The Logic Behind Earthing

Thread Starter

alexfrey

Joined Feb 23, 2019
22
I think I have started to understand the logic behind earthing but several questions exist in my mind. As I understand, we bury a copper rod in earth where we want to make earthing (also neutral of transformer which supply electricity to us).

But as I read, earth isn't a good conductor like copper. So it means that it has high resistance and if our earth rod is far away from transformer , doesn't this earth way between our earth rod and transformer rod cause significant voltage drop ? won't it make us not feed our circuit as much as it needs ?

Also what is the reason of ground loop. I can't animate it in my mind ?

Thanks.
 

crutschow

Joined Mar 14, 2008
27,898
The neutral of a transformer carries the line current.
The earth ground carries no significant current under normal (non-fault) conditions so the earth resistance is not a problem.
The earth ground is just to prevent the power line from floating to a high voltage, keeping it at "ground" potential.

Ground loops are normally local in instrumentation and audio setups and cause errors or hum in the signal.
They are caused when there is more than one ground connection causing a "loop" which picks up a small AC voltage by magnetic induction from the power line.
They have nothing to do with the earth connection.
 

WBahn

Joined Mar 31, 2012
26,398
I think I have started to understand the logic behind earthing but several questions exist in my mind. As I understand, we bury a copper rod in earth where we want to make earthing (also neutral of transformer which supply electricity to us).

But as I read, earth isn't a good conductor like copper. So it means that it has high resistance and if our earth rod is far away from transformer , doesn't this earth way between our earth rod and transformer rod cause significant voltage drop ? won't it make us not feed our circuit as much as it needs ?

Also what is the reason of ground loop. I can't animate it in my mind ?

Thanks.
With rare exception, the earth is not expected to carry the load currents. The earthing is done primarily for safety considerations.

But, having said that, while the resistivity of the earth is often pretty high, the size of the conductive path quickly grows to such dimensions that the total resistance can be very low.

For instance, say that the resistivity of the ground in a region was 10,000 Ω·cm. A very rough rule of thumb is that the effective resistance between two grounded electrodes can modeled as a cylinder whose length and radius are equal to the distance between the electrodes.

So for two electrodes 100 m apart, the effective resistance between them is going to be a fraction of an ohm.
 

MaxHeadRoom

Joined Jul 18, 2013
23,780
Back when I practiced in the UK, the neutral was not referenced to the earth ground at a residence, the ground was obtained either by metalic water pipe or a ground rod.
We had to Megger test the earth conductivity by testing between the neutral, which was grounded back at the transformer star point and the local ground conductor, it had to conform to a minimum low resistance to pass inspection.
A Ground Loop in the context of industrial machinery is where different parts of the machine are not reliably bonded together at a Earth ground point causing a potential difference between some of these points and result in influencing low voltage control circuits when earth currents flow.
At one time, decades ago, local telephone communication was achieved using one conductor, the return path was earth ground.
Max..
 
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nsaspook

Joined Aug 27, 2009
8,770
https://www.ecmweb.com/content/building-powerful-grounding-foundation

The average power consumption for the fabs is ~2 kW/m2 so electrical system grounding on power dense tool sets is extremely important for all the above reasons. Most have their own redundant multi MW sub-stations with static ground grids bonded to the building ground (usually a ufer ground system in the entire foundation) dedicated for plant operation.

Intel Oregon plant power.
https://www.axiomimages.com/aerial-stock-footage/view/AX53_104
https://www.axiomimages.com/aerial-stock-footage/view/AX52_116
 
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Thread Starter

alexfrey

Joined Feb 23, 2019
22
The neutral of a transformer carries the line current.
The earth ground carries no significant current under normal (non-fault) conditions so the earth resistance is not a problem.
The earth ground is just to prevent the power line from floating to a high voltage, keeping it at "ground" potential.

Ground loops are normally local in instrumentation and audio setups and cause errors or hum in the signal.
They are caused when there is more than one ground connection causing a "loop" which picks up a small AC voltage by magnetic induction from the power line.
They have nothing to do with the earth connection.
Thanks for respond. If there is no connection to ground why we care floating voltage ? I mean, there is no way for current to go back transformer so why is it important to have not floating voltage. I may understand floating term wrong. As I understand it has no connection to earth ground. So we can measure voltage difference in two point in circuit. If that is not true understanding. Could you please explain it ?
 

