Earthing of appliances with conductive cases

Thread Starter

u0362565

Joined Dec 10, 2019
11
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.
 

BobTPH

Joined Jun 5, 2013
2,207
You are not misunderstanding anything. It would indeed be dangerous without the fuse, which is why the fuse is required by the electrical code.

Bob
 

SamR

Joined Mar 19, 2019
1,655
Electrons flow on the path of least resistance. The Safety ground to earth directs that flow to earth ground. If the flow is excessive the fuse/breaker opens stopping the flow.
 

MrChips

Joined Oct 2, 2009
19,932
If the line (live) wire were to come in contact with the earth ground the fuse or circuit breaker should open.
If the breakers did not exist then, yes, a person might still receive a shock depending on the resistances of the wiring and any faults in the wiring.

When we moved into our house I discovered that the ground wire at the GFCI (ground fault circuit interrupt) was clipped because the installer could not be bothered to trouble shoot the wiring that had an actual fault behind one of the outlets.
 

Thread Starter

u0362565

Joined Dec 10, 2019
11
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?
 

SamR

Joined Mar 19, 2019
1,655
Electrons follow the path of least resistance. Copper wire has very little resistance and the human body much more resistance. Therefore for safety, the earth ground is connected from the equipment chassis directly to earth ground since that would be the least resistant path. Even when energized/HOT touching the safety ground or equipment will not shock you. Equipment is actually tested for "voltage leakage" to ground and if above a very low level will not pass UL certification.
 

MaxHeadRoom

Joined Jul 18, 2013
19,777
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..
The Earth ground should always be electrically common to chassis and neutral conductor, regardless of fuse or breaker trip.
Max.
 

Thread Starter

u0362565

Joined Dec 10, 2019
11
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.
 

SamR

Joined Mar 19, 2019
1,655
Earth/Safety ground does not conduct until there is a leakage fault from the supply. Normal current path is from HOT to Neutral on 120VAC circuit. Safety/Earth ground is connected to the normally unpowered chassis to conduct current in case of a fault in the main power supply inside the device to earth ground. A bit different in 240VAC US/Canada circuits where both of the legs from the panel are HOT. There it is HOT/HOT/NEUTRAL/SAFETY Ground. Used to be just 3 wire HOT/HOT/GND.
 

TeeKay6

Joined Apr 20, 2019
569
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?
All faults are not equal. In the kettle example, suppose the fault was a leak in the kettle that allowed fluid (water?) to connect the heating coil to the outer housing. That is not a direct short and might very well not blow a fuse, but it might very well cause injury to the user who touches that housing (and some other contact to complete a circuit)--were the housing not grounded. Other faults that might not blow a fuse or breaker would include shorts created by burned material such as a crisply fried insect and by very thin "stray" wires (e.g. from the use of stranded conductors).
 

SamR

Joined Mar 19, 2019
1,655
I remember when electric hand tools had no ground and their chassis/outer body was cast from aluminum. Eventually, they would start to shock you when you pulled the trigger on them. No big deal, it was only a tingle. IF YOU WERE LUCKY!!! You just kept using it until the tingle became painful then went and bought a new one. Caused a lot of deaths until plastic bodies and grounding wires came along. When they finally came out with 3 prong plugs most people cut the 3rd grounding prong off with a pair of side cutters because there were no 3 prong grounded outlets.
 

MaxHeadRoom

Joined Jul 18, 2013
19,777
Now many kitchen and other appliances are fitted with 2 pin plug and no ground as supposedly they conform to the Double Insulated standard, I sometime have my doubts on some, steam irons etc?
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
19,777
One of the first things I noticed when coming to N.A. from UK was the large usage of un-grounded appliances, 2 pin socket use in UK had virtually dissapeared at the time I left.
Max.
 

Thread Starter

u0362565

Joined Dec 10, 2019
11
Thanks guys, I have a tendency to read into things too much. I can accept the practical aspect that the earth wire provides a relatively resistance free path to ground as it's the path of least resistance. However I often find this statement/fact misleading and I think this is why you mention old hand tools SamR that you're technically not immune from shock, the current through you will be much lower than the earth wire as you have much higher resistance but we cannot say no current will take this path.
 

SamR

Joined Mar 19, 2019
1,655
Standing in seawater wouldn't help but in a normal environment with shoes on it would be negligible to the point it would not even be felt, which is the point. The other point is where you put your other hand. Hand to hand conduction is not good.
 

MisterBill2

Joined Jan 23, 2018
4,929
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.
You have exactly understood the purpose of that bthird grounding wire, which is to prevent a hazardous voltage from being present on any acessable part of a device. Yes, it should pop the fuse or trip the breaker.
 

crutschow

Joined Mar 14, 2008
24,130
In the US (and probably in most places) the safety ground wire is the same gauge as the hot wire, so a short circuit to ground would generate roughly half the line voltage from the safety ground to earth ground until the fuse blows / breaker opens.
 
A point to know is that N and Ground are tied together at one point for the "house".

If there is no fault, then G at the kettle is at 0V.

During the actual fault using that kettle ground as the fault path, it is not.

In the case of a CATV system, the ground is a reference.

A "hospital" or other equipment may have two such paths. The reference ground and the safety ground and it's not an either/or situation. Look up "isolated ground" receptacles.

GFCI's will detect leakage as well as shorts to the case.
 
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