Actual resistance of ground

Thread Starter

Spacerat

Joined Aug 3, 2015
36
Hello,
I am still a novice in electronics and was pondering on the idea of earth ground.
We generally connect our appliances to the ground for safety in case the hot wire, due to a short circuit, touches the metal case. IF we accidentally touch the case, the current through our body then ground then back to the source would be small...

We assume that the case connection to the ground has a very very small resistance so that junction is not the issue. The negative terminal of the power source is also connected connected to ground. Essentially, planet earth plays the role of a huge conductor....But wouldn't earth (dirt, etc.), as a conductor, have a very large resistance?
For example, if I took a 12 battery, a resistor, a lightbulb and closed the circuit with planet earth instead of using a simple wire, I don't think the bulb would light up...Am I wrong? This should be true at DC and AC

Thanks!
 

blue_coder

Joined May 7, 2016
36
Earth as in soil isn't a very good conductor, no. However, it's normally something like 25% moisture, and the cross sectional area is absolutely huge! For electrical testing of TT earthing systems (with earth stake) in the UK, we have to measure the Zs to size protective devices (RCDs). The maximum it's usually possible to go to is a resistance of 1667 ohms (50V / 0.03A) as 50V is considered safe, and the 30mA being the cut off for the lowest common RCD.

Your 12V battery idea would work fine, provided you have good enough earth stakes. A nail poked into the ground probably won't work. You also still need one wire obviously.
 

SamR

Joined Mar 19, 2019
4,117
Household and commercial earth grounding is ultimately due to the installation of a driven ground rod or attachment to buried metal water supply piping. It is then tested for conductivity and resistance in the megohm range. Typical ground rod is a 10' length of copper clad steel 5/8 to 3/4" in diameter. Some areas require multiple ground rods to pass code inspection.
 

MaxHeadRoom

Joined Jul 18, 2013
24,975
Earth as in soil isn't a very good conductor, no. However, it's normally something like 25% moisture, and the cross sectional area is absolutely huge! For electrical testing of TT earthing systems (with earth stake) in the UK, we have to measure the Zs to size protective devices (RCDs). The maximum it's usually possible to go to is a resistance of 1667 ohms (50V / 0.03A) as 50V is considered safe, and the 30mA being the cut off for the lowest common RCD.
Back when I started out in the UK, the neutral was grounded at the star point of the distribution transformer, but was not allowed to come in contact with the neutral in the panel, as it is done in N.A.
A ground source had to be provided at the dwelling using either the metallic water service or ground rods.
We could not use any ground provided by the service provider.
Therefore we were required to meter test the resistivity of the ground from the grounded neutral at the transformer through the neutral conductor back through earth ground, the reading had to be less than 20Ω, IIRC.
Now however, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.
With the advent of non-metallic water service, RCD's became the norm in the UK.
 
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BobTPH

Joined Jun 5, 2013
4,738
Hello,
I am still a novice in electronics and was pondering on the idea of earth ground.
We generally connect our appliances to the ground for safety in case the hot wire, due to a short circuit, touches the metal case. IF we accidentally touch the case, the current through our body then ground then back to the source would be small...

We assume that the case connection to the ground has a very very small resistance so that junction is not the issue. The negative terminal of the power source is also connected connected to ground. Essentially, planet earth plays the role of a huge conductor....But wouldn't earth (dirt, etc.), as a conductor, have a very large resistance?
For example, if I took a 12 battery, a resistor, a lightbulb and closed the circuit with planet earth instead of using a simple wire, I don't think the bulb would light up...Am I wrong? This should be true at DC and AC

Thanks!
If line shorts to the appliance case, and that case is connected to the ground wire, you have a short going from line to case to ground wire to neutral wire at the breaker box. The earth is not in the circuit and the breaker trips immediately.

Bob
 

SamR

Joined Mar 19, 2019
4,117
Here in the US, the chassis "Safety" ground is connected from the chassis to the safety ground wire to the ground buss in the panel. The panel neutral and ground buss both are connected to an "Earth" grounding (driven rod or otherwise) which is tested for conductivity to code specification before passing electrical inspection and receiving a Habitable Certification (passed all local building code requirements) by the local regulatory building inspector.
 

