Help me understand chassis ground

[STS4]

Hi guys,

I am having a really hard time understanding chassis ground, best illustrated by the example of a car's electrical system:

The negative terminal of the car's battery is connected to the chassis to provide return path for all current of all the loads.

So basically, the chassis of the car is a conductor and when the car is started - it becomes a conductor that is part of a closed circuit. If this is the case, why don't you get shocked if you open the hood while standing barefoot on the ground?!? Or if you are wearing shoes - don't you become just another "load" in this closed circuit?

What am I missing here?

 

[chuckey]

if you grabbed hold of the car chassis, for current to flow, you must complete the circuit back to the battery positive.
Frank

 

[betwixt]

As Frank stated - to get a shock you need current to flow and that requires a path to flow through. It doesn't matter what the REAL voltage of the chassis of the car is, all that matters is it's voltage relative to the other circuits inside the car. In any case, it is unlikely you would get an electrical shock from only 12V.

Similar example: if you are insulated from any other conductive paths and touch the 'live' pin of your wall socket you will not get a shock despite it being 115/230V. Your whole body would adopt the 115/230V without you even noticing but if you touch something else that offers a return path fror the current you could be instantly killed. Don't try this at home!

Brian.

 

[STS4]

if you grabbed hold of the car chassis, for current to flow, you must complete the circuit back to the battery positive.
Frank

But the circuit is complete - the ions flow from the + terminal of the battery, through various loads, then through me, then to the - terminal of the battery.

Similar example: if you are insulated from any other conductive paths and touch the 'live' pin of your wall socket you will not get a shock despite it being 115/230V.

Your example is an open circuit, but when I am touching the chassis of a running car I would be touching a closed circuit, right?

Or is it that, yes, I do become part of the circuit, but my resistance is so great compared to the chassis of the car that a very tinny amount of current flow through me, hence I don't feel anything?

------------

I would like to add a side question here, so I don't litter the forum with my stupid questions

Your whole body would adopt the 115/230V without you even noticing but if you touch something else that offers a return path fror the current you could be instantly killed. Don't try this at home!

I've been wondering about this, as well. When I check my resistance by holding each probe of an ammeter I come to about 1.5MOhms. Let's say that my body is just 1 M Ohms for ease of calculation. Why do you run the chance of getting killed if you touch the 120v hot wire while grounded, when the total current comes out to a miniscule amount: 120/1,000,000=0.00012 amperes?

 

[betwixt]

There is an old saying: "it's volts that jolts but mills that kills", mills referring to milliamps. The current through your body is so small you wouldn't notice it, it would take a higher voltage than a car battery produces to drive enough current for you to be harmed.

If you have high resistance you probably wouldn't be harmed by 120V but try measuring the resistance again with wet hands and you will find your body resistance is much lower so more current would flow. I believe more people are injured by the physical effects of electrical shock than the passage of current, it's the sudden contraction of muscles that throws you into a potential danger. For example, getting a shock while handling machinery or electric tools.

Brian.

 

[Tunelabguy]

But the circuit is complete - the ions flow from the + terminal of the battery, through various loads, then through me, then to the - terminal of the battery.

No, the current flowing through the various loads has no way to get to you. Those loads are connected to the chassis. You also happen to be touching the chassis, but by then the current is trying to get back to battery "-", which an easy zero ohms path through the negative battery cable. Your touching the chassis does not help them one bit in getting to where they want to do. That would be a detour, and electrons are not interested in taking detours.

Your example is an open circuit, but when I am touching the chassis of a running car I would be touching a closed circuit, right?

Yes, you are touching a closed circuit, but you are not part of the closed circuit. In terms of roads, think of a cul-de-sac, whereas the path the electrons are taking is the expressway. It goes somewhere useful to them.

Or is it that, yes, I do become part of the circuit, but my resistance is so great compared to the chassis of the car that a very tinny amount of current flow through me, hence I don't feel anything?

No, even if you pave a cul-de-sac and make it 6 lanes wide (give it low resistance), no one is going to drive down it if they only have to do a U-turn to get where they really want to go.

I've been wondering about this, as well. When I check my resistance by holding each probe of an ammeter I come to about 1.5MOhms. Let's say that my body is just 1 M Ohms for ease of calculation. Why do you run the chance of getting killed if you touch the 120v hot wire while grounded, when the total current comes out to a miniscule amount: 120/1,000,000=0.00012 amperes?

That is true if your hands are nice and dry. You probably won't even feel a 120v shock if your hands are very dry. But if you sweat just a little bit, and if you hold tighter onto the probe of the ammeter, you will see a resistance much lower than 1.2 MOhms. For details on how much current is dangerous, see this website

 

[STS4]

Yes, you are touching a closed circuit, but you are not part of the closed circuit. In terms of roads, think of a cul-de-sac, whereas the path the electrons are taking is the expressway. It goes somewhere useful to them.

How am I not part of the circuit??? By touching the car chassis I have connected myself in a parallel circuit, relative to the chassis, i.e the current will flow through me>chassis>battery negative and chassis>battery negative with the majority of current bypassing me, of course because of my resistance; just like if I connected 2 resistors in a parallel circuit - one 1K ohm and the other 1M ohm, most of the current will go through the 1K ohm resistor, but some will also go through the 1M ohm, as well.

That would be a detour, and electrons are not interested in taking detours.

No, even if you pave a cul-de-sac and make it 6 lanes wide (give it low resistance), no one is going to drive down it if they only have to do a U-turn to get where they really want to go.

I don't understand your analogy. If it worked as you described - parallel circuits with different resistances would never exist - the electrons would always go down the path of less resistance.

 

[j33pn]

I have connected myself in a parallel circuit,

You are not connected in parallel until you have grabbed the chassis AND the +12V terminal of the battery.

And the point of the culdesac analogy is that it is a dead end. Like a water pipe cut-off and capped, no water (current) flows through it, regardless of how much water pressure there is.

 

[STS4]

I mean in parallel to the chassis.
Here are a couple of doodles I made to better illustrate what I am trying to understand:



I don't understand how I would not be part of the circuit. If you substitute "hands & body" with a simple wire, you would expect current to flow through it, right? So it should flow through me, as well, alas in much smaller quantity, since my resistance is huge. Isn't this exactly how an LED, for example, would connect on a breadboard, i.e. the positive and negative wires of the LED will sit on top of a conductor?

 

[j33pn]

Per the drawing, you are hooked in parallel with a conductor, so the amount of current flowing through you will be inversely proportional to your body's resistance vs the conductors resistance between the two points your are holding onto it, assuming that is part of the current path back to the battery if we are talking about a car chassis - there's no headlight current flowing through the rear bumper. Your resistance is very high, so you see very little current (none) and the conductor has a very low resistance and sees a very high current (all of it). The voltage drop required to move the current from one part of the wire (or chassis) to another part is essentially zero, which is the same voltage your body is seeing.

If you put your hands on each side of the headlights - you would have to get at the electrical connector attached to the headlights to do so - now your body will see the car battery's 12V across it (minus whatever it takes to move the current through the car's wiring and chassis), because all of the battery's voltage is going to overcoming the resistance of the headlight filaments. You won't feel anything though, b/c 12V is not enough to shock you.