Electrical Protection Devices in Mines

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Electrical Protection Devices in Mines

• Nancy Cains
• 9804903d

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• The largest danger in an underground mine is
  fire.

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• Electricity is now widely used in the mining
  industry and issues such as poor maintenance,
  misuse or lack of awareness and risk taking all
  increase the risk of electrical fire and possibly
  death.

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• Electrical protection in the mining industry is
  designed to detect and disconnect faulted
  electrical circuits. Protection devices are
  important in providing a safe working environment
  for mine workers.

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• Electrical circuits can fail in a number of ways.
  The two most common failures are
• .Earth faults
• .Short circuits

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Dangers and Injuries

• SHOCK
• Effect produced on the body by electric current
  passing through it.
• Effect depends on current strength (which in turn
  depends on voltage) the path the current takes
  through the body, the surface resistance of the
  skin and several other factors.

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• DEATH
• The most common cause of death from shock is
  suffocation
• Minor shocks may not be serious, but can lead to
  serious consequences, for example, the muscle
  contraction which they cause may lead to falls
  from working platforms or ladders.

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• BURNS
• These are caused by the passage of heavy current
  through the body or by direct contact with an
  electrically heated surface. They may also be
  caused by the intense heat generated by arcing
  from a short-circuit.

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• EXPLOSIONS OR FIRE
• Where flammable gases or vapours are present,
  special care is necessary in the design and
  selection of electrical equipment. In such areas,
  all equipment should be fully flameproof.
• Fires may start from sparks, short circuits or
  overloading

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Types of Protection

• Instantaneous short circuit
• Over-current
• Earth fault
• Earth leakage
• Earth leakage personnel protection
• Earth continuity
• Earth fault lockout and
• Overload

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Instantaneous Short Circuit

• Tailing cables on electric face equipment in
  underground coalmines undergo more serve service
  than most other cables in industry.
• Cables are subject to extreme tensile forces,
  severe abrasion and frequent flexing, twisting
  and crushing.

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• As a result of this electrical faults in trailing
  cables occur more frequently than in other
  industries.
• Of the various faults the short circuit has
  proven to be the most hazardous.
• Energy expended in a short circuit in a trailing
  cable is capable of igniting lose coal and coal
  dust.

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• The frequency of short circuits in trailing
  cables and the potential hazards associated with
  this occurrence make adequate trailing cable
  short-circuit protection very important.

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• Instantaneous settings for circuit breakers are
  now used to provide short circuit protection for
  trailing cables.
• It is possible to select various short circuit
  pick-up levels according to the coordination
  needs.

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• Instantaneous breaker automatically trips when
  there is a short circuit and if it were set too
  high, the breaker would not stop electrical
  current until dangerous amounts had flowed to the
  equipment.
• A delay of only fractions of a second in tripping
  the breaker would increase miners' exposure to
  the hazards of fire, electrical burns or shock.

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Earth Fault Protection

• Earth fault protection is used in industrial
  situations were we are primarily concerned with
  high voltage equipment where the primary voltage
  system is assumed to be solidly earthed at the
  supply end of incoming cables or overhead line.

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• Earth faults are most likely to be high-impedance
  faults and fuses can only be relied upon for
  earth leakage protection on very low current
  circuits.
• The most common earth fault protection is
  Residual Current Protection.

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• If all the conductors in a cable pass through a
  ring type transformer there should be no magnetic
  field to energise the ring and therefore no
  secondary voltage or current.
• However if some current returns by another path
  the voltage and current will appear in the CTs
  (current transformer) secondary circuit to
  energise the relay.

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• Sometimes it may be necessary to prevent an earth
  fault in the system, which should be cleared
  elsewhere, from operating the internal protection
  of a transformer. This is done by balancing
  residual current against a direct measure of the
  earth return current. This is called restricted
  earth leakage protection and responds only to
  faults in the area, which is protected.

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Overload

• Overloads in an electrical system are hazardous
  because they can produce heat or arcing.
• If too many devices are plugged into a circuit,
  the electrical current will heat the wires to a
  high temperature. The temperature of wires may be
  high enough to cause fire and if the insulation
  melts then a fire may follow.

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• In order to prevent too much current a circuit
  breaker or fuse is placed in the circuit. Too
  much current in the circuit will cause the
  breaker to trip and open like a switch. A fuse
  will on the other hand melt internally opening
  the circuit and shutting off the current. Both
  circuit breakers and fuses aim to do the same
  thing open the circuit to shut off the
  electrical current.

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• Breakers and fuses, which are too big for the
  wires they are protecting, are dangerous. The
  overload in the circuit will not be detected and
  the current will not be shut off. Circuits with
  improper overcurrent protection devices or no
  overcurrent protection devices are hazards

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Earth Leakage

• There are statutory requirements for earth
  leakage protection. Earth leakage protection
  attempts to protect personnel when they are
  providing the only path for fault current between
  an active conductor and the earth.

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• When natural earthing is very good or there is a
  high conductivity earth conductor, very heavy
  earth return currents may flow and this may
  damage expensive underground equipment or cause
  fires.

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• If the fault impedance forms a major part of the
  earth-loop impedance there may be dangerous local
  voltage rise.

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• Earth leakage protection is of two types
• 1. Detection of the out-of-balance current or
• 2. Consequent difference of voltage between metal
  casing or sheaths of apparatus and conductors.
• The first one is referred to as current operated
  protection and the second as voltage operated
  protection

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• Voltage operated circuit breakers detect the
  difference in potential between the casing of
  protected apparatus or its earth-continuity
  conductor, and an earth electrode at a suitable
  point not in the immediate vicinity.
• This can give protection against fatal electric
  shock but not against damage or fire risk from
  overcurrent.

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Earth Continuity

• If equipment is supplied with electricity through
  a trailing or reeling cable such as a jumbo then
  the risks associated with the use of the
  equipment can not be addressed by the use of a
  residual current device (RCD).

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• Earth continuity equipment that will isolate the
  power supply if a break in the earthing circuit
  occurs must be used in conjunction with the
  equipment

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Conclusion

• Protection Devices in Mines are critical.
• Lives will be at risk if these are not installed
  or not installed properly.
• Ensure qualified electricians only, service
  electrical equipment.