Electrical safety in mines is a topic which in my opinion doesn't get the attention it deserves. Arc flash and shock are still the hazards that exist but due to the nature of mining, certain situations offer a much greater risk of incident than those in a typical factory.
There are some fundamental reasons why electrical safety in mines is a little different than most places and this article aims to point out a few of those unique situations so you will be better prepared to handle them. I'm not going to go into the details but just touch on some of the high points and hopefully peak your interest to research further (or just ask questions in the comments).
Mines move around
I'm not a geologist but I do know that after cutting a "room" in an underground potash mine the ground around you on every side wants to close back in. Now it's not like an Indiana Jones movie or anything but if you were able to turn off all the machines around you and stand very quietly in a freshly cut opening you would hear a cracking and popping similar to the sound you hear when pouring milk into a bowl of rice crispies. That's the sound of the ground compressing and trying to fill that hole back in.
You could imagine the difficulty this adds to the electrical distribution system. Picture what would happen if the cable trays in a mill were constantly shifting and moving around with enough force to sever a cable in half. Cables are constantly being pinched, scraped, bent, and crushed and sometimes this can lead to a lethal shock just waiting to happen.
Cable design has been improved over the years to include a protective armor underneath the coating of the mine power feeder cable and certain monitoring systems are in place to detect any issues. However, I have seen cables with pin holes in them that have gone unnoticed by the monitoring system, that was of course until the cable was drug through a puddle... then everybody knew it had a hole in it! Kaboom!
Miners move around
If you still picture the Seven Dwarves when someone is talking about a mine then it's time to update your mental picture. Today's miners are sitting in the cockpit of a 250-ton machine that look's like a tank with a huge cutter head attached on the front. These machines can cut through the ground at tremendous rates and are powered by electricity.
Imagine a 5,000-volt extension cord that's about the width of a baseball and 1000 feet long plugged into a substation at one end and the mining machine at the other. Now imagine that when the mining machine gets to the end of that cord you need to plug another one in. It's the same theory as using an extra extension cord in your back yard when the skill saw just won't reach the back fence but when you are plugging and unplugging 5,000 or 15,000 volts you might want to make sure you know what you are doing.
This is where a pilot circuit comes into play. Inside the power feeder cable, there is a smaller conductor used solely for the "pilot circuit". It is constantly being watched by an upstream device that sends a signal out and waits for it to come back. If a plug were to be uncoupled (or the cable severed as in the above case) then the pilot circuit would be broken and the device would tell the power to shut off.
The ground isn't the ground
I once heard someone say that grounding is the "black magic" of the mining electrical world and it's all due to this one concept. The ground isn't the ground. Well, as far as your main electrical substation and the associated ground grid is concerned. Depending on the type of mine you are in or whether it's a surface or underground mine the ground you are standing on may or may not be the same as the system ground that you would find at the main sub.
What ends up happening is that the system ground is carried to all the equipment through the mine power feeder cables and trailing cables. The ground conductors are monitored closely and the system is tripped if a fault is detected.
The bottom line
The bottom line is that the mining world offers many challenges to electrical safety and one needs to be well aware of what they are. I've only highlighted three of them in this article but there are plenty more. To find out more you could get a copy of the standard M421 Use of electricity in mines or simply post your comments or questions below.
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