Electric Shock From Automobile Body

The electrical system for vehicles is normally 12 volts much too low to cause a shock. You can get a shock from friction of the seats, usually the tires are acting as insulators and therefore are not grounding the car. Tires with too much carbon can have low insulating properties and cause more shock to happen.

The simplest answer is that the "ground" is only the electrical system common, and no voltage potential exists in reference to anything else, except the "hot" (positive in a negatively grounded system) part of the electrical circuit, and the ignition circuit when the engine is running.

For you to get a shock, a sufficient voltage potential would have to exist between you and the car body, and even the "hot" side of the wiring in a car is typically only 12-15 Volts. At lower voltages, below a few hundred volts at least, you have to complete a circuit in order to get a shock and the voltage has to be high enough to overcome your body's resistance: 15 volts is too low to sense as any kind of shock in any case.

At much higher voltages, the capacitance of your body is sufficient to allow the passage of current.

You can get a shock if a static charge has built up on you or the car. You could often feel a shock and see a spark between the all metal car key and the door lock from such a static charge, on a typical winter night, which is one reason they often put plastic on the handles of car keys.

Also, you could get quite a shock from an old "trick", long ago outlawed, that was to hot wire the car body to a self operating ignition coil (a Model T Ford ignition coil was one such self operating source of high voltage). While effective at preventing theft by the unwary, it would run down the battery if left on too long.

Greg

Greg.

It is by putting high voltage coil output to the ground with extra cable.

thus, this make the earth voltage higher than car body it self.

You cannot put high voltage coil output to shock you self to touch car body.

Been there done that.

Regards,

A. Kimiadi

USA

A. Kimiadi,

I have no idea what you tried but I can definitely tell you you don't need any cable to earth ground because I've done exactly what I described and many similar pranks as a mischievous youngster. Perhaps you were using the coil in the car's ignition circuit, but that is not what I described. Model T coils had a vibrator that created the high voltage output separately from any points or electronic equivalents in the distributor. This is not something I would attempt on modern cars for fear of possibly damaging some electronics, but in the early 60's there were none.

Greg

THANKS GURU

as said by coleagues earlier, the electric system of vehicules is usually 12V. and it is DC voltage. when you see the voltage range capable of causing electric shock, you will realised that 12 V is very minor. talking about earthing, unless there is a fault, one touching and earth cable has norisk of electric shock.

The potential (electrical, gravitational, thermodynamical) is defined unless an arbitrary additive constant. So, one should always say of "difference of potential, d.o.p": what voltage REFERRED to xx?

In the cars, the max d.o.p (except the H.V. to the sparking-plugs) is 12-15 Volt (ELV, Extra Low Voltage), too low for electric-shock hand-to-hand.

If you absolutely wish to feel electric-shock with ELV, try with a 1.5V battery, one pole connected to the tongue, the other to an internal point of the mouth.

More interesting is the problem of the ESD (Electro-"Static"-Discharge) between your finger and the metal of the car.

The phenomenon is more frequent in January-February, also with 100 % R.U., when the content of water-molecules/volume is very small.

By triboelectricity, or you (with shoes of rubber) or your car are at different potentials: a contact (with spark) cancels the different charge distribution.

The shock is painful not for the amount of energy involved, but because the spark is concentraded in an extremely small area.

In order to avoid this, I usually discharge my-self, in respect to the soil and the car-metal, clasping a key or a coin: the current of the spark is then distributed on a larger area.

Your house is connected to one side of the power company's mains too, and it's 120 volts or more! The reason you don't get shocked by your car or your house is because it's only one side of the source. Some cars use the negative side of the battery to connect to the chassis while others use the positive side. It is possible to be shocked by 12 volts, but usually your fingers don't conduct well enough for you to notice. Ever stick the connector end of a 9-volt battery to your tongue? It tingles!

On the other hand, while your body may be at the same potential as the earth (more or less), which is where the power company connects one side (or center tap for those who know) of the secondary side of their transformers. 120 volts or more is plenty high enough to cause a shock if you touch a "hot" wire in your house. (Has anyone ever told you to not stand barefooted in a wet basement when using electrical equipment?) Likewise, if you touch a ground or neutral wire and a hot wire at the same time, you won't have to wonder whether or not you're making good contact with the earth . . .

