Ah ok so it is a short to ground in essence, thanks for clarifying. I think it's the term electrically common that got me in this scenario because the earth path is only electrically common with the ground once the fuse trips and it becomes open circuit? While the current does flow through the earth wire it's not electrically common with the ground-there is voltage..
Can I consider the point where the live touches the casing as a branch point for a simple parallel circuit,I.e. the current has 3 possible paths, through the load, through the casing and then earth wire or through the person. If each is a parallel branch then there has to be the full voltage drop across each branch? But that would mean the person could get a significant shock depending on their resistance?

Yes, you have those three parallel branches and all three receive the same voltage -- but NOT the line voltage. The connection from the case to the ground (and then to return on the supply) is very low and so nearly all of the current flows through that without developing a significant voltage. You actually have a voltage divider formed by the effective resistance of the wiring from the power supply to the case and the effective resistance of the wiring back to the supply via the ground connection. In theory this could result in a significant fraction of the supply voltage appearing across the load and the person, but in practice the amount of current that would need to flow to make this happen is so large that it blow the fuse or trip the breaker long before this happens -- which underscores why the fuse or breaker MUST be there.

 

Thanks for answering that Wbahn, helps clarify things in my head at least.

 

Hmm ok not sure I get why the earth wire is common to ground in a fault situation and current flows through the earth wire. If there is current there is a difference in potential? Therefore some aspect of this isn't electrically common with the ground.

For a moment lets speak in absolute terms:
Ground = the earth itself. (the term ground has more than just this meaning, but in your example, ground is earth)
Earth = um - the earth! Considered to be at an absolute zero voltage (though there can be disputes over this)
Hot (or Line) = the main source of power to a device.

In terms of absolutes - if there's an accidental connection to the metal casing from the Hot - the Ground (or you can call it "earth wire) conducts (in absolute terms) to ground. (yes, there are potential faults, line resistances and such - but lets ignore this for the sake of understanding)

IF there's such a short circuit - as has been stated, the fuse or breaker should trip. Something I didn't notice anyone address was what might happen IF there were no fuse or breaker. In such a case without a breaker, current would continue to flow from Hot to Ground continuously until the wire itself burned out. Potentially causing a fire. But again, in absolute terms, lets forego the fire hazard for a moment; If your pot is properly grounded and there's a short internally, touching it will present no hazard to you whatsoever. Someone already touched on wearing shoes and using a defective tool. Yes, you can use such a tool and not get a serious shock. AS LONG AS YOU DO NOT GRAB SOMETHING THAT IS GROUNDED. I've seen safety videos where a person was using a power drill with an electrical fault. There was no shock to the user until he rested the palm of his hand on a convenient pipe, which WAS grounded. In the video the actor received a lethal shock. HAD THAT TOOL BEEN GROUNDED first, the breaker should have popped. Second, in the case of a failure of the breaker, the user would not have been shocked. The ground wire will safely conduct all energy to ground (again, I'm speaking in terms of absolutes).

The real matter is "Absolutes". There are no such conditions where you can absolutely depend on ground to conduct all energy safely away from the user. Poor connections can elevate the potential energy a user can get should he touch the tool and a good ground. Long wire runs - since wires have resistance, those long runs can have significant resistance, and there's the potential you might have low enough resistance through your body to conduct a small amount of current. I'm not certain, but I believe I've heard it takes as little as 6 mA of current (0.006 amps) to stop your heart. So it wouldn't take a big fault in the ground system to allow a user the chance of being harmed.

As I mentioned earlier, there are plenty of definitions in electrical and electronic terms the word "Ground". And ground doesn't mean absolutely zero volts. Birds land on high voltage wires without harm. From their standpoint, they are at the same voltage as the power line. But without a path to a lower potential there is no shock. Should a bird manage to touch a 50KV line and a 25KV line at the same time - neither of which are at zero volts, there's a potential difference of 25,000 volts, and yes, that poor little birdie will have its goose cooked. So "Ground" doesn't necessarily mean "Zero volts".

But from what I've read of your initial understanding, you're on the right path. Just don't confuse the terms "Earth" and "Ground" to be different things. In the case of house wiring, they mean the same.

 

you can use such a tool and not get a serious shock. AS LONG AS YOU DO NOT GRAB SOMETHING THAT IS GROUNDED.

Learned that lesson the hard way 60 years ago. LOL

 

@SamR Glad you're still here.

I can't count the number of times I've taken a full on 120 VAC from hand to hand. Fortunately in all circumstances, muscular clampdown caused me to pull away. Had I been holding tightly onto something energized, I'd not be here.

 

Old aluminum cased 1/4" drill. If you used it one hand it would only tickle you a little bit unless you put your other hand onto the piece of metal you were drilling. Dad finally 86ed that deathtrap.

