Mine trailing cables are used to supply mine mobile equipment such as miners, draglines, shovels, drills, bucket wheels, conveyors, etc. They are typically 5kV to 25kV cables and sometimes as high as 72kV. In addition to being high-voltage, they also carry significant current, providing a tremendous amount of energy in megawatts and horsepower. These can create dangerous situations that have killed many mine workers over the years.
This danger is countered by the cable design, manufacture and integrity. Modern cables are all shielded, with a braid of grounded metallic sheath around the insulation surrounding each conductor. There may or may not be an outer shield around all of the conductors that would also be grounded. There is a very, very high safety factor inherent to the insulation of the cables, but from electrical failure – not mechanical abuse. When there is an insulation failure, there would be a blast of current from the phase conductor to its shield, hopefully tripping the protection system.
The protection is provided by protective relays monitoring these ground faults, or any phase-to-phase fault, and tripping the supplying circuit breaker. Additionally, in the cable there will be a separate wire called a pilot wire (a.k.a. ground check). This will be series monitored and, should any break occur in the pilot wire, it will trip the system out, based on the assumption that when the pilot wire is separated, the cable must have been damaged or separated.
It is customary to have high-impedance grounding to limit fault currents as well as reduce voltage-to-ground during an electrical contract to a survivable level. Due to the dangers of these systems, prudent companies will ensure they have high-speed protection.
Once the cables are installed, the danger is further countered by safe work procedures, safe work practices, PPE and training. As the machines move, the cable also has to be moved, which increases the risk to the workers, as the cables are moved both by machine and by hand.
In our experience, human contact with these cables elicits two responses: first, a highly emotional response: “These are incredibly dangerous and stupid to even touch”; or, second, a ration response based upon the safety factors described above, and the fact that those cables need to be moved. That, plus the fact that incidents are rare.
While researching the subject, I found an accident that happened to an experienced mine worker who received a shock from a 5kV trailing cable that was operating at 4160V phase-to-phase, and 2400V to ground. The trailing cable had been hanging too closely to a moving conveyor, and the conveyor belt rubbed through the outer insulation. The worker inadvertently touched the cable at the damaged spot and received a severe shock.
Technically, it is unlikely the belt rubbed through to the conductor as that would have bridged between the conductor and the ground sheath surrounding the conductor insulation. It is more likely that there was a remaining thin layer of insulation around the conductor that allowed a charge to be developed, and this was the source of reduced shock.
2400V-to-ground is higher than the voltage used in the electric chair; even momentary direct contact would cause skin burns. Since the worker in this case was not burned, I believe there could not have been a direct contact. Regardless, a different current path could have cause heart failure, so there is definitely a danger… but I will explore that in my next column.
Until next time, be ready, be careful and be safe.