MaxHeadRoom

Joined Jul 18, 2013
23,780
Here is a paper published a while ago by the U of Penn.
If safety is an issue, a circuit or equipment should be earth grounded for protection.
If low voltage, generally 50v or lower especially where galvanic isolation occurs, grounding is not required.
Max.
 

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be80be

Joined Jul 5, 2008
2,057
@ crutschow He hit the nail on the head
I really liked that post.
There is a lot of people that don't understand earth grounding
We install a lot of 480 volt 3 phase power which is fun but to get 208 and 120 you have to add transformer.
They use MDP for that but the last building we built are own MDP.
You have a transformer with 4 taps one is neutral. You wire one side with 3 phase 480
the low side comes out with the 4 taps neutral and 3 at 120 volt each.
This is where grounding is needed you ground the neutral tap if not it will float.
I've had guys forget the grounding and let the smoke out.
Here In the US there is still one wire power out in rural towns.
The house transformer gives you 2 voltages 120 and 240 believe it or not it's still used.

Like the above pic just that T2 would have 3 taps.
I made a little video how well the earth works as ground.
I wish I was a good writer
 

MaxHeadRoom

Joined Jul 18, 2013
23,780
There is a whole raft of articles and Utube vid by Mike Holt out there on Grounding and Bonding.
I got my copy of Eustace Soares book on grounding from Abebooks for a $1.00, as new.
Earth Grounding is all about Safety in high voltage circuits!
Max.
 
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nsaspook

Joined Aug 27, 2009
8,770
Thanks for respond. If there is no connection to ground why we care floating voltage ? I mean, there is no way for current to go back transformer so why is it important to have not floating voltage. I may understand floating term wrong. As I understand it has no connection to earth ground. So we can measure voltage difference in two point in circuit. If that is not true understanding. Could you please explain it ?
We care about floating voltage because the imperfect electrical universe is a way for current to go back to the transformer (potential source). That path might be through your body and/or sensitive equipment or components in some random system of imperfect but electrically conductive connections. When we bond circuit common or neutral to earth ground we eliminate a large slice of those imperfect but electrically conductive connections by providing a much better directed solid connection for that voltage to return to the transformer (potential source).
 

pmd34

Joined Feb 22, 2014
520
So where do those isolation transformers you use when using power tools outside fit into the scheme of things? And why not use the same idea all the time, and prevent the danger of electric shocks?
 

Yaakov

Joined Jan 27, 2019
3,565
So where do those isolation transformers you use when using power tools outside fit into the scheme of things? And why not use the same idea all the time, and prevent the danger of electric shocks?
Almost all power tools are "double insulated" making it impossible for the case to become connected to the live mains. For metal housings, the earth ground is connected to the case so if the live mains should become disconnected inside or short to it, the path to ground is much lower impedance than through the user.

An isolation transformer makes it so the 0V reference for the live mains is not the earth ground, so a grounded user is not a return path. This is good, but it is bulky, expensive, and inefficient. The double insulation scheme is much more practical.
 

MaxHeadRoom

Joined Jul 18, 2013
23,780
Even double insulated tools are used on external outlets using GFI, in N.A. at least, these sockets are referenced to earth ground, in the eventuality that the unit receives water damage or immersion for example.
Same for Bathroom appliance use and kitchen outlets within 6ft of a sink..
Max.
 
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mvas

Joined Jun 19, 2017
538
SWER, Single Wire Earth Return, must send all "return current" through the Ground Wire and through the Earth because their is no Neutral Wire.
The SWER type Electrical Grid is used in Australia, Brazil, New Zealand and Alaska.

One side of the Primary Winding of the Local Transformer is connected to the HV Hot and
the other side of the Primary Winding is only connected to a special and separate High Voltage Ground Rod.

Single Wire Earth Return ...
https://slideplayer.com/slide/7427106/24/images/4/How+does+it+work.jpg

So, there is one situation where all of the "return current" for the Hot Wire must flow through the Ground Wire and through the Earth.

The Neutral Wire of the secondary winding of the Local Transformer will typically carry MOST, but not all, of the "Return Current" from the Hot wire(s) from the premises. The Neutral Wire of the secondary winding of the Local Transformer is grounded at the pole and then the Neutral Wire is re-grounded at the Main Panel of each building. The path between all of the grounding points is in parallel with the Neutral Wires. Since the paths are in parallel, some "Return Current" must flow through the Ground Points and the Earth.

Local "Return Current" flowing in the ground path is a side-effect of the USA Grounding design.

