Handling mine trailing cables
There are four types of tests typically done on the insulation of these cables. The first is a manufacturer’s test, then an acceptance test once the cable has reached the field. After that there will be maintenance tests on a recurring basis, followed by tests that are done when a failure occurs and the cable has been repaired.
Causes of failure include too much tension when moving the cable, mechanical damage, overloading the cable, or poor splices and terminations. The fault location will have an effect on the amount of energy released. When it is at the end of the cable, close to the mining machine, then the impedance of the cable will limit the fault current and potentially delay the reaction of the protective device.
When it is close to the supply end of the cable, then there will be more current and, therefore, a faster response – but also a larger failure.
An internet search, especially of the American Mine Safety & Health Administration, shows numerous electrocutions that have occurred over the years on these cables. It is important to identify the parts of a cable to determine where the faults could happen. The parts of the cable could be described as:
1. The end junctions or terminations
2. In the length of the cable where there is damage
3. Splices in the cable
4. In the length of the cable where there is no damage
No. 1 is the domain of the electrical workers and definitely dangerous. Safety in this area is outside the scope of this article, although it is important to note the danger of junction boxes in wet areas. I had a supervisor describe a 15kV junction box that was completely submerged yet did not trip their system. (If you are under the impression that water and electricity do not mix, try submerging a live trouble light in a plastic pail of water. It would be good to bring your shaver as you will grow a beard before a 15A breaker trips. The water will be energized as hundreds of milliamps will be flowing, creating a definitely lethal situation, but water and electricity, unprotected by a GFCI, mix quite completely.)
Damage in the length of the cable, No. 2, could be external or internal. External damage would generally be caused by a machine or materials damaging the outer sheath. External damage, whether in an open pit or an underground mine, can be caused by vehicles or equipment driving over the cables. In the dark, it is very easy to make a mistake and drive over one of these cables.
This damage could be on the outer protective covering, or down into the shield, or it could even be through the shield and into the internal insulation around the conductor. When the damage goes through the shield and insulation of the conductor, then there will be a fault from that phase to the shield; the protective relay should detect this and trip the system out. When the damage does not go that deep, then the cable is permanently weakened and is likely to eventually fail at that point. This external damage can be detected by keen observation given sufficient light.
There is a concern that a cable could be damaged and energize the ground around it, but this is unlikely. If the cable had the conductor insulation violated and the conductor exposed, the system would soon trip. General contamination would infiltrate the violated area causing tracking current; this would rapidly increase to a flashover and trip the protective system. You can expect, though, that damage through to the conductor would cause an immediate flashover and immediate trip.
It would be highly unlikely for one of these cables to be damaged to such a point that the external cover, plus the sheath, plus the primary conductor insulation could all be damaged and a live cable exposed to atmosphere, without having tracking current cause a flashover and trip the relay. In this highly unlikely circumstance, workers would encounter tingle voltages on the circumference of the subsequent ground gradient surrounding the fault. It is unfortunate that these would be masked by insulated boots.
Splice failures, No. 3, occur from high-resistance connections that can cause electrocution, hand burns and fires. These poor connections are easily detected in their early stages by feel or infrared scanning. Splices also fail due to internal short circuits and, again, it is highly unlikely to have leakage current external to the splice… but common sense dictates to stay away from them, other than for inspections.
More on this in my next column. Until next time, be ready, be careful and be safe.