MisterBill2

Joined Jan 23, 2018
11,556
"Ground" is a mythical zero potential zero resistance plane that serves as a reference for voltages. Actual "ground" is a bit different. As others have mentioned, it is close to low resistance to the neutral of our power distribution system.
In the local real world, an adequate earth ground connection will be a low enough resistance connection between it and the power distribution system neutral to trip the circuit protection device between the line side and the accidental connection to the safety ground (which I call the "green wire ground") .
As has been mentioned, in much of the USA the safety ground and the neutral circuit are tied only at the distribution panel, where also some connection towards earth ground is also tied. Prior to plastic water piping this was the cold water supply pipe from the outside water connection, currently it is a driven ground stake in most areas.
In many buildings with correctly installed wiring there may be a volt or more difference between the neutral line and the safety ground measured at points some distance from the primary distribution panel.
My intention here is to explain in more detail what "ground" is all about.
What is very amazing at times is to see a downed power line sparking to "dirt", where there is clearly no distinct conductor preset. I saw that at fairly close distance when a wire downed in a storm some days previous started arcing to ground in my garden. It would spark for a while until that section had dried and was not conducting, and then as the breeze moved it spark again a few inches away. That was very impressive.
 

MaxHeadRoom

Joined Jul 18, 2013
24,975
"Ground" is a mythical zero potential zero resistance plane that serves as a reference for voltages. Actual "ground" is a bit different. As others have mentioned, it is close to low resistance to the neutral of our power distribution system.
This is why when I came to N.A. I had to get used to the fact the use of the term 'Ground' covered circuit ground etc, AND Earth GND, i.e. no real distinction.
In the UK, we used the term, 'Earth', as in run a live pair together with an Earth.
Many even use the earth GND symbol on schematics when intending just a power common point, or 'zero' voltage .
.
 

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MisterBill2

Joined Jan 23, 2018
11,556
The earth ground symbol s the easiest to draw and that is probably why it is used so much. Also, that was on the first electrical symbol templete that I ever had.
 

MaxHeadRoom

Joined Jul 18, 2013
24,975
The earth ground symbol s the easiest to draw and that is probably why it is used so much.
They are all rather easy to draw? Just that most get them wrong, including publications that should know better, Art of Electronics for e.g.

This is often the one of the alternates that Should have been be used as opposed to earth.
1641673964136.png
 

MisterBill2

Joined Jan 23, 2018
11,556
Of course, to make circuit analysis simpler it works to draw all connections as wires like all of the other connections. At one job I had to do tat because the PCB layout person had no understanding of electronic or circuits, she only knew the PCB artwork program.
 

SamR

Joined Mar 19, 2019
4,117
Wait until you run into Isolated Grounds. It used to be seen mainly for computer systems but now with all the instrumentation in hospitals are very common there. Usually seen as Orange colored 3 prong receptacles.
 

nsaspook

Joined Aug 27, 2009
9,435
Wait until you run into Isolated Grounds. It used to be seen mainly for computer systems but now with all the instrumentation in hospitals are very common there. Usually seen as Orange colored 3 prong receptacles.
Sure, we have plenty of equipment with one ground for electrical reference, and a 2nd for safety (metallic grounding).
1641676809248.png
 

nsaspook

Joined Aug 27, 2009
9,435
Where is the Logic or signal ground version??
Reference/common electronic circuit point grounds.
http://lednique.com/ground-earth-chassis/
The ground symbols indicate the generic reference point. Even if there is no earth or chassis connection it is common to refer to one point or voltage in the circuit as “ground”. In equipment where electrical isolation is provided between sections of the circuit two or more ground symbols may be required to indicate which ground the components are connected to.
1641677857683.png

http://www.normservis.cz/download/view/iec/info_iec60417iso7000_DB.pdf
 

SamR

Joined Mar 19, 2019
4,117
After using LTS, I use the simple inverted triangle for my circuit sketches. Much simpler and easier to draw. If I were doing a formal schematic, I would use the more descriptive symbols.
 
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