If I have a relatively large dc voltage source of 100V to 200V and I touched only one of the wires would I get a shock? If I did get a shock would there be any difference between touching the positive or negative?
Short answer: No, you won’t get a shock.
Long answer: You might get a shock, but probably not.
The only way you’ll get “shocked” is if some of the current flows through your body. The current has to flow from the negative terminal to the positive and it has to flow through a conductor. Wires are extremely good conductors and air is an extremely poor conductor (hence it’s an insulator) but honestly nothing is 100% conductor or 100% insulator. The human body is kinda in-between. It conducts electricity, but not very well (unless it’s wet).
As long as you’re only touching one of the terminals (it doesn’t matter which one) then current can’t flow unless you are also touching the other terminal, or something connected to the other terminal. This is where it gets tricky, because everything touches everything.
For example, if you have a 200V DC power source sitting on a table and you touch the positive terminal with your right hand, the question becomes what’s touching the negative terminal? Certainly, the air in the room is touching it, but air is a poor conductor. If the power supply is in a metal box, and the negative terminal is touching the box, and the box is touching the table, and the table is touching the floor, and the floor is touching your foot, then yes it’s possible that current could flow from the negative terminal through the box and the table and the floor and through your body to the positive terminal.
But in reality that’s not much to worry about unless the table is made of metal and the floor is wet.
It is often true that the negative terminal is connected to the sides of the box. For example, in most automobiles, the negative terminal is connected to the car’s chassis. So if you touch the positive terminal with your right hand while your left hand is touching any part of the chassis, then yes current could flow through your body. But if you touch the negative terminal of the car, then it doesn’t matter whether you’re touching the chassis or not.
Thanks for the quick answer. It confirms what I thought but I wasn’t 100% sure about. Now what happens if the source is AC and not DC? I seem to recall a couple of times when I was careless with AC and got a shock even though I remember only touching one of the wires.
Same principle with AC, except there really isn’t a positive or negative terminal.
I feel obliged to point out that a 200V DC supply is a very dangerous thing. With AC you can let go- with DC sometimes you can’t.
I’ve been in the business a while, and I treat 200V of either kind as a lethal voltage.
It would also matter if the AC in question is a stand alone source or the AC power grid. If stand alone I don’t believe AC or DC would matter. If the AC power grid there are many more connection paths.
If you’re really, truly only touching one of the wires, then it shouldn’t be an issue. Case in point: high voltage transmission line maintenance, in which a helicopter deposits a maintenance worker directly on a line operating at several hundred thousand volts AC. The worker has to wear a conductive suit (his own personal Faraday cage) to fend off problematic induced currents due to the field around the wire, but the suit won’t save him if he manages to grab a ground contact while also hanging onto the AC line; he’ll literally fry. But as long as he’s only in contact with the AC line, he’s got no problem.
If you’re standing on the floor, then it’s a crap shoot as to whether your shoes (and the floor) are conductive enough to allow lethal current to flow through your body.
If you’re working on the 110V wiring in your house, then there’s also the question of whether you’ve grabbed the neutral line (which should be at the same potential as ground) or the hot line. When you look at a standard 3-prong wall outlet, only one of those contacts is carrying 110V AC. If you don’t know which is which you might grab the neutral contact, feel nothing, and think “hey, 110V is no big deal.” At which point you may be in for an unpleasant surprise if you grab the hot (110V) contact.
If you got shocked while touching only one wire, then that means you must have been touching something else which was indirectly touching the other wire.
This can happen if a house is wired improperly.
What’s supposed to happen is that there are three wires running to each outlet: hot, neutral, and ground. The hot wire alternates from about +150V to -150V and back again 60 times per second, while the neutral wire stays at zero. On average, you’ve got 110V that can flow from the hot wire while it’s negative or to the hot wire when it’s positive, but only if there’s a complete circuit touching both hot and neutral. The ground wire is LITERALLY connected to the ground (as in dirt) either by connecting it to the metal pipes, which in turn touch the dirt, or by connecting it to a metal spike which is driven into the dirt.
But on older houses, or houses which have been incorrectly wired, you might find that the neutral wire is connected to the ground even though it’s not supposed to be. If that’s the case, and you touch the hot wire, and some other part of your body is touching something which is indirectly touching the ground, then you can get shocked.
You thought you were only touching one wire but indirectly you were touching two.
Assuming a house is wired properly with three wires: hot, neutral, and ground…
If you touch hot and neutral, you will get shocked.