In fact, homes have completely lost their Neutral Connection between the house and the the Pole Transformer and still appear to function normally. The broken Neutral Wire can be masked by a very low impedance local ground rod / water pipe connection, which can then carry ALL of the "Return Current". During a Home Inspection the voltage between "L1 & Neutral" is compared to "L2 & Neutral" to identify a bad Neutral connection. An elderly couple, the neighbor of of my parents, had a bad neutral connection to the pole for 10 years. My father convinced them to report the problem to the Power Company, and they replaced the old 3 wire type drop with a new Triplex Drop - problem fixed. The clue was some lights got slightly brighter, while some lights got slightly dimmer when the loads changed in the neighbor's house. All of those homes used a low impedance ground connection to their metal cold water pipe, which allowed any one home to lose its Neutral Wire connection to the pole and yet still function "normally". The bad home sent ALL of its Neutral "Return Current" into the metal water pipe, back to Ground Rod at the Pole and back up the Ground Bond in neighbor's Main Panel.

So, if you disconnect a ground wire you might just find a very lethal AC Voltage between the Ground Wire and the Ground Clamp !
 

djsfantasi

Joined Apr 11, 2010
7,865
<off-topic, a little>

Back in the 80s, I was a director of an outdoor playground construction project. We had contracted for two onsite engineers, whose time was extremely restricted. If we postponed for any reason, we’d have to wait several months to reschedule. There were 2,000 volunteers, rented or donated equipment, storage areas that were negotiated for specific dates, a special electric panel installed by the electric company... Delay was not an option.

As it turned out, that weekend we had a Nor’easter roll in. A Nor’easter is a storm that backs into New England from the northern Atlantic Ocean and known for its ferocity.

Everyone showed up. Construction ensued. But the GFCI breakers in the panel kept tripping from the water, delaying construction. Even the enclosure couldn’t keep out the storm. We had a compressor nearby, and would blow out the breakers periodically and sometimes again when a breaker tripped.

We got through it. 2,000 volunteers completed a children’s wood playground in 24 hours over two days. It was an incredible effort.

<story end>
 

be80be

Joined Jul 5, 2008
2,057
We use the beast to test Neutral it works by putting a 50 amp load on both L1 and L2 you flip it on and the voltage will drop on one line if you have a weak Neutral. Most house fires are from a lost neutral and house with poor grounding on water lines.
They now make you install 2 grounds rods 6 feet apart thats a real life saver.
 

Tonyr1084

Joined Sep 24, 2015
6,344
My dad grew up in the great depression. He learned that you didn't just go out and buy something you needed - either you BUILT it OR you went without. Many of the lessons I learned from him had to do with DIY sort of stuff. I remember he once built an electric lawnmower. It consisted of a washing machine motor with the blade welded to a bobbin that could bolt to the motor shaft. One afternoon I was cutting the grass - barefoot. Even though the machine did not malfunction, I still noticed a considerable amount of vibrations going through my body as electrons were finding a path to ground through my bare feet in the cool and somewhat damp grassy ground. Back then MUCH of what was built did not have grounding protection. And it was a period of time when we were learning much more about electricity and safety. With AC, there was no polarization. Either leg of the plug could be connected to the hot line; and many lights within the house had a switch controlling neutral from the ceiling light. Many times I've learned the lesson to shut the breaker off because the switch might turn the light off, but there could still be power at the junction box - usually metal - and usually grounded. Touch that and the hot wire when you THINK it's off because the switch is off and you'll quickly learn not to do that again - until, like me, you THINK you're safe enough.

Had our power been supplied from a transformer NOT referenced to ground then it's likely I would not have gotten the stockings I've gotten over the years. Still, and has been mentioned, with high voltages, even if there is no circuit, there can be significant discharge to ground. Look at lightning. It's not a circuit, there is no closed loop. Yet, it discharges to other clouds and to ground. Even you've experienced static discharge, when you cross the carpet in your Sunday shoes and grab the door knob and get that annoying, and sometimes painful hit from the discharge. AND the door knob is not grounded. It's just at a different potential.

What we call zero volts is just what we call it. Actually, and we may never know this for sure, but it's possible that what we think as zero volts may actually be some electrical existence far above (or below) what we think is zero volts. But the earth is a pretty big doorknob, and when charged sufficiently we can get a life threatening hit from some high potential to a much lower potential.

For low voltage applications, grounding is not so critical. Except when it involves radio waves or other apparatus that requires connection to earth ground. But generally, the #1 reason for grounding is for safety. Why chance it when you don't have to?
 
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