Why do we ground a circuit to frame ?will we not get electrocuted when we do so, upon touching the frame ?

 

Which has the path of least resistance? A grounded chassis or you standing in shoes? Hint, your bodies resistance is much higher than a copper wires.

 

It's not so much a matter of grounding a circuit to a frame as bonding the frame back to the source.

By bonding one point in the transformer secondary supplying the system to the equipment grounding conductor - most often a neutral conductor which is common between all phases - that system becomes solidly ground-referenced. That is to say, there is deliberate electrical continuity between the equipment grounding conductor (green wire or green/yellow in EU) and the ungrounded circuit conductors ("hot" wires) through the transformer windings.

In this way, if a fault occurs between one of the ungrounded conductors and an equipment frame which has been solidly bonded back to the transformer via the equipment grounding conductor, a complete circuit having very low impedance will be formed. The voltage to ground experienced at the frame will be minimized due to the low impedance back to the source, and sufficient current will flow to operate the overcurrent protective device protecting that circuit. Once the overcurrent protective device opens, the (relatively) low shock potential will be eliminated.

Much better than the alternative of an un-bonded frame sitting at line voltage indefinitely until someone wanders past and happens to touch it as was often the case prior to the 1960's.

As they say, code books and engineering standards are written in blood.

 

Which has the path of least resistance? A grounded chassis or you standing in shoes? Hint, your bodies resistance is much higher than a copper wires.

Okay so you are saying that even if we touch the frame, since its conductivity is much higher, the current will keep flowing through it and then to the earth and not our body?
If yes to above statement, then how about grounding car battery to chassis of a car? The chassis of a car is isolated from earth. Why do we not sense any electricity upon touching to car's body even if the battery is grounded to car chassis? I know I am missing major concepts, but what.

 

It's not so much a matter of grounding a circuit to a frame as bonding the frame back to the source.

By bonding one point in the transformer secondary supplying the system to the equipment grounding conductor - most often a neutral conductor which is common between all phases - that system becomes solidly ground-referenced. That is to say, there is deliberate electrical continuity between the equipment grounding conductor (green wire or green/yellow in EU) and the ungrounded circuit conductors ("hot" wires) through the transformer windings.

In this way, if a fault occurs between one of the ungrounded conductors and an equipment frame which has been solidly bonded back to the transformer via the equipment grounding conductor, a complete circuit having very low impedance will be formed. The voltage to ground experienced at the frame will be minimized due to the low impedance back to the source, and sufficient current will flow to operate the overcurrent protective device protecting that circuit. Once the overcurrent protective device opens, the (relatively) low shock potential will be eliminated.

Much better than the alternative of an un-bonded frame sitting at line voltage indefinitely until someone wanders past and happens to touch it as was often the case prior to the 1960's.

As they say, code books and engineering standards are written in blood.

Thanks a lot for the effort, but I am only a beginner who is trying to study electricity and electronics himself. Are you able to simplify your answer for a lay person? I couldn't understand it...

 

By connecting the exposed parts of a metal cabinet to earth/ground the cabinet will be at the same potential (voltage) as everything around it - the floor, the water pipes, etc. If you touch the cabinet whilst standing on the floor, both are at the same potential, and no current will flow, hence no electric shock.

The car battery whilst connected to the car body is connected to the ground around the car by a resistance - mainly the dirt and any water on the tyres. The car body will then be at the same potential (voltage)as the ground around the car. If you stand on the ground and touch the car, the potential of your hands (on the car) will be the same as the potential of your feet (on the ground) - no current will flow and there will be no shock.

Remember Ohm's law - the current flowing between any two points is the difference in voltage between the two points divided by the resistance between the two points.

 

They say a picture's worth a thousand words.

Maybe this will help illustrate a little bit.

EGC - Equipment grounding conductor - bonds equipment back to the source
GCC - Grounded circuit conductor (Neutral in this case)
MBJ - Main bonding jumper - the deliberate bonding point between EGC (ground) and GCC (neutral)
GEC - Grounding electrode conductor - system ground potential reference

 

I suppose we ought to have mentioned how much current is required before you notice it.
The maximum permitted by law before it is considered dangerous is 700uA.
It will take about 30mA to stop your heart if you are unlucky. If you are lucky, the current will flow through the sweat on your skin and won't go anywhere near the heart, but don't push your luck! Being sweaty can save your life!