If you touch hot and ground, nothing will happen.
If you touch neutral and ground, nothing will happen.
If you touch just one wire, nothing will happen.
Pretty much any time you get shocked while touching just one wire, the truth is that you are actually touching two wires but one of them might be indirect. This is true whether we’re discussing AC or DC. Current can’t flow unless you have a complete circuit.
Are you sure about this? I thought the neutral wire was grounded at the transformer.
I believe you are mistaken about touching hot and ground and NOTHING will happen.
You’re going to get somebody killed.
“ground” and “neutral” lines are physically connected just outside the house, staked to the dirt as you note. The only difference between the two is that the neutral line continues from there back to the transformer. When you measure the voltage between the ground and neutral pins at an outlet, they should be equal, provided there is no current flowing through the neutral pin at the time you make your measurement. If there is current flowing (i.e. you’ve got something plugged in and drawing power), then the return current flowing through the neutral wire will raise its voltage a bit; then you might see a couple of volts difference between neutral and ground. But if a house is wired properly with three wires, then the difference should never be more than a few volts.
Which means:
If you touch hot and neutral, you will get shocked.
AND
If you touch hot and ground, you will get shocked.
Residential AC systems are grounded. The neutral wire is physically connected to an earth ground connector close to the breaker box. If you touch the “hot” wire while touching something that electrically connects to earth ground (a water pipe, aluminum siding, etc) then you’ll get shocked.
If your DC power source is grounded, then it will have the same basic issue. You will be able to safely touch the grounded wire but not the ungrounded wire.
If your DC source is not grounded, then you can safely touch either wire alone and not get shocked. You will only get shocked if you touch both wires.
There are ungrounded AC systems out there as well. They are called “isolated” systems since they are isolated from earth ground. They are used in hospital operating rooms and other locations, especially those locations defined as “wet” locations. Navy ships will also often run isolated AC systems. Isolated systems require constant testing and maintenance to insure that they remain isolated. Isolated systems are safer since you can touch either wire and not get shocked, but aren’t practical for residential AC service since mother nature likes to randomly insert ground connections into your AC system by doing things like having branches grow or drop onto the wires.
the neutral wire is connected to the ground at your electric meter or your service entrance (where the wires come into your building).
if you touch the hot wire and anything that is conductive which connects to the earth then you will get shocked.
if you touch the hot wire and the ground then you will get shocked.
Okay, I’m confused. If the neutral wire and the ground wire are connected to each other, then why have three wires at all? Also, I thought the phrase “ground fault” described the condition where the neutral wire and the ground wire get connected somehow and that this is an undesirable condition which GFCI outlets are supposed to detect and then react by disabling themselves. Am I wrong about that?
You first.
Which means it’s at ground potential – usually called zero volts. You might recall that hot is fluctuating between +170V and -170V, and back, sixty times per second.
Except at the main panel, where neutral must be bonded to ground.
This would happen anyway, unless the earth itself was mysteriously pulsating at 120VAC.
Please tell me you haven’t been wiring houses.
Under normal circumstances, current flows from the neighborhood transformer to the hot wire, then to the neutral wire, then back to the transformer (I’m speaking as if this is DC; the reality of course is that in a residential AC system, the current flows back and forth). The connection of neutral to ground just outside the house assures that the voltage on the neutral line doesn’t “float” up or down from earth/ground over time to the point where it could deliver a dangerous shock if someone touched neutral and earth/ground (e.g. a kitchen faucet).
A ground fault happens when electricity finds some path back to the transformer other than the neutral line, e.g. through the ground and back to that grounding spike outside the house, and then up the neutral line to the transformer. GFCI outlets/breakers detect when there is a mismatch between the current flows in the hot line and neutral line, indicating that current is going somewhere it shouldn’t.
A short-circuit between hot and neutral won’t trigger a GFCI device, since the currents in the hot/neutral lines are equal (but it should trigger a breaker, whether it’s GFCI-equipped or not, due to over-current).
A short-circuit between hot and ground will trigger a GFCI outlet/breaker because you have mucho current in the hot line and none in the neutral line.
If a sloppy electrician chooses to work on a live circuit, and touches the hot wire with one hand and the neutral wire with the other hand (and doesn’t touch ground due to wearing good-quality electrical safety boots), the GFCI outlet/breaker won’t trip because the current in the hot line and neutral line are equal.
A short-circuit between neutral and ground might or might not trigger a GFCI outlet/breaker. Remember, under the worst of conditions there’s only supposed to be a few volts of difference between the ground and neutral pins at the outlet, but depending on the sensitivity of the GFCI device, this may drive enough current between the neutral and ground pins to trigger it.