 

For a moment lets speak in absolute terms:
Ground = the earth itself. (the term ground has more than just this meaning, but in your example, ground is earth)
Earth = um - the earth! Considered to be at an absolute zero voltage (though there can be disputes over this)
Hot (or Line) = the main source of power to a device.

In terms of absolutes - if there's an accidental connection to the metal casing from the Hot - the Ground (or you can call it "earth wire) conducts (in absolute terms) to ground. (yes, there are potential faults, line resistances and such - but lets ignore this for the sake of understanding)

But from what I've read of your initial understanding, you're on the right path. Just don't confuse the terms "Earth" and "Ground" to be different things. In the case of house wiring, they mean the same.

Aye, there lies the rub!
In the UK where I got my training, we always used the term Earth for a grounded conductor, unlike N.A. where the term ground is used even for conditions where the object referred to is not connected to a earth ground, even the correct symbols are rarely used.
Here is an old U o fPenn document that shows it.
Max.

 

Yea. My story. I was probably 15 and sanding my car on my side with no shirt on and barefoot with a flat bad extension cord.
I got the bad extension cord caught between two toes. I could not let go. I thought about it and decided to scream. I was able to divert that shock so I could let go.

It's like on your body, what hurts more hurts. It overshadows the small stuff.

 

Aye, there lies the rub!
In the UK where I got my training, we always used the term Earth for a grounded conductor, unlike N.A. where the term ground is used even for conditions where the object referred to is not connected to a earth ground, even the correct symbols are rarely used.
Here is an old U o fPenn document that shows it.
Max.

Our group has adopted the term "Green Wire Ground", which defines the physical arrangement and implies that it is fairly much at the intended neutral potential.
And our HIPOT tester also has a test function to verify that that greenwire connection is adequately attached to the item frame and does not become disconnected in the event of a short circuit to the line wire. It is called the "ground integrity test".

 

Hi all, this is my second post in relation to the textbook available on this website. It's very good but still finding I need some clarification on some of the points it makes. I've linked to the page below:
https://www.allaboutcircuits.com/textbook/direct-current/chpt-3/safe-circuit-design/

I'm good down to the example that shows a kettle with a conductive case but the case has been connected to the ground via the earth wire in power cable. This is described as being electrically common with the ground and thus 0V. If the live wire were to somehow touch the casing would current flow between the hot wire the case and through the earth wire back to the 0V terminal? Because it creates a short circuit not passing through the load? A person would be safe because the short would trip the fuse in the plug or at the MCB? And the current flows through the earth wire rather than the higher resistance option of the person? If the fuse/MCB didn't exist wouldn't you have a dangerous short circuit and a shock hazard in the kettle casing? Because of the short circuit. That would not be electrically common though so I must misunderstand what's happening here. Thanks for the help on this all.

Keep in mind that there is still a parallel path for current to flow. One path is through the person in contact with the case & a grounded surface. The magnitude of the current is generally low, but depends on several things including the size of the grounding wire Grounding conductor wire that is too small risks a dangerous voltage drop from the case of the appliance to earth ground. The person in contact with the case could be in contact with a properly grounded surface (conduit, water pipe, etc.). Also, fuses & standard circuit breakers are not designed to protect a person from electrical shock. That is the job a GFCIs & GFCI breakers. If the grounding conductor wire in the receptacle is rated per NEC (NFPA 70), then there's not much to worry about, although I've been "bit" as a result of poorly terminated grounding wires. I was lucky. The current wasn't enough to injure me even though it passed through my chest. It sure was a wake up call. Now, while troubleshooting, I usually perform Lockout Tagout first (& verify zero-voltage state with PPE) & measure resistances & fix any problems I find, then power back up & test voltages (while wearing electrical PPE per NFPA 70E).

When in doubt, lock it out. Be safe!

Pat McFarland
Formerly an industrial electrician/ technician (15 years)
Currently a Plant Engineer responsible for electrical safety (20 years)

 

Keep in mind that there is still a parallel path for current to flow. One path is through the person in contact with the case & a grounded surface. The magnitude of the current is generally low, but depends on several things including the size of the grounding wire Grounding conductor wire that is too small risks a dangerous voltage drop from the case of the appliance to earth ground. The person in contact with the case could be in contact with a properly grounded surface (conduit, water pipe, etc.). Also, fuses & standard circuit breakers are not designed to protect a person from electrical shock. That is the job a GFCIs & GFCI breakers. If the grounding conductor wire in the receptacle is rated per NEC (NFPA 70), then there's not much to worry about, although I've been "bit" as a result of poorly terminated grounding wires. I was lucky. The current wasn't enough to injure me even though it passed through my chest. It sure was a wake up call. Now, while troubleshooting, I usually perform Lockout Tagout first (& verify zero-voltage state with PPE) & measure resistances & fix any problems I find, then power back up & test voltages (while wearing electrical PPE per NFPA 70E).

