A reader writes:
I have heard on the worksite (construction; I'm working through college as a part-time fetcher and carrier) that if a power line falls, or someone drives a crane into power lines...
...you should move away from the danger site by taking tiny little steps, or even jumps with your feet together.
But I have also heard that I need to go somewhere and ask for a bucket of compressed air, or a "long weight", or a box of right-handed pipe elbows, on account of we only got left-handed ones here.
Is the pogo away from the power line thing just another way to make people look stupid? Doesn't the electricity get grounded into the... ground?
Jay
Oddly enough, this is actually good advice. It may not be necessary in a particular situation, but better to look a bit of a dick and survive than stride away in manly fashion and die.
It's all about the voltage gradient. Connect a power line to ground by cutting it or leaning some metallic object against it and the electricity doesn't just magically vanish at the contact point. If the contact point were an actual earth stake driven deep into the ground then even a quite major power-line short pretty much would disappear right there, but if all you've got is some cable draped on the ground, or big voltages buzzing to earth in scary arcs from this and that part of the frame of a piece of construction equipment, then it's sort of like pouring water onto level ground. Some soaks in at the point where it hits, it spreads out and some more soaks in, it spreads out more and even more soaks in, et cetera.
In this analogy, voltage maps to the depth of water on the surface. The closer to the contact point(s) a given piece of ground is, the higher the electrical potential at that spot will be. This is where the analogy breaks down, though, because you will come to no harm if one of your feet is in two inches of water and the other is in one. If one of your feet is on a piece of ground charged to twenty thousand volts and the other is on a ten-thousand-volt spot, though, you'll have a ten-thousand-volt potential from foot to foot, and you'd better hope your shoes have thick rubber soles with no nails.
Here's an occupational-safety video, as cool and stylish as such videos tend to be, explaining this:
The best thing to do is stay in the vehicle and let the electricity pass around you; the metal frame of a truck is way more conductive than a human, and you're probably sitting on an insulating seat anyway.
If you're in a sparks-and-fire situation best viewed from a considerable distance...
...though, then hopping out of the vehicle so that you don't touch the vehicle and the ground at the same time, and then pogo-hopping away, is the best chance you have to avoid becoming a very crispy critter. (If you don't want to see the severely charred body of a fork-lift operator who lifted his fork into power lines, don't click here.)
Tiny mincing steps can work about as well as pogo-hops, and may be safer in construction-site terrain. The idea is to get away without falling on your face and enjoying the large potential difference that now exists between your knees and your nose.
Even an actual earth stake may become less and less effective as a current sink if a lot of power passes through it for long enough, because that'll heat the area and boil out the water that makes the ground usefully conductive. The same applies to vehicles that are shorting power lines to ground; as the arcing and burning progresses, the area under the vehicle gets drier and less conductive, and the danger zone expands. Usually the power's cut off pretty quickly, but not always.
(For this reason, dry sand and most kinds of desert-dry ground are a bad place to hammer in an earth stake. Since you'll find water just about anywhere if you dig deep enough - this is the Great Secret of Dowsing - you can get around this problem by using a really long earth stake, provided you have some way to pound it into the ground. Hammering in an ordinary earth stake and pouring water around it will work just fine... until the water drains or evaporates.)
Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.
And then commenters will, I hope, correct at least the most obvious flaws in my answer.
20 August 2012 at 3:43 pm
In the local fire service, we've been taught that it's preferable you take shuffling steps by sliding your feet no more than a 1/2 length forward at a time. This is safer than hopping as it helps you to maintain balance, and by shuffling (as opposed to actually stepping) your feet don't break contact with the ground. But, as Dan said, any movement is a last resort and you're far better off staying put and waiting until you're certain the power's off.
If you work for the electricity authority, though, it would seem you're allowed to do all sorts of wild-looking things. I've attended a couple of incidents where overhead power lines have broken and come right down to the ground. People think the fire brigade knows best what needs to be done so they ring us first, but all we've done is set up a 25m+ exclusion zone to keep everyone out of the danger area. When the electricity guys showed up they walked confidently up to the broken ends, grasped one in each hand, then loudly declared, "Yup, she's dead all right!". I'm assuming they had the benefit of prior knowledge...
20 August 2012 at 9:41 pm
Dan, you are entirely correct about avoiding large steps. There's also the option of *running* (which may come more naturally than shuffling), in which case there are times when you have *zero* feet on the ground.
What's interesting is that in such a situation, the voltage gradient will exist no matter what the intervening material, be it dirt or the steel plate deck of an oil platform. Shorting a high-voltage cable in the latter scenario is far more likely to result in tripping a breaker, of course. However, it also means that sending electrical signals isn't as trivial, since even under normal conditions, "ground" at one location can be some number of volts DC higher or lower than "ground" at another, and the same applies for AC.
I used to be an instrumentation engineer for a company that built such oil platforms, and having a large facility with a large grounding system (pretty much anything metal is grounded) means you can't use earth as your signal ground. In fact, in the instrument cabinets, there are commonly two or three grounding bars--one for power, one for safety, and one for all the instrumentation. They get tied together at some point, but only at one point. Ground loops are a pain.
21 August 2012 at 8:55 am
This, exactly. It's what I learned in the transmission and distribution overview class I took when I was working for San Diego Gas & Electric many years ago, and I have retold that piece of advice many times because it's so strange.
By the way, this is why electrical substations have a buried copper grid underneath the entire "footprint" of the secure area inside the fence - if something connects to ground, everything will be at more or less the same electrical potential and it won't be an issue for anyone working inside the fence.
It's also why crackheads who try to dig up the copper wire buried a foot or so under the gravel end up being found dead occasionally.