The Engineer's Place for News and Discussion®

 Previous in Forum: questions Next in Forum: Loop Check

### Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

### Rating Vote:

Active Contributor

Join Date: May 2012
Posts: 10

### Question

05/31/2012 2:53 PM

when I checked the continuity between the clean earth and dirty eath with mutimeter RED probe on Clean bus bar and BLACK on Dirty busbar,it showed open.But when I changed the probe with Black on clean and Red on dirty it showed closed.WHY? can you please clear it.?

Interested in this topic? By joining CR4 you can "subscribe" to

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Oct 2008
Location: 34° 34' 21.60" N, 92° 55' 42.28" W
Posts: 21342
#1

### Re: Question

05/31/2012 3:04 PM

Is this related to the previous question?

__________________
Luck comes and goes. Skill is forever. Intelligence either is, or it ain't. lyn
Active Contributor

Join Date: May 2012
Posts: 10
#2

### Re: Question

05/31/2012 3:15 PM

It can be a part of the previous question.Even otherwise also.

2
Power-User

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 132
#3

### Re: Question

06/02/2012 1:00 PM

You may be experiencing a DC voltage drop between the two earth busbars. Ohmmeters use an internal DC power source to push direct current through the circuit being tested. If the circuit being tested contains its own DC potential, the meter will register different amounts of resistance depending on which way you connect your meter.

When measuring resistance on any circuit that might have potential, you should always first set the meter to measure voltage (both DC and AC) and check for the presence of voltage before trying a resistance measurement. If there is voltage present, don't try to measure resistance!

__________________
"fallor ergo sum" (I err, therefore I am)
Active Contributor

Join Date: May 2012
Posts: 10
#4

### Re: Question

06/03/2012 12:23 AM

Thanks for your reply. But let me make it clear that it is these two points where I want to test are two earth bus bars.Clean earth and dirty earth.There is no potential on these bus bars.So can you explain now please.

Power-User

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 132
#5

### Re: Question

06/03/2012 5:40 PM

You may *think* there is no potential between these two busbars, but you won't really know unless and until you check with a voltmeter. The very fact that one of them is called "dirty" earth while the other is called "clean" earth presupposes a difference of potential (at least during certain conditions) between them. If these two grounding busbars were always at exactly the same potential, there would be no need to have both of them in your system!

The difference of potential I am referring to may only be a few tenths of a volt, but it can be enough to skew the resistance indication of most digital multimeters. Always measure voltage between two points before trying to measure resistance, unless you know for a fact that there can never be potential between the two (e.g. measuring resistance of a resistor removed from any circuit).

__________________
"fallor ergo sum" (I err, therefore I am)
Active Contributor

Join Date: May 2012
Posts: 10
#6

### Re: Question

06/04/2012 1:10 AM

Thank you very much for your reply.I totally agree with your answer.But let me ask ,"will it happen in the sameway In case if there is no potential". I mean two rods connected nowhere.Can you please clear my doubts given below.

1)In a panel Clean earth and Dirty earth bus bars are separated, but the loop is connected togethet at the end of all the connections.Why?

2)What is the resistance between Dirty eath and Clean earth with respect to the common earth rod.?

3)In the field junction box the cable shields are not connected to the common earth.Why?

Power-User

Join Date: Sep 2011
Location: Bellingham, WA
Posts: 132
#7

### Re: Question

06/06/2012 4:36 PM

The only other causes that come to mind explaining why your multimeter would read resistance differently with polarity is if you did not make good contact with your meter probe during one of the tests, or if you have an intermittent fault in your multimeter (or in the test leads).

As for your other questions, it appears they were answered well in another post by another user. Did you find his answers unclear? Your question #1 is somewhat unclear to me, as I do not know what you mean when you say "loop is connected together at the end of all the connections." A diagram would be helpful here.

__________________
"fallor ergo sum" (I err, therefore I am)
Interested in this topic? By joining CR4 you can "subscribe" to