Resistance reading.

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Rahim84

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Hi all, im Testing Foundation Fieldbus cables for instrumentation. From the positive to negative the requirements are at least 50Kohms. Today I was only reading 42.4 so obviously this is a low reading. But when I switched the meter from auto to manual I get .05Mohms... I'm using a Fluke 125 B.. does anyone know why this happens...


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.05 Megohms on a digital meter is really anything from .04 to .06, which in turn is 40 kOhms to 60 kOhms.
When you switch the meter from automatic to manual ranging it is your responsibility to select the appropriate range for best accuracy.
As to why the default setting of the manual range is what it is, I have no idea.
 
You sure you are using the right tool for the job? Seems like this is an insulation test in which case you should be using a Megger.
 
No this isn't an insulation test, a Meggar isn't required. This is instrumentation cables. Fluke 125 B, FBT-6, and Hart 475 communicator are my test equipment. But thanks for your responses


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But while I got you guys here, can you help me understand a 120/208 system? How do you get 120 from line to ground from either phase but 208 from any two?


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Rahim84 said:
But while I got you guys here, can you help me understand a 120/208 system? How do you get 120 from line to ground from either phase but 208 from any two?


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The 120/208 supply is derived using two phase lines and the neutral of a three phase, four wire, 208Y/120 service.
The two lines are each 120 volts to neutral, but since they are 120 degrees out of phase with each other instead of 180 degrees the line to line voltage is only 208.
To see it graphically, draw two lines of length 1.20" at a 120 degree angle to each other at their common endpoint. The other ends of the lines will be 2.08" apart.

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I can understand that concept sounds simple enough, I'm still not sure how this works tho. Maybe because I need more theory or just to understand the relation transformers a little better. Is there a pictorial diagram that can support this?


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