In my last column, I identified the parts of a cable determine where faults could happen: 1) the end junctions or terminations, 2) in the length of the cable where there is damage, 3) splices in the cable, and 4) in the length of the cable where there is no damage. Let’s get to the last one, No. 4.
It is far more common to have an internal fault in the length: possibly a manufacturer’s defect, but much more likely from external abuse. We have one client with many decades of operation, and we know its operators often abuse the cables during moving. Their procedure calls for safety hooks or loops to be put in at least two places on the cable a certain distance apart but, for ease and speed, the will often just put one loop around the cable and start pulling with the machine. I have had operators describe seeing the cable twisting and looping back on itself as this is happening, though the cable does not fail immediately.
The difficulty this creates is that the operators know that the cable can take a tremendous amount of abuse and so, for their own speed and efficiency, they will abuse the cable. Abuse such as this will typically end up creating internal damage, as the flexing will eventually cause the cable to break down. It is difficult to convince a person to quit abusing cables when they’ve been doing it for years with no immediate failure.
Interestingly, although the acceptance test voltage might be 36kV on a 5kV cable, trying to intentionally fail the cable with either an AC or DC hipot would actually take well over 100kV. As we train mine operators about the dangers of these, we explain the type of damage that can happen in a cable, and what they should be watching for. It is a lot easier to do this in the day light, but they should be concerned anywhere close to a junction box or a splice. When faults are in the length of the cable, and operators do a visual inspection, moving a high-voltage trailing cable by hand would have a higher safety factor built into it than either a 120V or 600V cable.
It’s important to evaluate the literature and the accidents and determine where the accident occurred. Many accidents have occurred where the victim has been working at the junction box. Contact with a live system at the junction box does not apply to a risk in moving a mine trailing cable or touching it. A failure of a splice because it was poorly done also does not impact the danger of moving a perfectly good, undamaged cable in the length.
When I have had operators tell me they have seen cable flashes (and they describe a huge flash), I always ask them “How long ago was that?” I typically get the response, “About ten years”, which is a lifetime of technological change. Most clients now have very sensitive, high-speed relays that operate fast enough to minimize the arc flash.
Companies have to make the decisions on how they are going to operate their system, but at minimum, there should be the fastest relays as possible on the system, then train the operators on the dangers and how to perform inspections. Then, they have to ensure inspections are done properly.
Where operators occasionally hand-bomb mine trailing cables, there are many protective products available. Some companies require rubber gloves and, while you cannot go wrong with them, they are truly annoying for continuous heavy work, worse in hot weather, and contaminants such as tar sand can quickly destroy them. Many companies require ropes, slings and saddles properly spaced apart, but the abusers will continue to abuse them. There are D-rings that counter this with the proper bending radius in the design. Other companies will use insulated hook sticks or tongs, but dry conditions will quickly marginalize the insulating value.
We get both emotional and rational responses in our own instructor group. We have one very experienced substation electrician/electrical technologist who is absolutely adamant he would never touch the outside of a live 15kV cable. We have another instructor with the same credentials who just shakes his head and says it’s just a big extension cord. Both of these men each have over 40 years of high-voltage field experience. Who is right? Actually, both.
The bottom line is, there is a tremendous amount of danger when you are dealing with high voltages and high currents. At the same time, there are large extension cords, and when they are installed correctly and treated properly, then there is no more danger to them than touching a low-voltage cable. But hey… that’s a rational response.
Until next time, be ready, be careful and be safe.