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Extend Oscilloscope leads to 100 feet

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nstech

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I would like to verify measurements taken by another person with a Fluke Scopemeter and would appreciate some feedback. Measurements were taken at various outdoor points such as a well casing and metal tin of a building to see if there was any voltage/frequency present.
Here is how it was hooked up: A ground rod was sunk in the earth approximately 100 feet from the Scopemeter. This was connected with a piece of RG-6 coax with the shield connected to the center conductor at the ground rod. Only the center conductor of the coax was connected to the scope ground connection. Two more coax cables approx 100 feet in length was routed to the metal siding on a building and an underground well casing. Again the shield was connected to the center conductor at these two points. Again, only the center conductor of the coax was connected to inputs A and B.
I believe there is a big problem with capacitance, reflective energy and impedance matching. These last 3 decades I have mostly lived in the electrical world and only stray into the electronics arena from time to time so I am hoping to get some insight into this connection method from some scope/rf experts.
It seems to me that the shield should be connected to the center conductor at the Scope end and/or connected to the shield from the ground rod. Any insight into precisely how scope input leads can properly be extended with RG-6 coax would be appreciated!
 
This sounds like a right can of worms!

Can you tell us what it is that you are expecting (hoping?) to find?

I suspect that there is not just one simple answer to this.

JimB
 
This sounds like a right can of worms!

Can you tell us what it is that you are expecting (hoping?) to find?

I suspect that there is not just one simple answer to this.

JimB

Yes, you have a good question: Typically when measuring for 60 hz AC voltage using an RMS Voltmeter from earth to equipment that is connected to the electrical system you might pick up a few tenths of a volt or so because of normal voltage drop on the premise electrical system. I am expecting that taking these measurements using a 1 M input impedance scope connected to 100 feet 75 ohm coax in the configuration I describe will give highly erroneous readings that indicate voltages and frequencies that aren't really there but are a result of the improper connection method. On the other hand, it might be connected perfectly correct but I am not sure. Thanks for your response!!
Dave K
 
scope inputs are 1Meg input impedance, and between 5 and 30pf in parallel with that input impedance. the capacitance of 100ft of RG-6 is 2nf (20pf/ft*100ft). the turnover frequency for 2nf and 1Meg is 79.5hz. 50 or 60hz is within this range, but harmonics of 50 or 60 hz are not. what are you trying to measure? it sounds like you're trying to do some kind of differential voltage measurement.
 
Have a look at the attachment.

If I understand your set-up correctly, I would connect as per the top sketch if I were using a simple test meter.
One thing to bare in mind when using newer high impedance digital meters, they can respond to high frequency noise (radio transmissions etc) picked up on the long single wire.
In many ways, an older moving coil type meter is sometimes better in a situation like this.

If I wanted to use an oscilloscope of some kind, I would connect it as per the lower sketch.
I would not use the scope probes, I would just use a coax cable connected normally with a BNC plug to the scope input.
At the remote end, I would connect the centre of the coax to the device under test.
Back at the scope, I would connect the outer of the coax to the test earth spike.

So, that is how I would go about it.
What I would do or change after having done this... I have no idea!:)

JimB
 
scope inputs are 1Meg input impedance, and between 5 and 30pf in parallel with that input impedance. the capacitance of 100ft of RG-6 is 2nf (20pf/ft*100ft). the turnover frequency for 2nf and 1Meg is 79.5hz. 50 or 60hz is within this range, but harmonics of 50 or 60 hz are not. what are you trying to measure? it sounds like you're trying to do some kind of differential voltage measurement.

Trying to measure anything that is there. I would expect to measure a half a volt 60 cycle. I'm guessing this 60 cycle might appear modulated if there are electonic ballasts or variable frequency drives in the building.
 
Extend scope leads

Have a look at the attachment.

If I understand your set-up correctly, I would connect as per the top sketch if I were using a simple test meter.
One thing to bare in mind when using newer high impedance digital meters, they can respond to high frequency noise (radio transmissions etc) picked up on the long single wire.
In many ways, an older moving coil type meter is sometimes better in a situation like this.

