Yow, i didn’t really want to get into this, but comments so far submitted are forcing my hand.
Electricity does some pretty strange things and 2 similar situations can result in 2 quite different outcomes. There is an old saying in Alaska that goes, “The only thing predictable about a bear is that he will be unpredictable.” The same can be said of electricity. My advice, expect the worst and you will never be disappointed. Treat every situation where you encounter possible energized lines, equipment or devices as if electrocution were possible. Hire a licensed electrician or wireman if you do not know 100% what you’re doing. DYI is a wonderful thing, but keep in mind, more people are killed by 120v in the US than any other voltage, simply because more people are exposed to it than any other voltage. I once caught an 80 year old woman trying to remove the base of a broken light bulb with a pair of needle-nosed pliers, and the socket was energized! On the other end of the spectrum, i have stopped a 3 year old boy from trying to stick a fork in an outlet.
To specific points raised in previous posts:
To paraphrase ftg (and elaborated on by engineer_comp_geek), pure water is a very poor conductor. But pure water is very hard to come by, a raindrop cannot form without a speck of dust. Although the danger may be overrated as engineer_comp_geek states, the danger is still present under the right conditions. Of course the thing is, what are the right conditions? To illustrate, in the sidewalks in Las Vegas (and, i’m sure, cities across the nation) are secondary junction boxes that are used to feed traffic and street lights. Many of these boxes have metal lids. Pedestrian traffic tread on these lids continuously, especially along Las Vegas Boulevard (The Strip) in front of the hotel/casinos. On Saturday, 16 Aug 03, an unusually wet evening, a woman stepped on one of these metal lids in a puddle and fell face first into the puddle from no apparent cause. Would be rescuers were shocked as they tried to reach her. The cause, as best determined to date, is frayed conductors contacting the metal lid combined with immersion in water and the fact that the woman was wearing open-toed shoes. Click here for the full story. I think this is the type of ‘odds game’ engineer_comp_geek is referring to in his post. While this can be viewed as a ‘freak accident’, think about how many times you have heard this term used in regards electrical shocks and electrocutions. Kinda gives one pause…
I believe that the practice of grounding trucks was originally adopted as a means to ensure that the breaker would operate ‘for safety reasons’ and subsequent utilities adopted this ‘for safety reasons’ without explaining it fully. Thus, the ‘safety reasons’ may have been interpreted, in lieu of an explanation, to be to prevent electrocution, as everybody knows that ‘electricity will take the path of least resistance’. I believe that what panamajack is referring to is the breaker operation, but it does not go quite far enough. While i cannot say 100% (i ain’t got enough dimes to make the calls) most, if not all, utilities require that the reclosing function of the power circuit breaker be taken out of service before work is undertaken on or in proximity to energized lines. For those that have not the slightest clue as to what i’m talking about, power circuit breakers on distribution and transmission lines are set to ‘reclose’ on an overcurrent fault to allow the fault to ‘clear’. This prevents extended interruptions should a line experience a ‘transient’ fault, such as a lightning strike. In such a situation, the breaker will open, preventing additional damage to equipment, and reclose in a predetermined amount of time. Should the fault still be present on the reclose, the breaker will again open and either ‘lock-out’ or reclose again (this is dependant on many factors which we really do not need to go into here). In the case of a ‘persistent’ fault, the breaker will eventually reach the point that it will lock-out and the line will remain de-energized. In the case of a ‘transient’ fault, the breaker will reclose and the line will remain energized, thus saving many souls from the ordeal of suffering extended outages. The mere fact that the breaker trips should a truck get into a line is not sufficient to clear an injured person as the breaker will most likely reclose at least once. This is why utilities demand that the reclosing function be disabled (a practice commonly referred to as ‘putting the breaker on one-shot’) when working on lines. As to the current flow if a person were actually touching the truck, whuckfistle mentions that the parallel path and the proportional current flow need to be taken into account. Remember, in a parallel circuit, the voltage is the same on all paths, the current varies depending on the resistance (this is simplified), it is not uncommon, depending on how far you are from the source, for distribution circuits to have in excess of 10,000 amps of available fault current. With this much available, it would not take much of a proportional share to be deadly. And, no Desmostylus, i did not mean to state that the lineman was electrocuted because the truck was grounded, rather in spite of the truck being grounded. To further explain, the truck in question had an insulated upper boom (it was an insulated bucket truck), the lineman in the bucket happened to rotate the elbow of the boom (the un-insulated portion of the boom) into a distribution line at the moment that the lineman on the ground was picking something off the bed of the truck. As he was a parallel path for the fault current, he received a shock that was sufficient, in this case, to be fatal. The reclosing function on the breaker had been taken out of service, and the breaker did open as a result of this contact, but it was not fast enough to prevent the electrocution. If anything, this is more an anecdote to be aware of the potential for live line contact than a justification for insulated bucket trucks. Be that as it may, I have run into too many people that mistakenly believe that because something is grounded, it is safe to touch. I presented the anecdote as an example to dispel those beliefs.
