Down To Earth

I had been tasked to perform MPI on a concrete pump boom at an agricultural small holding. The paint had not been removed from the welds but was clean and free from cracking (and yes, I did measure the thickness). The booms were in an extended position with the tip touching the ground. This meant that a ladder had to be used in order to get to the main boom that was attached to the turret.

Down to earth image 1 here

Since the job had started in the late afternoon, the duration thereof ran into the night. When it got too dark to see without artificial lighting, one of the customers' labourers lent me an old halogen type floodlight that had been rigged to work straight from a 220V electricity supply i.e. fitting, cabtyre cable and plug.

2m up in the air, on the ladder, I energised my yoke and then sprayed the particles. The solvent ignited and the can became a mini flame thrower. I almost dropped everything and fell off the ladder!

My inexperience led me to believe that something was wrong with the yoke (the same inexperience that had previously led me to believe that my yokes switch was malfunctioning when in fact the neutral wire had broken near the housing). Fortunately, my office was about 10km from site. I went back there to fetch another yoke and while searching in the store for it, I came across a twin fluorescent tube light fitting that had been rigged for another inspection. I decided to take the light back to site with me.

Back on site, I plugged in the other yoke, climbed the ladder, energised and sprayed - action replay = fire. This time around the fright was manageable. I climbed down unplugged the old floodlight, connected my fluorescent light, end of problem.

After a long and tiring night, when packing up my equipment, another man who came to fetch the floodlight said to me "Boss, you must not use this light, it bites".

Years later, on retelling this story to an electrical engineer, I was informed that the floodlight could have had no earth function or was not connected properly or the electrical supply was not earthed. This could have caused the yoke to spark at the poles thereby igniting the solvent. I'm not entirely convinced - what causes the spark in the first place? In the past I have used yokes without wearing gloves and with not having a proper earth from the electrical supply, working on a cement kiln with 50+ circ welds was not a pleasant task.

Just the other day, I was working on a section of a new weigh bridge, with the grinder and welder following me and repairing the areas that I had marked up for repair. While the welder was welding (GMAW), I energised the yoke and you guessed it, sparks (arcing) was noticed between the poles and the steel. Any other suggestions?

The following was learnt:

Keith Cain