Here's an item that I have added to my urban survival kit. It is a voltage detector that works on the electrical field created by ac current. This is the "Circuit Alert GVD-504A" made by GB Instruments. It's a pen type thing about 5/8" square x 5" long. I think I paid something like $12 for it. You hold it near an energized conductor & it emits a beepbeepbeep sound. It will detect the electrical field in my desk lamp from 8" away.

I normally would assume that downed power lines are live, but in a major calamity you never know what may be happening. If you have a partially destroyed building, it may be useful to know if exposed building wires are live. Again, I say I would stay away from this if possible, but it may be necessary to pass through such an area to escape the debris or rescue someone.

Another neat trick is, if you suspect that something like a fence might be electrified by a downed line, you can check it by touching it with the BACK of your hand. If there is any electricity, you arm will jerk AWAY from the power. This works well for 110/220 type stuff, but not for use with high-voltage.

We used to test electric fences by holding a blade of grass to them, the grass had a pretty high resistance and would cause you to just feel a slight buzz.

Sounds neat, but does electricity cause your muscles to do the same thing everytime? It'd be a shame to fine out the hard way. I usually just assume all wires are live and act accordingly.

Incidentally, does anyone know how many amps it actually takes to kill a person? I always hear the "its the amps, not the volts that get you" warning, but just how many amps is too many? Haven't been able to find that, and I've looked all over.

Kyle

As an afterthought to this, here's another question:

Water's a great conductor, but how close would lightning actually have to strike the water to affect someone in the water?

Thanks,
Kyle

According to Wikipedia, 1 milliampere (1/1000 of an ampere) can kill you. In any case, it's a lot less than you think. It apparently doesn't take much to stop your heart or cause fatal fibrillation.

Oof. That's not much. <img src="/images/graemlins/shocked.gif" alt="" />

No. I believe that in general, DC usually causes your muscles to jolt once then relax, while AC generally causes your muscles to jolt shut and pretty much stay there. Keep in mind there are exceptions as electricity is inherently unpredictable, but this is what is generally thought when comparing AC to DC.


I believe the MythBusters use 6 mA through the heart as the threshold of death.


Actually, both amps and volts can kill you, but they do it in different ways. Volts kill you by disrupting the brain's involuntary nervous signals to breathing and pulse. In this case, many victims can be resuscitated by starting CPR as soon as possible. Amps, on the other hand, kill you by cooking your body tissues. The internal resistance of the human body causes heat, and CPR doesn't help that much for un-cooking you.

Close, but backwards. DC can cause your muscles to lock, but because AC's actual voltage varies, the muscle spasms between contracted and relaxed, letting you get your hand off. But in both cases, don't be surprised if one your buddies wacks you with a stick to save your butt.

But, nothing is written in stone. The big question that people don't think about is the current path that is taken through the body. If the heart or spinal column are in the way, there is real change it's "good night Irene". We also need to worry about the duration of the exposure, the wattage, because that factors in the seriousness of the burn.

And also how moist your skin is- dry skin is a pretty good resistor, moist skin... well, let's just say that powers of ten come into play with the comparison. That's why just brushing a cattle fence with the back of your hand just stings, but if you fall for the redneck dare and pee on it, they'll hear you scream in the next county- those things are DC. You're house has AC, so you'll yelp and swear and have something go numb, but most of the times you are ok. It's that fluke you have to be afraid of.

General rule of thumb, treat every circuit as if it were live until you know otherwise. It's just like a firearm. Although I'd never thought of using an inductive current sensor like that- attach it to the end of your staff, and you've got a pretty good sniffer if you have to cross rubble.

Oh, and the 1mA to kill has to be across a certain nerve in the heart, or in your brain stem. Again, the current path and skin resistance are major factors, but people have been killed with household and others have survived direct hits with lightning. Fun is playing with breakdown voltages, which is the point at which solid matter just kinda disappears. <img src="/images/graemlins/smile.gif" alt="" />

Thanks for pointing that out, ironraven. I wasn't able to find a source on GoodSearch before I posted earlier, but now I do have a quotable source.

From Lessons In Electric Circuits — Volume I — Chapter 3:
Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations. Thus, from the concern of becoming "froze on the circuit," DC is more dangerous than AC.

However, AC's alternating nature has a greater tendency to throw the heart's pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart stand still. Once the shock current is halted, a "frozen" heart has a better chance of regaining a normal beat pattern than a fibrillating heart. This is why "defibrillating" equipment used by emergency medics works: the jolt of current supplied by the defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover.

I think it is important to explain that the "touch with the back of your hand to see if it is live" advice should be taken in-context.

It is not a general-purpose method for testing if a circuit is live. It is just a good habit that MIGHT save you when you go to purposely touch something that is disconnected, turned off, verified with a meter or test light and SHOULD be perfectly safe to touch. As long as you intend to touch it anyway, make your first touch with the back of your hand.

Please don't use it to find out which breaker controls a circuit by having your wife in the basement turn off one at a time while you repeatedly touch the bare wire with the back of your hand.

I'm not an "electrical" guy, but I've been pondering this probe idea since yesterday and it makes me nervous and I'm not sure if it's for a valid reason. 0311, perhaps you can tell me if I'm worrying about nothing here. And I'm thinking about checking broken wires, not intact circuits here, because building collapse or some other calamity seems to be the scenario that you envision using it. Maybe I'm just confused about where the ground in an AC system needs to be.

Couldn't a wire be "dead" according to this probe, but then you actually get shocked if you touch it because you just completed the circuit to ground? I guess I'm thinking that this probe only works when current is actually flowing at the time you test it. No current = no EM field. It seems to me that it won't tell you if the line is actually not hooked up to a power supply or not. Does this question make sense?

No. Those probes actually detect an electric field, not a magnetic one. The voltage on the wire is still AC, and it will still generate a faint electric field, even if no current flows. I have had one, and they work well. The only problem is that the probe needs to be less than an inch away to detect anything. It is pretty useless for detecting the electrified puddle or tree, unless you are paranoid and use this thing on every object before you touch it.

Interesting... So, I could hold this probe up to an empty household socket and it would detect that it is live, right?

I know that this is an AC probe, but can an open DC circuit also be checked this way? Seems like DC characteristics tend to be more "common sense" than AC, so maybe my layman's thinking is correct for DC.

AC in the US alternates at 60hz. That means that the polarity reverses 60 times/second. So, you have 1/60th of a second for the reversal, & the voltage drops to zero for an infinitly short period of time. We rarely see DC at high voltages, so AC is the think to be concerned about.

The back of the hand idea is a good one for taking no chances with a circuit that is "known" to be disconnected. This is the last test for safety AFTER you are certain that you have disconnected it.

The probe will detect that an extension cord is live if it is plugged into a AC outlet but has nothing plugged into it. The probe will detect flowing AC as far away as 8". Yes, it will detect a live condition in an empty light socket. Loose building wire is what I am concerned about. A house may be destroyed in a tornado, but the electric may still be connected. Before I start chopping debris to free a victim, I want to know that the power is off. You might not get a shock, but crossing the conductors with an axe head could start a fire. AFAIK, DC cannot be detected this way.

Correct. You can detecd DC current using a hall-effect sensor, but if there is no current, then you can't see it until it bites you.

No worries, though. In a household situation, you will never see any DC voltage large enough to kill you. You only have to start worrying in an industrial setting, but even that would be rare.