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Question about Ground Loops


V8meathead
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Well I've searched Google, and wikipedia.  I have a pretty good idea of them.  But I need some more examples or something.  I guess I'm not really picking up on the idea fully.  Any of you guys have any information on them that could help me out?  Practical stuff or Book stuff.  Doesn't matter to me.  Thanks. 8)

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I know and that's exactly why you shouldn't connect the neutral to earth in your house because an earth loop would be created causing the sort of problems discussed in the link you've posted.

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I read the following from the link above:
A ground loop in the power or video signal occurs when some components in the same system are receiving its power from a different ground than other components, or the ground potential between two pieces of equipment is not identical.

my questions are:
(1) please draw an example for TV and PC and say amplifier, show me how exactly  some component in the system is receiving its power from a different ground than other components.
(2) How to measure the ground potential between two pieces of equipment? If using AC voltmeter, across what points I should measure?
thank you very much

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The answer to question #1 depends upon the schematic of the individual device. Some devices ground the chassis and some do not.

The answer to question #2:
Set your multimeter to ohms and check the resistance between the ground of the two connected devices. For example, the resistance between the metal chassis of the two devices. If there is resistance, you have a ground loop problem. If you get a reading of zero ohms, both devices have the same ground potential. Next step is to check all internal connections to chassis. Make sure they are not loose, frayed, broken or corroded. If all is good, and all internal grounds are at the same potential as the case of both devices, you do not have a ground loop problem between these two devices.

MP

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any set (like TV) have a plug to connect it to electrical power
each pluge has 3 terminals, phase neutral and ground (earth).
Now, my question is: Is the metal chassis connected directly to the Ground (Earth) or not?

And if i want to measure the resistance between the metal chassis of the two devices, i think the two devices must be plugged but OFF. Is this not affect the meter
thanks

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I read the following on:
http://leminstruments.com/grounding_tutorial/html/soilresistivitytest.shtml
"The depth that the test electrodes is to be driven is A/20" then the writer said:
"The ground stakes should be screwed in no deeper than 1/3 the distance from one another"
please refer to the page link above to see the figur.
i feel the two sentances above are in contrast

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CAUTION!
I don't know why you want to measure the resistance between two "grounded" chassis, but make absolutely certain that the electrical outlets are wired correctly.

If the electrical outlets and the equipment are wired correctly and their insulation is good, then there won't be any current in the electrical ground wire to cause a ground loop.

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Hi audioguru


CAUTION!
I don't know why you want to measure the resistance between two "grounded" chassis, 


MP tell me this: "Set your multimeter to ohms and check the resistance between the ground of the two connected devices. For example, the resistance between the metal chassis of the two devices. If there is resistance, you have a ground loop problem. If you get a reading of zero ohms, both devices have the same ground potential. Next step is to check all internal connections to chassis. Make sure they are not loose, frayed, broken or corroded. If all is good, and all internal grounds are at the same potential as the case of both devices, you do not have a ground loop problem between these two devices.
"
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When you connect together two grounded pieces of equipment with a shielded cable then you usually have a ground loop due to leakage current in the shield. Some people cutoff the electrical ground pin which is dangerous of an amplifier or oscilloscope to eliminate the ground loop. I disconnect the shield at the sending end and make sure that their chassis's are connected with the electrical ground.

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I read The following from: http://www.leminstruments.com/grounding_tutorial/html/soilresistivitytest.shtml
Measuring Soil Resistivity 4 - Pole Method

The calculation of this measurement can be simplified by converting distance in cm to distance in feet giving you the following equation:

      p = 191.5AR
Where: p = the average soil resistivity to depth
          in ohm - cm
      A = the distance between electrodes in feet
      R = the measured resistance value in ohms
          from the test instrument
For example, you have decided to install 10' ground rods as part of your grounding system. To measure the soil resistivity at a depth of 10' requires that the spacing between the test electrodes is 10'. The depth that the test electrodes is to be driven is A/20.

Look at this"The depth that the test electrodes is to be driven is A/20."

then he said: The ground stakes are positioned in a straight line equidistant form one another and at a distance between one another reflecting the depth to be measured. The ground stakes should be screwed in no deeper than 1/3 the distance from one another.

he confuse me A/20 or no more than 1/3 A

and in the figure he present (see below) he put a and in formulas he use A

please help me to understand it
thanks

post-2833-14279142556417_thumb.jpg

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Hi Walid,
I've never built a an electrical substation nor a telephone system's central office.
Most electronic circuits I work with and build don't need an earth ground. A few radios I built were connected to the electrical receptacle's ground, and very sensitive radios were connectd to my cold water pipe which is an excellent earthed ground.

Maybe you need to consult with your professor, classmates or electricians on an electrical forum.

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MP, you are correct in the sense of small equipments in the same workspace.


This is also true of a tower with a multitude of meteorological equipment out in the elements... Or a weather station in the middle of nowhere...all the way down to a monitoring station in a populated city. Not just small equipment. The star grounding is no insurance. Corrosion will find it's way into any connections if the instrument is exposed to the elements. Even in a lab, I see corrosion build up on connections. Thus, ground will take a different path through other equipment to find the least resistance.

MP
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hi all
u all must know that i am the only person among tens in my work who has the chance of conncting with u and the natural result of this must make me the more understanding person and this is not occur till now
ok i am in advance but slowly.....

now i have a FLUKE 112 TRUE RMS MULTIMETER and a video monitor and a video casitte, there is a hum in the picture, how to do some measurements to fix it?

Greetings...
Merry Christmas.... to all
walid

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Hi Walid,
Here in Canada, my TV and my video cassette recorder/player aren't grounded so there isn't a ground loop. If I see hum in my picture then I use a better-shielded cable connecting them.

I don't think you can measure AC mV low enough to identify a very low level of hum. You also can't measure an extremely low resistance that would cause a ground loop.
The magnetic field of your monitor or some other device might be radiating into your video player causing hum. Try separating them 60cm.

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You also can't measure an extremely low resistance that would cause a ground loop.


Hmmm....it is done all the time to fix sensors, dataloggers, etc. I know you are not familiar with the meteorological area, but you might want to read up on this before making such a comment.

MP
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Why you enlarge the problem, try to forget the datalogger for a monent, my question is very simple
i ask if you work in TV corporation and you notice that there is some bars in the picture and a hum in the audio and u want to prove to the others that there is a ground loop what you tell them and what measurement you must make to support yourself.
thanks

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I do not see where sensors connected to a logger is any different than all of the different peripherals of a TV working together.
If all components do not have the same ground potential, none of your voltage references are the same from one end of the device to the other. If the shielding is not good on all points that are protected from spurious frequencies, you will have this type of noise as well. These are the two main concerns in grounding.
Normally, one goes through the system with a scope in TV equipment to see where the signal is no longer clean. Then further investigation usually provides an answer as to why.

MP

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