It’s a safety feature. The neutral should never be connected to something that is exposed to a human, like an appliance chassis. The appliance frame is connected to ground. That way, if there ever is a fault in the system, instead of a lethal voltage being present on the frame, a circuit breaker will blow.
I remember getting shocked as a kid, before 3-wire systems existed, when I played my record player in the basement. One side of the AC was connected to the chassis, but you couldn’t tell which side since the plugs weren’t polarized. If I plugged it on one way, I got shocked, the other, not. Many old tube radios had this kind of connection, too.
ETA: Machine Elf said it longer, but better.
The separate ground wire is a safety feature. Appliances with metal cases are bonded to the ground wire which should normally not carry any current. If a current-carrying hot wire should come loose inside the appliance and touch the case, the current will be conducted via the low-impedance ground wire rather than via the (relatively) high impedance person who might be touching the appliance.
So that raises the question of why not just connect the neutral wire to the case? What would happen if the receptacle was wired backwards? Now your appliance case is connected directly to the hot wire! Another possible failure mode is if a wiring fault causes the neutral wire to break. Now the chassis has no connection to ground, but a broken hot wire could cause it to be energized. The redundant ground wire separates potential fault currents from normal return current which makes things a lot safer.
Believe it or not, GFCI’s don’t even require a ground wire! A ground fault occurs whenever the current returning on the neutral conductor is less than the current coming out of the hot conductor. It’s called a ground fault because it is assumed that this missing current must be passing through something else (potentially a person) to the physical ground. This can especially happen in wet places like kitchens, bathrooms, or garages where humans’ resistance is substantially reduced.
A GFCI has sensitive electronics inside which will shut off current whenever such a ground fault is detected. Other things can trip a GFCI, such as large inductive loads (e.g. refrigerator motors.) That’s why you generally don’t want to plug refrigerators into a GFCI receptacle.
Because GFCI’s are so sensitive to current faults, they are not actually required to be grounded. This is why GFCI’s can be used to replace two-prong receptacles in old houses where adding a ground wire may not be practical. (In this case, the GFCI must have a sticker affixed to it that says “NO EQUIPMENT GROUND.”) It’s perfectly safe to plug most three-prong appliances into a GFCI without an actual ground connection.
It’s a serious concern with electric guitars, too. Just ask Keith Relf… though you might have trouble getting an answer.
Why we have three wires has mostly to do with what happens when things break.
Let’s say we have a typical home appliance of some sort, and it happens to have a metal case. If we don’t connect the metal case to anything, then if the hot wire manages to break and touch the case then the entire case becomes electrically hot. You touch the case of the appliance and something that is connected to earth ground (your kitchen sink maybe) and WHAMMO. Shocked.
So for safety, we need to connect the case to the “ground”, which is the neutral in this case since this is the older 2 wire system.
Now, if the hot wire shorts to the case, it blows the breaker. If the neutral shorts to the case, no biggie. It’s already connected to the case. If the hot wire breaks, then the thing just doesn’t work. No shock hazard, no biggie. But, if the neutral wire breaks, then you now have an electrical connection from the hot wire, through the device, to the floating neutral, and that neutral is connected to the case. So the case floats up to the full AC voltage and becomes hot. Touch the case and something grounded, and again, WHAMMO.
So instead of using the neutral as our protective ground, we run a separate protective ground. Now we have the modern 3 wire system. If the hot breaks, no biggie. The device just stops working. If the neutral breaks, the case is still connected to the protective ground, so no biggie. If the protective ground breaks, as long as the other wires aren’t broken or shorted, no biggie. If the hot shorts to either the neutral or the case, it blows the breaker. So no biggie. If the neutral shorts to the case, nothing much happens, so no biggie. So in any single failure, you can’t get shocked. The only way you can get shocked is if you have multiple failures simultaneously. So this scheme is much safer.
A ground fault happens when the electricity from the "hot’ wire finds another path back to ground other than through the neutral wire. Since this path might include you in it, it’s a safety thing. A GFCI measures the current going through the hot and neutral, and if they don’t match, it shuts the electricity off. So, for example, if you are using a hair dryer that has something seriously wrong with it and the electricity goes through your hand and into the sink, the GFCI detects that all of the current going out the hot wire didn’t come back through the neutral and shuts off the electricity, hopefully before it throws your heart into fibrillation and kills you.