Do you and your team know what to do if a piece of equipment is struck by lightning or comes in contact with an electric transmission line? Quick and proper action could save serious injury or even loss of life.

Since mine sites are often at high altitudes and have no trees, mining equipment is often the highest point and a target for lightning. It is also common for truck beds and crane booms to accidentally contact electric lines.

Lightning and high voltage transmission lines contain an enormous amount of electrical energy. When equipment is energized by a lightning strike or contact with a transmission line, that energy goes to ground through the tires, the path of least resistance being the ply cords which in most radial tires are steel. The very large surge of electricity through the ply cord material creates heat that can weaken the bond between the ply and the surrounding rubber. Since an inflated tire and wheel assembly is a pressure vessel, any compromise of the integrity of the vessel could lead to a catastrophic event.

Further, the heat created causes expansion of the gases and can turn any moisture inside the tire into steam which can very quickly increase the inflation pressure beyond the capacity of the now damaged tire, causing a rapid air loss event. This however is only half the story.

The Hidden Danger

Heat can also initiate a process that is called pyrolysis which you could roughly think of as “de-vulcanization”, in other words, the chemical bonds in the strong tire rubber compounds begin to break down. Pyrolysis is exothermic meaning it generates heat and this heat speeds the process of breaking down and weakening the rubber compounds while also heating up the gas and moisture in the tire/wheel assembly, causing an exponential rise in inflation pressure.  Pyrolysis can take place very quickly or, it can take several hours to reach the point where the inflation pressure exceeds the strength of the now weakened casing. A tire that is undergoing pyrolysis gives no outward sign and that is the hidden danger after equipment is energized with electricity!

tire-damage

27.00 R 49 tires after a lightning strike. The four rear tires blew out immediately. The left rear out tire was propelled 250 m and the wheel 300 m. The “hidden danger” is in the front tires. 

What to do:

  1. Get everyone away from the equipment immediately and create a safety barrier around it as large tires, wheels and hubs have been found over 300 meters from the equipment after these events. The most critical areas are those 45 degrees outward from the end of the axle.
  2. Stay away for at least 24 hours as there are records of large tires having catastrophic air loss up to 21.5 hours after electrical contact as a result of pyrolysis.
  3. After at least 24 hr. scrap all tires on the equipment. The tires may look OK but they may have damage to the chemical bonds which cannot be seen. 

What is the safest way to get the operator off the equipment? Stay tuned.