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Earth is really not shocking

by infomike
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infomike
#1
Feb17-13, 04:56 PM
P: 44
It appears to me that the reason that you can be shocked while contacting a high voltage line and the earth is not because of any inherent electrical quality of the earth, but because the power grid has been connected to earth at multiple grounding points. I have concluded that if the grid were not connected to earth then, under ordinary circumstances, I would not be shocked by any electrical line as long as none of those lines did not contact the earth.

Is this reasoning correct?
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Aussielec
#2
Feb17-13, 06:19 PM
P: 15
Quote Quote by infomike View Post
It appears to me that the reason that you can be shocked while contacting a high voltage line and the earth is not because of any inherent electrical quality of the earth, but because the power grid has been connected to earth at multiple grounding points.
Correct.

If where talking about "grounded system" your neutral will be connected to the earth's surface at the transformer. And depending what country your in the supply line neutral might also be connected to earth at regular intervals and at the installation.

This creates a situation where the potential difference between your active(hot) conductors and the earth will be equal to your active to neutral potential.






Quote Quote by infomike View Post
I have concluded that if the grid were not connected to earth then, under ordinary circumstances, I would not be shocked by any electrical line as long as none of those lines did not contact the earth.

Is this reasoning correct?
Not quite correct.

The reason being is that there may be a significant capacitance between our supply system and the earth's surface. This voltage could be upwards of several hundred volt's depending on the size of the system, the earth's resistance and dielectric constant.

Just because the grid's not connected to the ground does not mean we can not be electrocuted. And is one of many reasons why solidly grounded systems are prefered over non grounded systems.
my_wan
#3
Feb17-13, 10:15 PM
P: 863
Note that the reason the white wire in a typical US house does not shock you is because it IS connected to the ground. That ground is in effect the only different between the white and black wire in a typical AC system. Otherwise, in an AC system both wires alternate between both + and -.

When you get a static shock from a doorknob the charge was allowed to build because it lacked a ground connection. Hence you get shocked because you provided that ground. Lightning strikes for the same reason, once the charge builds enough to overcome the electrical resistance of the air.

Removing the ground will only make it more dangerous, not less.

Drakkith
#4
Feb18-13, 02:45 AM
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Earth is really not shocking

Per wiki: http://en.wikipedia.org/wiki/Earthing_system

In electricity supply systems, an earthing system defines the electrical potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply, and regulations can vary considerably among countries. Most electrical systems connect one supply conductor to earth (ground). If a fault within an electrical device connects a "hot" (unearthed) supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth (e.g., by standing on it, or touching an earthed sink) will complete a circuit back to the earthed supply conductor and receive an electric shock.

A protective earth, known as an equipment grounding conductor in the US National Electrical Code, avoids this hazard by keeping the exposed conductive surfaces of a device at earth potential.
Make no mistake, touching a live wire WILL shock you whether that wire is grounded or not. The purpose of the ground is to keep equipment connected to the power lines grounded in case of faults. That way you don't touch your microwave and have it shock you with a bazillion volts. Instead of having a buildup of voltage the device will have current flow to ground from the lines and probably trip the breaker.
infomike
#5
Feb18-13, 09:26 PM
P: 44
So it's possible to get a shock while using a common portable generator that is not connected to ground? This is assuming that the rubber tires of the generator are not good conductors

I experimented by starting my generator and measuring the voltage from one of the hot slots of a receptacle to the soil. My meter read about 20 volts.
davenn
#6
Feb18-13, 09:47 PM
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Quote Quote by infomike View Post
So it's possible to get a shock while using a common portable generator that is not connected to ground? This is assuming that the rubber tires of the generator are not good conductors

I experimented by starting my generator and measuring the voltage from one of the hot slots of a receptacle to the soil. My meter read about 20 volts.
no you shouldn't. for the same reason a 1:1 mains isolation transformer wont give you a shock between either secondary (output) line and ground.
Thats the whole idea of having ground isolation
You should only get a shock if you touch both lines ( wires)

BUT ... I suggest what is possibly being alluded to, is that at some point if the voltage potential is high enough, there will probably be a discharge between either leg through you to ground ( what voltage ? I dont know 500V ? 1000 V more ? and am not going to personally experiment to find out haha)

cheers
Dave
davenn
#7
Feb18-13, 10:07 PM
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just an afterthought about the generator

And I really hope the various manufacturers DONT do this
that is.... at the generator power outlet tie the neutral and ground together
Then you would have the same situation as with you mains power at home

Personally I would never buy a portable gene if the neutral and GND were tied together
(if they existed)

Dave
rcgldr
#8
Feb18-13, 10:57 PM
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High voltage lines carrie 500,000 to 750,000 volts. That's enough to electrocute someone even if isolated from ground, such as a helicopter. Even if completely isolated, antenna effect through a persons body could result in electrocution from a high voltage line.