A little explanation of the ‘freezing effect’ that panamajack refers to is probably in order also. Contact with alternating current will cause muscles to contract, meaning that if you were to touch an energized conductor with the palm of your hand, your hand would close around the conductor rendering it, if not impossible, than extremely difficult to let go. DC, conversely, has the opposite effect in that it forces your hand open. There is an old saw that ‘recommends’ that you test an AC circuit with the back of your hand and a DC circuit with your palm. I’d like to meet the bonehead who dreamed this up just to see if he still has his hands. Of course, as nothing is ever that simple, the above mentioned effects hold true only so far. With AC, the ‘freezing’ effect applies to about 5000v. At voltages higher than that, the violence of the ‘jolt’ (i.e. the raw energy) tends to throw you clear, not always, but generally.
Now, as to the amount of current (and voltage) necessary to kill a person: again, it depends and the trick is in trying to determine on what it is dependent. panamajack mentions that you need about 25,000v to get ‘really dangerous’. OSHA, on the other hand, requires that insulating rubber gloves be worn on any voltage in excess of 50v. This illustrates the 2 extremes of the dependencies i keep mentioning. In perfect conditions a person can come in contact with 25,000 volts with no ill effect, whereas conditions sufficiently adverse will cause a dangerous shock with as little as 50v. In reality, in near-perfect conditions, a person can come in contact with voltages in excess of 765,000 volts with no immediate ill effects (no need to go into the long term effects of exposure to EHV fields in this little missive). The proof of this can be seen in that companies such as Haverfield Electric Air Team routinely do helicopter live-line maintenance on lines energized to 765,000v. Although the chances of you actually witnessing this are slim, next time Understanding Electricity airs on the Discovery Channel, make sure to watch as HEAT is featured on the show. Though there may be a tendency to guffaw about the ‘dangers’ of 50v, keep in mind that a shock does not have to be deadly to be dangerous. Think about the surprise you would feel if you were shocked with 50v, then think of your reaction to that shock, then think of that reaction should you be standing on a ladder 20’ above the ground over a pile of broken roofing tiles that you were replacing… Also, ask any welder how much fun it is to weld in the rain.
Both panamajack and engineer_comp_geek give current levels that illustrate relative dangers, but again, these figures are not absolutes. engineer_comp_geek qualifies his figures by mentioning that these are figures that are ‘presently agreed on’, and panamajack sites a reputable source, but keep in mind that these are still generalities (thus the chance percentages in panamajack’s post) and though 5mA may be ‘safe’, is that dependant on duration? I can’t honestly answer that, but it is logical to assume that the longer the current flows, the greater the danger of harm. Hopefully engineer_comp_geek can provide the information. The 5 amp guarantee is not a sure thing either, there have been instances involving severe shock that have caused power circuit breakers to operate or fuses to blow where the shocked individual lived, though not always in perfect health.
The above points were certainly not intended to cast aspersions on any posters, ftg, panamajack, Desmostylus, whuckfistle, and engineer_comp_geek have consistently submitted well written, thoughtful (and thought-provoking) posts on any number of topics and i always enjoy reading their submissions. The purpose of this rather long-winded missive is basically to get a very simple point across: Do not assume anything in regards dealing with electricity. When in doubt, hire competent help and stay in the clear.
Fiat Lux,