 

The statement that electricity follows the path of least resistance is a misnomer.

Electricity follows all paths of resistance proportionally.

E.g. a non-current-limited circuit having 120 volts potential to ground will flow through the sweat on your skin *and* through your heart simultaneously given the opportunity.

 

My guess is that the most common shock would be between finger to foot, and be current limited by the impedance between foot and ground, and the second-most common between two points on the same finger. I wonder if people in 230V countries get fewer shocks because they are more careful?

 

I wonder if people in 230V countries get fewer shocks because they are more careful?

Well, I've worked with more people than I can count who think it's a good idea to splice circuits live, so there may just be something to that argument, lol.

As far as common shocks, it varies widely. I've heard tell of someone getting bits of their ear blown off when they touched a 277V circuit because said ear was touching a metal stud. People being unable to let go of a live conductor because their other hand is latched onto the panel tub, etc...

Nasty all around.

 

Well, I've worked with more people than I can count who think it's a good idea to splice circuits live, so there may just be something to that argument, lol.

As far as common shocks, it varies widely. I've heard tell of someone getting bits of their ear blown off when they touched a 277V circuit because said ear was touching a metal stud. People being unable to let go of a live conductor because their other hand is latched onto the panel tub, etc...

Nasty all around.

It pays to work with a colleague. Not just so that when you recoil from the shock, they can point and laugh and say "Was that a bit live, then?"

People seem scared of three-phase, but by far the most common shocks are phase-to-ground, so in that respect the 400V phase-to-phase is almost irrelevant.
However, 384V of 80Ah lead-acid batteries needs some respect.

 

I would argue that 120V is the deadliest voltage. It's prevalent everywhere (at least in NA) and people are desensitized to it.

"It's only 120... just get it done!"

Complacency kills.

The real danger from three phase systems is that most three phase circuits (again, at least in NA) have low impedance and are fed from big gear not too far away - and therefore often have a lot of incident energy available in the event of an arc flash. 10~63kA from a 1 meg unit-sub any day of the week.

 

I've had several of the plant electricians tell me that they touch feel for 120V live circuits. I didn't argue, just walked away with the willies thinking about it and shaking my head. Luckily in my career none of them were ever electrocuted. Suspect their supervisor may have had something negative to say about the practice and not all of them were prone to do so.

 

I've had several of the plant electricians tell me that they touch feel for 120V live circuits.

That's rather less common in 230V-land!

 

I've had several of the plant electricians tell me that they touch feel for 120V live circuits. I didn't argue, just walked away with the willies thinking about it and shaking my head. Luckily in my career none of them were ever electrocuted. Suspect their supervisor may have had something negative to say about the practice and not all of them were prone to do so.

People take shortcuts, I understand that. My primary live electrical safety shortcut at this point in my career is to have another person do the job.

 

I've had several of the plant electricians tell me that they touch feel for 120V live circuits.

Back in the early 1900's the code book suggested exactly that sort of practice if I'm not mistaken. It also suggested that some people might not have the pain tolerance for it and should wear leather shoes or stand on rubber while doing it.

Technically if you know what you're doing you *can* get away with it. But that doesn't necessarily mean it's a smart thing to do. That's like seeing if a gun is unloaded by pulling the trigger.

 

I was working in an old house, originally built in the 1600s and remodeled several times.
The electric was through armored cable. I was holding a steam pipe to give me leverage and accidentally touched the armored cable.

Full 120VAC hit me. Fortunately, I was able to jump off the ladder and break contact.

Lucky!

 

Someone didn't use listed AC cable connectors and/or didn't bond their boxes.
Spend enough time doing electrical work and you learn not to trust the guy who came before you.

5 times out of 10 it's either a homeowner special, a maintenence guy or some hack with a license and the gall to call themself an electrician.

Remember... half of the doctors in the world graduated at the bottom of their class. The same is true of every other trade.