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Anonymous Poster #1

questions

05/31/2012 2:42 PM

1) why Clean earth and dirty earth are seperated and connected together at last.?

2) what is the resistance between the clean earth wire / dirty earth wire with respect to the common earth rod.?

3) Field instrument cable shields are not connected to the earth in the field junction box. why?

4) Fiels instruments are not connected to earth.why?
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PWSlack
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#1

Re: questions

06/01/2012 3:52 AM

A1) So that any current appearing on the dirty earth during a fault event has the least chance of appearing on the instrument earth and affecting the instrumentation readings during that event.

A2) As low as is practicable.

A3) So that circulating currents ("ground loops") are not set up within the instrument cable shields. These currents might otherwise affect instrument readings by inductive interaction between the shields and the signal-carrying conductors in each cables.

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