Just think about birds on a high voltage wire. They don't get electrocuted because they are in fact at the same potiental as the high voltage wire. But if they ever touch two wires at the same time, then they're toasted!

MidniteFighter

Lets not mix apples and oranges here. The Voltage in th car is D.C. And the house is A.C. You can still grab both terminals,plus and minus of a 120 volt battery pack and not get a shock. Dierct current must have a good conductor to flow and the human body is a lousy conductor. Alternating current on the other hand doesn't need as good a conductor, hence you can get a bute of a shock from house power. To put my .02 onto the original question, you asked why you didn't get shocked with all of the electronics grounded to the car body. Grounded is the magic word. No voltage potential. There are points in some of the electronics where you can get a hell of a shock but not at ground.

I would most strongly advise you NOT EVER to try to veryify by
experement that you will not be shocked by placing your body across
120 V. D.C. - that voltage level can be lethal if enough current is available.

Medical advice is that 0.002 A. (2 Milliamp.s) across the heart can stop it.
The body does 'dilute` the current by spreading it out over parts thicker in
cross section, but it's not advisable to take risks here.

As to the 'car` question: The potential difference is between the battery
and the frame 'ground`. There is little or no potential to the 'ground` you
are standing on.

O.T. : Low level D.C. 'shocks` are less perceptible than those from A.C. because
of the constant rather than alternating flow.

Pragmatist,

I agree.

I would also add that AC is typically stated in terms of its RMS voltage which for a "perfect" sine wave is the peak value divided by the square root of 2 (or the RMS * 2^1/2 ). Meaning that 120 VAC is really 170 Volts at its peak values.

Therefore for potential shock value, 120 VDC should be compared with an AC voltage much closer to/at 85 VAC.

Regards, Greg

Bill,

I agree with the thrust of your argument, but your house is in fact connected to both sides of the power company's mains. The various ground connections, from those at the generators themselves to the ones in your house, and all the grounds in between are "reference" grounds and carry only a very small amount of current overall.

Regards, Greg

Greg,

Thanks for the input, but I wasn't making an argument. I was trying to keep things simple.

If you had read my post carefully, you would have noticed that I said, "On the other hand, while your body may be at the same potential as the earth (more or less), which is where the power company connects one side (or center tap for those who know) of the secondary side of their transformers."

I am a professional electrical engineer with somewhere around 30 years experience.

Bill,

Its usually guesswork here as to the intent (as in your keeping things simple, or someone else overstepping their knowledge) of a post. My intent is here is to learn whatever I can from others, and share the knowledge that I have.

Even though I addressed my reply to you, my comments were actually directed to those in the community at large who may have incomplete knowledge of grounding's role. We are both aware that many people have relatively little understanding about details of the AC distribution network (because it's really very complicated), and that the nuances of grounding are the source of much misunderstanding.

In other words, my reply was to clear up possible misunderstandings of that part of your post rather than a rebuttal to you. Had you defined which side of the power main you were talking about, rather than just "house connected to one side ...." I would have understood immediately what you meant, as I do now, but I wasn't at all sure then.

On the other hand, I'm sure that many understood exactly what you meant: there is no "one size fits all'' for the level of detail or simplification. I tend towards too much detail, and am therefore more likely to misread a simplification as potentially misleading.

When I find what could be ambiguities or flat out mistakes in my own posts, you'll find I respond to that post immediately, and in the same strident style I use for other responses, the only difference being my discomfort prods me to throw in an excuse or two .

Regards, Greg

The answer to the question posed is simply that you MUST become part of a CIRCUIT to get a shock.

You must touch the positive lead of the battery (or a wire attached to the positive lead) at the same time as you touch the chassis to get the shock.

(I have a very long and involved story about how I demonstrated that you cannot get shocked as long as you only touch ONE terminal of the 550 volt generator lines at a time, and nearly gave my Tech Sergeant heart failure! Not that I recomend it you understand....)

(Oh, and don't get complacent just because it is "only" 12 volts....it is not the voltage its the current, and 12 volts will push plenty enough current to melt the ring right off your finger!)

Static charges are another thing entirely, and is why toll booth money takers don't like touching people's coins! But thats another topic entirely.