acceptable methods of providing undervoltage protection are shown on Addendum Sketch #1.
      5. Ground monitor. A ground wire monitoring system or alternative method approved by MSHA.
  15. Secondary Receptacles
    1. Grounding through a contact of the coupler and receptacle or coupler threads.
    2. Ground connection to each output receptacle will be solidly connected to the ground limiting resistor without relying on the power center frame as the conductor.
  16. Ground fault interrupter on all 110 volt and 220 volt receptacles.
  17. Drawings 1-7 (PDF) are intended to illustrate these specifications.
Addendum Sketch #1 (PDF) amends these drawings relative to UVR protection.

[1] Lexan 940 TM is a fire retardant Lexan.