When in doubt, lock it out. Be safe!

Pat McFarland
Formerly an industrial electrician/ technician (15 years)
Currently a Plant Engineer responsible for electrical safety (20 years)

If the green ground is ten ohms and the skin contact resistance is 1000 ohms then the current ratio is 1/100. and usually the skin contact resistance is greater than 1000 ohms. And since we are talking about household appliances there is not the high voltage arcing to consider.

For industrial machinery the first requirement is that only those who understand what they are dealing with and know how to be safe should ever be involved. And they still need to focus on what they are doing.
The enclosure plus the safety shielding requirements for some countries look a lot more like they are protecting drunks bent on self destruction.

 

@SamR Glad you're still here.

I can't count the number of times I've taken a full on 120 VAC from hand to hand. Fortunately in all circumstances, muscular clampdown caused me to pull away. Had I been holding tightly onto something energized, I'd not be here.

The voltage that electrocutes (kills) the most people in the U.S. is 120vac. This is mainly because it's so prevalent. The fact remains that it can & does kill.

 

The voltage that electrocutes (kills) the most people in the U.S. is 120vac. This is mainly because it's so prevalent. The fact remains that it can & does kill.

Unless I missed something, no one has stated, implied, or hinted otherwise.

 

The voltage that electrocutes (kills) the most people in the U.S. is 120vac. This is mainly because it's so prevalent. The fact remains that it can & does kill.

Yes, that's definitely true. It's also true that most people who die from drug overdoses die from the drugs they are abusing. Kind of obvious.

It's no doubt I've been lucky to have not ended up in the morgue from electrocution. Nevertheless, just because I've survived a number of shocking's doesn't mean it's safe to be electrocuted. Nor does it mean I'm saying it's OK to be shocked. It's not! You're right, 120 is more deadly than 12 volts. But in the right conditions people have been killed by 5 volts. Case in point (rumor) a young girl had her cell phone (or tablet or whatever) plugged into the charger (5 volts) while taking a bath. The device slipped into the tub and (again, according to rumor) was electrocuted. If this is true, I'm sure it wasn't the result of 5 volts but rather a difference in potential and a poorly designed charger, and possibly a non GFCI outlet. Though rare, it's possible. I suppose.

The most common voltage we deal with every day from the time we get up to the time we go to bed is 120 VAC (US) and 240 VAC (European and others). So death from electrocution will be most common among the most common things we deal with. Lion tamers are more often killed by lions because they're more often in the cages with these dangerous animals.

While we all appreciate the statement of the obvious, we also recognize the importance of being aware of our surroundings. Not sure if it's this thread or another I've read but I learned that when on wet pavement that may be energized by HIGH voltage you should keep your feet as close together as possible while attempting to exit the danger zone. So even "Obvious" stuff isn't always so obvious to others. So thank you for pointing out that 120 VAC is dangerous.

 

The one hand in pocket rule,

and

Don't make connections to be measured on live equipment unless you have too.

I worked on 15 kV 1.5 A e-gun power supplies and 100 Kv @ 0.1 A alone most of the time, I was probaby a 12 YO kid working on TV's with HV and 600 Vdc supplies.

 

So death from electrocution will be most common among the most common things we deal with.

Statistics show that the most prevalent deaths from Electrical sources is due to flash-over burns, rather than direct electrocution.
Max.

 

That is why you NEVER stand in front of an electrical control panel bucket when you energize a circuit. And always secure the bucket door before you do. Plasma flash is nasty stuff.

 

That is why you NEVER stand in front of an electrical control panel bucket when you energize a circuit. And always secure the bucket door before you do. Plasma flash is nasty stuff.

And I always turn my face away when plugging things in, especially for the first time after a repair. I got an uncomfortable flash burn just plugging in an extension cord once. It blackened my fingers and copper flashed my fingertips.

 

That is why you NEVER stand in front of an electrical control panel bucket when you energize a circuit. And always secure the bucket door before you do. Plasma flash is nasty stuff.

Didn't see it personally, but an electrician was opening a 480 VAC 3ø panel when the nut holding the panel cover on came loose and fell and struck two legs of the 3 phase. The resulting explosion shook the building. Luckily, as you said, the technician was not standing in front of the door when that happened. But I remember the building shaking from the boom.

 

In one Industrial plant I worked in we had 4kv sub stations in various places, the requirement to operate the distribution disconnect switch was via a 8ft insulated pole that slid over the disconnect handle and rubber gloves.
Max.