Safety and health newsletter for the Oregon construction industry

September 16, 2014


An accident waiting to happen.

spider box

Let's assume you are working at a construction site and your supervisor asks you to get four spider boxes and four 50-foot temporary power cords from the back of the equipment trailer and hook them up. (A spider box is a portable power supply typically fed by a 125/220-volt, 50-ampere cord set; it usually has a 50-ampere outlet to hook up downstream spider boxes.)

You have not hooked up spider boxes before but you think it is just a matter of plugging them together with the four temporary power cords.

You notice that one of the cords has the female plug missing, but because the cord is going to be installed directly into an existing 220-volt electrical panel for power to the spider boxes, the female plug is not necessary. You notice that the other end of the cord is the male end, but it looks like it is OK, so you string the power cord out to the first spider box and plug it in to one of the receptacles. Then you find a journeyman electrician and ask him to install the uncapped end of the cord into the electrical panel. The electrician says, "Sure thing."

Now you start hooking up the remaining three spider boxes to the first one using the other temporary power cords. As you are crouching over the last spider box in the series, attempting to plug in the male end of the power cord into a female receptacle, you hear a pop, see an arc at the cord cap, and feel a tingle in your left hand, which is braced against the spider box. What did you do wrong?


You did not realize the plugs of the temporary power cord were reversed. After the electrician installed the female end of the cord in the electrical panel – a potentially lethal mistake – the cord (and the male plug) were energized with 50 amps and 220 volts. Because you incorrectly set up the first spider box, so were all the other spider boxes in the downstream series (the male end of each power cord connected to a spider box, and the mail receptacle in each spider box, were energized). If you touch the male end of a plug, or a male receptacle in the series, the chances are good that you will be shocked or electrocuted.

This incident actually happened at a construction site in Portland not too long ago. Fortunately, the worker was wearing nitrile gloves and work boots with thick Vibram soles when he was connecting the spider boxes, which spared him from a severe shock.

Factors that contributed to the incident:

What the company did to prevent the accident from happening again:

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