Guidance about trailing and reeling cables

What is a trailing or reeling cable?

Trailing and reeling cables are electrical power cables for mobile apparatus, such as large excavators, draglines, stackers and reclaimers. The cable allows the mobile apparatus to move without disconnecting its electric power supply.

A reeling cable is specifically designed to be frequently reeled on and off a cable drum or reeler (e.g. an iron ore rail-mounted reclaimer), whereas a trailing cable is specifically designed to be moved with the mobile apparatus (e.g. a coal mine dragline, mineral sands mobile processing plant).

Requirements for trailing and reeling cables

Trailing and reeling cables pose an elevated safety risk (e.g. a damaged trailing cable can lead to earthing faults and workers being exposed to electric shock hazards). They require specific design mitigation measures, which are detailed in the Mines Safety and Inspection Regulations 1995.

Design standards applicable to trailing and reeling cables

Trailing and reeling cables must conform to Australian Standard AS 2802 Electric cables ? Reeling and trailing ? For mining and general use (other than underground coal mining) and incorporate a pilot core arranged to cut off the supply of electricity in the event of a break in the earthing circuit.

Once an earth continuity relay and associated connections of the protection system are installed, the pilot core allows pilot earth loop or earth continuity monitoring protection to function.

Refer to r. 5.21 of the Mines Safety and Inspection Regulations 1995 for more details.

There are currently no underground coal mines in Western Australia so AS 1802 Electric cables - Reeling and trailing - For underground coal mining purposes does not apply.

Australian Standards are available from Standards Australia.

Why is a pilot core included?

The inclusion of a pilot core allows pilot earth loop or earth continuity monitoring protection to function. It is important to realise, though, that this protection method is not the same as pilot wire differential protection (where the current entering and leaving a cable is measured and compared).

Rather, earth continuity protection functions by establishing a loop current through the pilot core, returning through the cable earth. This is typically achieved using an earth continuity protection relay to apply a low voltage DC signal between the pilot and earth of the cable. By measuring this signal, the resistance of the circuit can be determined and protection operation initiated for variance from a pre-tested value. This variance is indicative of a break in the earth return path of the cable, and possibly the cable itself. With this arrangement, an end-of-line resistor is required to avoid a short circuit between the pilot cable and the earth.

Functions required for the earth continuity protection system

For the overall earth continuity protection arrangement, the system must perform the functions described below.

  • The protection system must monitor the status of the earth continuity for any break in the earth return path. Typically, this will require a status indication of earth continuity healthy, open-circuit or short-circuit.
  • In the event that the status of the pilot earth continuity circuit is open-circuit or short-circuit, an interlock must immediately initiate automatic de-energisation of the mobile apparatus.
  • When a fault does occur, the system should require manual reset — the system should latch and thereby store the fault condition in order to prompt fault-finding and testing.
  • The protection system should ideally interface with the control system of the mobile apparatus such that a healthy status indication for the pilot cable is required prior to start.

Commissioning and testing required at installation

Earth continuity protection must be installed, commissioned and tested in accordance with the manufacturer’s instructions and recommendations. These instructions and recommendations are required to conform to Australian Standard AS 1747 Reeling, trailing and feeder cables used for mining ? Repair, testing and fitting of accessories.

Refer to r. 5.21 of the Mines Safety and Inspection Regulations 1995 for more details.

Australian Standards are available from Standards Australia.

Records and details of electrical installing work carried out at the mine are required to be recorded in the electrical log book. This includes commissioning check sheets and test records, which must be kept. An entry should be made in the logbook indicating where the sheets and records can be found.

The earth continuity protection relay itself should have a label affixed indicating the test date and who conducted the tests, including company details.

Refer to rr. 5.13 and 5.14 of the Mines Safety and Inspection Regulations 1995 for more details.

Safe work practices

Trailing and reeling cables need to be installed, located, supported and protected in a way that:

  • minimises the risk of damage to the cable and to any connecting or coupling device
  • does not obstruct any access way (this may include a pit roadway)
  • separates the cable from other services at the mine.

In practice, this may require the use of protective berms and the use of visible markers for trailing cables.

Refer to rr. 5.19 and 5.21 of the Mines Safety and Inspection Regulations 1995 for more details.

Ongoing maintenance

Mines (including exploration sites) are required to have an appropriate maintenance system in place to ensure that electrical equipment and installations are maintained in safe working order.

A routine maintenance system must be implemented for testing earth continuity protection systems to confirm the integrity of the system and the pre-tested pilot and earth loop resistance value. This is critical because the system relies on the accuracy of this pre-tested value.

For surface operations, routine testing should be done every three months. For quarries, dredges (other than a floating treatment plant) and underground mines, this test must be done every month.

Refer to r. 5.27 of the Mines Safety and Inspection Regulations 1995 for more details.