difference between ground and earth

thingmaker3

Joined May 16, 2005
5,073
Please do not "hijack" other folk's topics.

"Ground" is a reference point for measurement. "Earth" is the most common ground reference. "Earth" is Earth, or a good conductor in good contact with the Earth.
 

Salgat

Joined Dec 23, 2006
215
Earth Ground is typically tied to anything conductive surrounding wiring in order to prevent faults in the wiring from using you as their path of choice. Ground is just a reference as thingmaker stated, think of it as the drain that sucks up all the electricity (as an extremely simplified view). Remember, earth ground is tied to a lot of the metal cases holding your wiring, and then tied to a pole thats several feet deep into the soil.
 

Dave

Joined Nov 17, 2003
6,960
This question has been asked in many guises before, a forum search for "ground" and "earth" will produce many results. One of the most recent was: http://forum.allaboutcircuits.com/showthread.php?t=6050

Note subtech's comment that "earth, ground, and common are about the three most misunderstood words in all of the electrical world" - quite fitting.

Dave
 

pilotshero

Joined Nov 29, 2007
10
Another way of looking at it is ground is the common return circuit in electronics equipment whos potential is zero. Also as stated before, a conection to earth by means of plates and rods, ie Home and Commercial wiring.
 

studiot

Joined Nov 9, 2007
5,003
There, and I always thought that 'ground' was the American for 'earth'.
You will certainly see in countless thousands of UK english circuits diagrams the word earth used in the same place as Americans (God bless their cotton socks) write ground.
 

David Bridgen

Joined Feb 10, 2005
278
There, and I always thought that 'ground' was the American for 'earth'.
Sometimes it is. On the other hand, sometimes it isn't.

And that's exactly the point of my write-up.

Its unqualified use is ambiguous, sloppy and imprecise.
 

studiot

Joined Nov 9, 2007
5,003
Sorry I can't agree with you DB, since your basic premise arises from a misconception.

but a very low resistance connection,
'earth' has never ever referred to a low resistance anything. It is not a measurement of resistance.

'Earth' refers to a body that has such a large charge sink/source capacity that for circuit purposes any current flows do not affect its potential. 'Earth' is a statement of Voltage.

As such, Racal does not deserve your vitriol.

Earthing resistance can range from milliohms to kilohms and be satisfactory and be within the wiring regulations.

Chassis connections are different again and attract a different circuit symbol in recognition of this fact.
 
Ground is something like THe reference you make while examining or making a measurement where as the earth is the most common ground reference.
 

studiot

Joined Nov 9, 2007
5,003
Perhaps you would like to calculate or measure the earth resistance at the earth connection of a Van Der Graff generator?
 

studiot

Joined Nov 9, 2007
5,003
I have always understood that ‘earth and earthing’ are English terms and ‘ground and grounding’ are American terms for the same thing.

Whatever you call it there are three main (electrical/electronic) uses for an earth and all rely on the fundamental property that the earth remains at constant potential. There is no requirement for current to actually flow, although in some cases there is flow.

The three main uses are

1) Screening and shielding
2) Voltage reference
3) Protective earthing

Confusion may arise because more than one earthing system may be employed to satisfy more than one of these uses in the same apparatus.

Taking each case in turn we see that.

Case 1 is the simplest with no intended current flow. A conductive (usually metal) screen is placed around or between signal carrying conductors. It is connected to something capable of holding it a constant potential, despite variations in the electromagnetic environment. This connection may be to a fixed point in the circuit itself or may be to an outside body such as mains earth. The screen on audio cables should be 'earthed' at one end only.
As such, this type of earthing is appropriate for AC but not DC.

Case 2 occurs when one point in the circuit is declared fixed, and made sufficiently beefy by comparison with the rest of the circuit, to make it so for the normal, non fault, currents. Normally one terminal of the power supply is chosen.
So for instance in the case of an automobile, one terminal of the battery is connected to the car body as the ‘beefy’ bit. This is obviously DC and may be referred to as ‘positive earth’ or ‘negative earth’. The automobile itself is obviously insulated from the planet earth.
With a single phase AC mains supply one terminal (neutral) is chosen and connected to something ‘beefy’ at the substation. In this case it is connected to the planet.

Case3 arises when the intended current is a fault current and occurs in conductors which do not carry current under normal operation.
So in some countries the external metalwork on mains powered apparatus is connected to a third conductor - the ‘earth’ wire. Current only flows in this wire under fault conditions and it is important that this current ( which is often large) does not alter the potential of the earth wire so it is connected to a very large source/sink namely the planet. The resistance (impedance) of this connection is important in this case and is termed ‘earth resistance’ in many national regulations.
It is important to realise that this resistance is a measure of the connection resistance to the planet at this point, not the resistance between separated connections to the planet. In particular not the resistance to the planet through the neutral.

Mains supply is therefore a system that employs two separate and distinct earthing types, case2 and case3.

Please note that these are all idealised cases - I have not covered imperfections or things going wrong with an earth.

I should perhaps also add that the term earthing or grounding is also used as a verb to refer to the removal of static charge from a person or object by momentary touching a metal object connected to an earth.
 
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