Earthing 1

[AhmedKassem]

What is clean and dirty earthing & purpose from them ?

 

[waross]

When communication techs find noise on their signals they often blame "dirty" earthing. It is easier to assume that the noise is coming from the earth than to clean up the signal. They will install a "magic triangle" ground which is not a very good ground and so may float with the noise on the signal and improve the symptom without actually addressing the root cause.
Consider:
An large industrial installation may have several hundreds or even thousands of pounds of copper cable in direct contact with the earth for hundreds of linear feet. In many installations, more than a few amperes of ground current will trigger the ground fault protection.
Compare that with the three driven ground rods ten feet long that comprise the "magic" triangle. Which is actually closer to the actual ground potential.
Dirty ground = hundreds of feet of cable in direct contact with the earth.
Clean ground = three ten foot long ground rods that may not provide that good a ground and may "float" with a noisy signal.
BTW In North America, the code requires both ground systems to be tied together in any event, negating the perceived effectiveness of the "magic" ground.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Generally the clean earth is one that is not expected to carry fault current. Its main purpose is as a reference electrode for electrostatic shields on screened conductors and similar.

Bill,

You will upset the disciples from the church of the magic triangle!


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If we learn from our mistakes I'm getting a great education!

 

[busbar]

Although somewhat dated, you may want to look over

http://www.everyspec.com/NIST/NIST+-+FIPS/FIPS_PUB_94_33007/

 

[waross]

Hi Scotty
When the first POS systems with data links were installed years ago they had serious issues with ground noise and ground continuity.
Many of the systems were installed in old buildings that were wired in the 40s and 50s when grounding was not reliable. There were many issues with the first bonding methods.
The data people ignored a provision in the code that stated that no device other than a ground protective device shall depend on a ground connection for proper functioning.
Our side of the industry also noticed that grounding methods were not reliable and upgraded grounding and bonding requirements.
By the time that the data people initiated the isolated ground regime, it was no longer needed for new installations. The code ground served as well.
Old installations still needed either an isolated ground or a ground method upgraded to current standards.
I have too many times done power supply installations for data and been told that there was a ground problem, when the installation exceeded the quality that the data tech was demanding.
Example;
This won't work. These two pieces of equipment need separate supply circuits.
Well, you have seven pieces of equipment in total and they are on seven independent circuits. What would you have me do now???
After the data folk instal the magic triangle, the inspector demands that we connect the power ground bus to the data/instrumentation ground bus with a very large jumper. The reason that not much fault current flows in the magic triangle is that it isn't a very good ground anyway.
My opinion is that data and instrumentation grounding methods are based on misconceptions and mistakes that have been passed down verbatum over the years. Copied from one text book to another. If you want to know about grounding talk to a protection engineer who must understand and calculate the various voltages and currents that flow in a ground system in the event of a fault.

Generally the clean earth is one that is not expected to carry fault current.

I like your definition Scotty. Although the magic triangle will experience the same voltage rise as the main ground grid, it won't carry much fault current. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbalex]

If you bond and ground things properly, the grounding electrode system shouldn't see much fault current either. Fault currents should bypass the grounding electrode system via the equipment grounding conductors. The grounding electrode system may see lightning strike currents or step & touch problems caused by overhead lines dropping to earth, but the primary fault path should be the equipment grounding conductors.

The NEC committee responsible for grounding was dominated by the telecom industry for over 40 years. It was their "signal" grounds they were primarily concerned about. We're only just recovering and discovering a properly installed safety grounding system is quite effective as a signal ground too.