Handling and management of surface trailing cables (update to Safety Bulletin 6)

Introduction

Trailing cables carry large amounts of electrical energy when supplying surface mining equipment, including pontoon pumps, draglines and shovels at voltages from 415V to 22kV.

In 2014 and 2015 the Queensland Mines Inspectorate received ninety reports of damage to high voltage trailing cables supplying draglines (64 incidents) and electric overburden shovels (26 incidents).

Incidents due to rocks falling from buckets and the spoil dumps rose from four in 2014 to ten in 2015 (eight of these occurred at draglines representing 30% of reported dragline cable incidents) – see photos.                       

Incidents of damage due to cable tractors improved from sixteen in 2014 to six in 2015, but this still represents 14% of the cable damage issues in 2015. Also, 2015 saw two very serious cable tractor-dragline interactions; refer Safety Alert No.319 for more information.

High potential incidents involving trailing cables at surface mines include:

• A rock fell from the back of a dragline bucket hitting the trailing cable, tripping power to the machine and exposing electrical conductors in the cable.
• A rock, balanced on the chains of an empty dragline bucket, dislodged and hit the trailing cable supplying the machine as the bucket returned to the pit. 
  In a similar incident, a rock landed on the cable cover pipe of the trailing cable, splitting the cable cover and cable resulting in exposed conductors.
• A shovel was relocating when the cable tower contacted the cable causing the machine to trip at the substation and exposing conductors in the cable.
• A cable tractor operator relocating a 22kV trailing cable was unaware the cable was incorrectly positioned as he operated the clamping mechanism, which penetrated and crushed the cable insulation causing arcing damage to the cable and cable clamp.
• A dozer operator, towing a 22kV trailing cable stand, was watching the wrong cable loop and consequently stretched and pulled the cable apart at a repair / splice joint.
• Visibility was poor while cleaning up along the trailing cable and the operator was watching the cable out of the side of the dozer. He noticed the cable move and power to the shovel tripped, as the dozer had picked up the cable loop. 
• An operator lined up a dozer and proceeded to cross over a dragline cable at a steel cable crossover. Part of the dozer track plate entered into the opening at the top of the crossover and damaged the trailing cable.
• A dragline drag chain contacted a smaller rock in spoil which dislodged a larger rock that rolled down onto dragline bench hitting the trailing cable.

This discussion will cover handling, management and general operation of the cables, and seeks to raise awareness of potential hazards from principally high voltage trailing cables at surface mines. It also highlights areas that may need attention to ensure risks are at an acceptable level.

Potential Hazards

Machine operators have a higher potential of injury from trailing cables as they are among those most exposed to the cables while they are energised. Potential hazards from surface trailing cables include: 

• Uncontrolled release of energy from arcing caused by insulation failure, and explosion due to pressure rise inside apparatus for example plugs, caused by insulation failure. This could result in severe burns or being hit by projectiles; serious and potentially fatal injuries.
• Indirect contact with electricity, where a person receives an electric shock, not from direct contact with a live conductor, but by touching a part that is energised under fault conditions, such as receiving an electric shock after touching the outer sheath of a cable - the shock due to leakage current to the outer sheath caused by insulation failure. Potential outcomes include electric shock, burns and death.
• Direct contact with electricity, where a person touches an energised conductor in the cable, such as a person contacting with the receptacles inside an energised plug or touching a conductor inside a damaged cable. Potential outcomes include electric shock, burns and death.
• Fire, such as a damaged or faulty cable catching fire, which subsequently spreads to other equipment or the surroundings.
• Internal energy sources - biomechanical energy; such as excessive forces placed on the body through manual tasks. Potential injuries include sprains, strains and permanent disablement.

Potential Risks

Under normal conditions, risks from an energised trailing cable are low, but this can change through:

• Inappropriate work practices
• Poor maintenance and repair standards  
• Inappropriate selection for use; or 
• Inappropriate operational standards and practice.

Then the risks may become unacceptable! Injury, equipment damage or near hits may result. Incidents, damage or injury are indicators that risks are not being managed as they should!

Depending on where the machine is in its working cycle when cable damage occurs, power tripping may excessively strain mechanical components, for example, if power to the dragline trips while hoisting a full bucket and swinging, excessive strain may be placed on the boom and suspension ropes.

Another potential risk if the trailing cable is damaged while the dragline is in its regenerative cycle, such as plugging and lowering the bucket into the pit, is power may be back-fed into the damaged cable, causing arcing damage which continues for some time despite the electrical protection relays operating correctly and tripping the supply.

Typical surface trailing cable construction

Typical trailing cables have a durable, hard-wearing sheath designed to separate the power conductors from the external environment and allow the cable to operate and withstand mining conditions.

Generally an earthed barrier is located between insulated power conductors so if damaged a fault to earth (phase to earth) likely occurs before a larger power conductor to power conductor (phase to phase) fault. Earth fault current limitation devices are installed for all high voltage trailing cable installations.

Typical trailing cables also contain a small pilot wire, typically at the centre of the cable, used with protection relays to trip power in the event of cable breakage or plug separation, or prevent power up if plugs are separated or the cable is broken (pilot wire broken). Note: care must be taken with asymmetrical designs, having a pilot wire in the interstices of the trailing cable, to ensure capacitive and inductive coupling does not have the potential to reduce the reliability of protection relays.

Typical sources of cable damage

Typical sources of cable damage include:

• Machinery damage 
• • Machinery including dozers, cable tractors, graders, light vehicles and lighting plants driving or being towed over cables
  • Machinery striking cables e.g. grader blades, trucks with bodies up crossing under bridges 
  • Draglines set shoes down on cables and draglines/drills/shovels propel over cables. Soft ground causes machine to sink and contact the cable
  • Cable tractor engagement tools catching on cables
• Falling rocks from spoil piles, buckets swung over cables and flyrock strike and damage cables
• Termite/borer damage
• Excessive strain from pulling too much cable in one drag, towing excessive cable loops and debris caught on cables. Over tensioning cables can cause corkscrewing and damage earth conductors
• Operating inside the minimum design bending radius will damage the cable and reduce its service life, (check manufacturers guidelines for specific data)
• Wear to plugs and cable from chaffing while being towed
• Water ingress into plugs from damaged plug seals or plugs left unsupported on the ground
• Poor repair standards with repairs failing under service conditions
• Cable sheaths torn and punctured by sharp objects such as rocks, cable protection pipes, and dragline tub hooks, with pin holes from sharp rocks and redundant drag/hoist and dump ropes allowing water and dirt ingress.

Authorised by Russell Albury - Chief Inspector of Coal Mines

Contact: Neville Atkinson, Inspector of Mines , +61 7 49360130 Neville.Atkinson@dnrm.qld.gov.au

Issued by Queensland Department of Natural Resources and Mines