People that service high voltage lines wear a faraday cage like suit, transition onto and off a high voltage line from a platform on a helicopter, and use a probe to set the helicopter potential the same as the line. After servicing a line and once back in the helicopter, the helicopter will initially be at some random voltage (potential) depending on when the probe was disconnected, but I assume the voltage leaks away into the air and/or a spark occurs during landing. Link to youtube video:

http://www.youtube.com/watch?v=9tzga6qAaBA
infomike
#9
Feb19-13, 01:44 PM
P: 44
Quote Quote by davenn View Post
just an afterthought about the generator

And I really hope the various manufacturers DONT do this
that is.... at the generator power outlet tie the neutral and ground together
Then you would have the same situation as with you mains power at home

Personally I would never buy a portable gene if the neutral and GND were tied together
(if they existed)

Dave
What IS very common is for the neutral to be bonded to the chassis or frame. That's pretty much the same thing. The idea being that if you use a transfer switch, the neutral-ground bond at the service panel is disconnected.
xlsdx
#10
Feb21-13, 10:30 AM
P: 8
neutral != ground. neutral returns to the transformer, whereas ground returns to the ground . since the transformer is also grounded, eventually both paths complete the circuit, but going through ground is not the ideal return path... grounding your home electricity is more for safety than anything. you never really WANT anything to use the ground path
sophiecentaur
#11
Feb21-13, 10:39 AM
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Quote Quote by xlsdx View Post
neutral != ground. neutral returns to the transformer, whereas ground returns to the ground . since the transformer is also grounded, eventually both paths complete the circuit, but going through ground is not the ideal return path... grounding your home electricity is more for safety than anything. you never really WANT anything to use the ground path
Before you make definitive statements like this one, you have to declare where the statement applies. US and UK have very different practices. If I could have 1 (or $1) for every time there has been a heated misunderstanding because of this, I would be able to buy a new boat.
davenn
#12
Feb21-13, 02:03 PM
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Quote Quote by sophiecentaur View Post
Before you make definitive statements like this one, you have to declare where the statement applies. US and UK have very different practices. If I could have 1 (or $1) for every time there has been a heated misunderstanding because of this, I would be able to buy a new boat.
agreed ... a pretty "out there" statement

xlsdx, you didnt even specify what type transformer you were referring to or in what situation ???

Dave
Crazymechanic
#13
Feb21-13, 02:10 PM
P: 853
That's right in europe well in Uk also I guess the local (the last one before your home) transformers neutral is tied with ground and the whole metal casing of the place is tied to the ground.
Now I live in a apartment so i have my neutrals tied to the bigger common neutral in the utility box outside my doors and that is again tied both to the neutral going to the transformer and to the ground wire that is directly grounded at the utility box and every next utility box or transformer.

But basically as others have said you can ground the wires or not that doesn't matter that much in your scenario because electricity is smart she always goes through the path of least resistance so if there is a voltage potential somewhere and if you will be the best path for that potential to ground then I bet my whole money you will get zapped.More or less depends on the situation if your the only path to the ground or there are other ones and then the voltage is divided by those.
That's why it is wise to ground your equipment and when building a house or restoring one use isolated floor material.In the USSR times this was a national standard that all high rise apartment buildings must have a floor that is made of wood or some isolating material.( The KGB just wanted you alive not dead as that would spoil all the fun ... :D:D) Usually it was pressed cardboard on wooden armature tied to the reinforced concrete floor slab.In the case of mains voltage 240/220 volts this provided a good insulation so I have experimented when I was a kid I touched the live wire standing on this isolated floor and nothing happened because there was no path for electricity to go.Ofcourse it is recommended to not replicate this experiment as not all floors are the same.
infomike
#14
Feb21-13, 08:36 PM
P: 44
Quote Quote by Crazymechanic View Post
That's right in europe well in Uk also I guess the local (the last one before your home) transformers neutral is tied with ground and the whole metal casing of the place is tied to the ground.
Isn't that pretty much the same in the U.S.? All the pole transformers that I see have their neutrals ties to a multi-grounded neutral wire.
rcgldr
#15
Feb21-13, 08:40 PM
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Quote Quote by infomike View Post
Isn't that pretty much the same in the U.S.? All the pole transformers that I see have their neutrals ties to a multi-grounded neutral wire.
True but in the USA, the grid that carries power from the generator stations only has 3 wires, for each phase of the generator output. Earth ground is used as the return when the load betwen the 3 phases isn't the same. This can cause a problem with stray voltage in the local ground / earth at locations like dairy farms. Wiki article:

http://en.wikipedia.org/wiki/Stray_voltage
infomike
#16
Feb21-13, 08:43 PM
P: 44
Quote Quote by rcgldr View Post
True but in the USA, the grid that carries power from the generator stations only has 3 wires, for each phase of the generator output. Earth ground is used as the return when the load betwen the 3 phases isn't the same.
That is true for the high voltage primary transmission lines, but I was referring to the distribution lines from the local substations which consist of 4 wires. (3 mains, plus neutral)
sophiecentaur
#17
Feb22-13, 02:49 AM
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The HV distribution system config is irrelevant to this.
In the UK, it is forbidden to connect Neutral to Earth at the consumer end. All Neutrals are common and connected to a common Earth at the sub-station, where there is a single, three phase step down transformer. (Star config of course)

[Edit: there is, effectively, a four wire system (underground), with three phases, a neutral and the armoured sheath which is grounded. Each domestic property is connected to one phase and the neutral plus the sheath.]


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