ground to chassis question

Thread Starter

Volttrekkie

Joined Jul 27, 2017
63
I saw a you tube video that said that the grounding wire is connected to the chassis of an appliance so that if the hot wire should touch the chassis current will flow from hot to chassis to ground to neutral to transformer pole's center tap and not hot to chassis through you to earth ground? But if I touch the positive terminal of my car battery I don't get shocked unless I also touch the negative terminal. Why is this?
 
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dl324

Joined Mar 30, 2015
12,929
First, don't believe everything you see on YouTube. I've watched too many that were absolute garbage.

You attach earth ground to a metal chassis to protect people from getting shocked from line voltage electrical faults.

The 12V of a car battery is too low to present a shock hazard.
 

MrChips

Joined Oct 2, 2009
23,958
I saw a you tube video that said that the grounding wire is connected to the chassis of an appliance so that if the hot wire should touch the chassis current will flow from hot to chassis to ground to neutral to transformer pole's center tap and not hot to chassis through you to earth ground? But if I touch the positive terminal of my car battery I don't get shocked unless I also touch the negative terminal. Why is this?
Let us see if I can separate the two scenarios.
But if I touch the positive terminal of my car battery I don't get shocked unless I also touch the negative terminal. Why is this?
A car battery is 12V.
You can touch the positive terminal of the battery AND the negative terminal of the battery at the same time and you would NOT receive a shock.
That is because 12V is not a high enough voltage across normally dry human skin to create enough current to produce any sensation of electrical activity.

I saw a you tube video that said that the grounding wire is connected to the chassis of an appliance so that if the hot wire should touch the chassis current will flow from hot to chassis to ground to neutral to transformer pole's center tap and not hot to chassis through you to earth ground?
There are three wires from the electrical service panel, LINE, NEUTRAL, and EARTH (for single phase service).
AC power comes in on LINE and the return path to the panel is via the NEUTRAL.
No current should be allowed to flow through the EARTH connection (grounding wire). Hence for electrical safety, metal chassis of AC powered appliances or equipment should be connected to the EARTH connection.

Any malfunction that makes the chassis LIVE should cause the circuit breaker to trip. This is the primary protection for any user that is in contact with the chassis of the equipment or appliance.

AC mains voltage is high enough that if you touch the LINE connection alone you will get an electrical shock. You do not have to touch the NEUTRAL or EARTH connection to complete a circuit. There is sufficient leakage through your body, into your shoe, and into the floor to create a current path.

A properly designed, installed, and functional GFCI (ground fault circuit interrupter) is designed to measure any leakage current in the EARTH connection and any difference in current between the LINE and NEUTRAL paths.

Edit: btw, 120VAC has a peak value of 170V. At 240VAC, it is double 170V at 340V. Either can be lethal!
 

nsaspook

Joined Aug 27, 2009
8,531
Word to the wise, AC power comes in on LINE and NEUTRAL to power loads Don't trust NEUTRAL to be ground or earth bonded in some random electrical box without first checking with a good meter.
 

Thread Starter

Volttrekkie

Joined Jul 27, 2017
63
Oh, it must have been then because I touched the two battery terminals with a wrench while I was tightening them down. The wrench became extremely hot and burned me.
I have been shocked by 240V so many times it is not funny. One time it passed from one hand to the other through my chest. I am going to look into this further so I understand it 100%. Tired of getting shocked.
 

dl324

Joined Mar 30, 2015
12,929
Oh, it must have been then because I touched the two battery terminals with a wrench while I was tightening them down. The wrench became extremely hot and burned me.
When I was a teen, and didn't know better, I inadvertently placed a metal holder for my socket wrench on the battery in my engine compartment. There was a spark and I yanked the case off of the battery. Fortunately, it only burned a hole in the case; could have been worse if it got stuck and I couldn't remove it.

45 years later, I still have that socket set to remind me of my foolish youth.

When working with voltages above 50V, you should take necessary precautions. Trying to use one hand and touching things with the back of your hand if you're not sure if they're live.

I saw a person knocked on their behind by 220VAC when I was a teen and I've always been ultra cautious around it.
 

dendad

Joined Feb 20, 2016
3,890
I have been shocked by 240V so many times it is not funny. One time it passed from one hand to the other through my chest. I am going to look into this further so I understand it 100%. Tired of getting shocked.
That is concerning indeed.
If you have been shocked so many times, I think it is time you gave up on playing with electronics, at least until you take a very close and careful look at your work practices.
A good "rule of thumb" is to keep one hand in your pocket if you have to play with live mains.
Make sure your power is via an earth leakage device. And an isolation transformer is useful, but it will still not stop you getting dead if you get across both power lines.
 

ebp

Joined Feb 8, 2018
2,332
I have been shocked by 240V so many times it is not funny. One time it passed from one hand to the other through my chest. I am going to look into this further so I understand it 100%. Tired of getting shocked.
You are lucky you are still alive. With AC there is what is known as a "can't let go" current threshold. If you are holding onto something with your hand and get current above the threshold through your hand and arm it will prevent voluntary muscle control and you will not be able to open your hand and let go. The threshold is typically something in the range of 10 to 20 mA at 50 or 60 Hz. This is usually well below the threshold for fatal shock but there is always the possibility of the current increasing significantly if the skin of the hand gets damaged. Of course you also can't let go if the current is in the lethal range.

If you are regularly getting electrical shocks from AC mains you are doing something seriously wrong. If you are dealing on a regular basis with equipment that is or may be faulty then the way in which you deal with such equipment - you must assume it is dangerous until proven otherwise. If you are regularly using equipment that is known to be faulty then you need to quit using the equipment until it is repaired and made safe. People who work around dangerous voltages all the time don't get shocks.

You must remove any conductive jewelry when working around electrical equipment at any voltage if there is a possibility that it can deliver significant current. People get badly burned by rings or metal watch bands that short car batteries.
 
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