If I wanted to use an oscilloscope of some kind, I would connect it as per the lower sketch.
I would not use the scope probes, I would just use a coax cable connected normally with a BNC plug to the scope input.
At the remote end, I would connect the centre of the coax to the device under test.
Back at the scope, I would connect the outer of the coax to the test earth spike.

So, that is how I would go about it.
What I would do or change after having done this... I have no idea!:)

JimB

Thanks for the explanations and diagrams. This is how I would expect the connections to be made. I thought I would check as the person who's readings I need to check wired the extended probes "backwards".
Dave
 
I don't see how you will measure anything of value. If there is a potential on the case of the device, it will have to be quite high to give any sort of reading. Why not just measure it at the device. If you're trying to verify if the ground system is adequate, you need to do a "fall of potential" test. This test allows you test the ground system when it is energized.
 
I don't see how you will measure anything of value. If there is a potential on the case of the device, it will have to be quite high to give any sort of reading. Why not just measure it at the device. If you're trying to verify if the ground system is adequate, you need to do a "fall of potential" test. This test allows you test the ground system when it is energized.

I appreciate the information! In the situation I'm working on the points need to be simutaneously measured to verify if any events affect them together or separately, and they are all separated by 100 feet or more. Also, I'm trying to verify if readings taken by another party are valid so I'm trying to replicate his setup. I'm sure of the test points but was unsure of the wiring connection method although I was quite sure the connection as I described in my origininal post was wrong (connected by other person). As far as value, if I don't read anything it might be worth more than you can imagine :).
I've used the "fall of potential" before when needed but these days I use my Fluke 1630. Thanks.
Dave
 
The advice for extending the range of an air compressor is to add a longer hose, not plug it into an extension cord.

The advice for extending the range of an oscilloscope is to add a 100+ foot extension cord and use a decent probe. Considering the power requirements for the scope, the extension cord could easily be up to 500 feet with little voltage drop. Today's digital scopes can store bunches of waveforms. Why waste that capability if you have it?
 
The advice for extending the range of an air compressor is to add a longer hose, not plug it into an extension cord.

The advice for extending the range of an oscilloscope is to add a 100+ foot extension cord and use a decent probe. Considering the power requirements for the scope, the extension cord could easily be up to 500 feet with little voltage drop. Today's digital scopes can store bunches of waveforms. Why waste that capability if you have it?

Thanks for the response Dean. I know I didn't provide all the information as to my setup as it would take several pages. I do believe my question was answered by JimB as I was mostly hoping for verification of my thoughts which I received.
Certainly I would move the scope to the point of measurement if possible. During the years I repaired TV's, stereos and so on I connected the probe right to the component under test. At my test location there is a "test" ground rod over 100 feet from the facility to move it out of the influence of any ground current traveling between grounded buildings and utility grounds/neutrals. Other scope connections go to utility and premise neutral and ground. Also connected is equipment at various buildings. The readings will contain ground currents/ground voltage/harmonics from VFD's/etc. from a multitude of equipment in and around this industrial environment. Because these readings need to be captured and compared simutaneously the scope needs to sit in one spot out of harms way. I know we could use an A/D device at each location and send the signal via RS-485 or some other method. I mentioned in my original post that I was trying to verify the measurements taken by another person. I certainly wouldn't have strung 900 feet of RG-6 but that was what was used and I was trying to make sense of it. Thanks.
Dave
 
So if you have 8 signals to verify, all 8 have to be measured at the same instant for comparison among one another?
 
So if you have 8 signals to verify, all 8 have to be measured at the same instant for comparison among one another?

Yes....all 8 need to be viewed to help understand how an event on the utility or premise electrical system affects the monitored points. Using a couple 4 channel Fluke 190 scopes and/or a Dataq DI 718B data logger. Thanks.
Dave
 
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