Ohm reading from ungrounded conductor to grounding conductor

[meternerd]

Some of the better Fluke meters will tolerate voltage on a circuit when reading ohms, but it's a garbage reading. Others automatically switch to a volts scale if you try ohms on a hot circuit. I was called to a service by a lineman who had a Fluke but was reading weird ohms on the top jaws of an energized meter socket. I politely explained that he's lucky it was a Fluke, because people have been injured or killed doing the very same thing with a cheaper analog meter. A line side fault on a socket takes a LONG time to blow a primary fuse. Bottom line....you cannot read ohms on a live circuit. If the breaker is off, it isn't live. You can only read a transformer winding if the breaker is on but power is off. Question doesn't make sense to me. I'm betting the meter switched to volts and he thought it was still ohms. Look at the display and I'll bet it said V.

 

[MD84]

Doh! I have read ohms on a 120v circuit. I was attempting to read volts and I didn't verify I was on the AC Volts setting. I think it was fluke 179. Still works ok but I wonder...

What about checking for volts when you left the probes in the current configuration? Doh! Palm to face. I have done thought probably twice.

Doesn't the ohm meter on a dmm typically push a small current (1ma) through the circuit? It then reads the voltage across that circuit to determine ohms. The DC component of the AC circuit would have to affect the ohmmeters function.

I have used a DC conductance meter on live DC. It sends a 20 hz current through the battery.

 

[zach_zachary1@yahoo.com]

Doh! I have read ohms on a 120v circuit. I was attempting to read volts and I didn't verify I was on the AC Volts setting. I think it was fluke 179. Still works ok but I wonder...

What about checking for volts when you left the probes in the current configuration? Doh! Palm to face. I have done thought probably twice.

Doesn't the ohm meter on a dmm typically push a small current (1ma) through the circuit? It then reads the voltage across that circuit to determine ohms. The DC component of the AC circuit would have to affect the ohmmeters function.

I have used a DC conductance meter on live DC. It sends a 20 hz current through the battery.

That is basically what I did. It was a DOOH! moment:

I installed a new outlet at work this morning. An L15-20R. I powered up the circuit, then went to verify that the ground was ground before I plugged in a very expensive piece of equipment. I set my multimeter to ohms and stuck my probe into what I thought was the equipment grounding part of the receptacle and put the other probe on aluminum conduit that I ran. The meter rang continuity and showed 1.2 ohms. I then noticed that my probe was stuck in the hot/ungrounded part of the receptacle (which was hot). The first thing that I thought was I miss wired the outlet. So I opened the breaker and double checked everything. Both hots on the receptacle where wide open with respect to ground and the equipment grounding conductor was ground. Perfect!! The world is round again. Plugged in the equipment and it worked fine.

When I got home the first thing I did was check the outlets in my house. Samething. On a hot outlet 1.2 ohms from ungrounded conductor to the grounding conductor.

From the replies to my post:

A. I am lucky I didnt ruin my meter.

B. The readings were junk anyway because you can't ohm out live circuits.

C. For a good ohm reading from ungrounded to grounding conductor you have to open the OCPD on the primary side of the transformer.

Thanks everyone for their comments.

 

[ActionDave]

From the replies to my post:

A. I am lucky I didnt ruin my meter.
Correct. Although there are meters that guard against this.
B. The readings were junk anyway because you can't ohm out live circuits.
Probably true.
C. For a good ohm reading from ungrounded to grounding conductor you have to open the OCPD on the primary side of the transformer.
No. Wrong. Absolutely not true.
Thanks everyone for their comments.

There should be a low ohm reading between the grounded and grounding conductor no matter the what state the transformer is in.

 

[MD84]

C. For a good ohm reading from ungrounded to grounding conductor you have to open the OCPD on the primary side of the transformer.

There should be a low ohm reading between the grounded and grounding conductor no matter the what state the transformer is in.

No doubt there should be a low ohm reading between grounded conductor and grounding conductor regardless the state of the transformer.

Zach was referring to the ungrounded conductor.

 

[ActionDave]

No doubt there should be a low ohm reading between grounded conductor and grounding conductor regardless the state of the transformer.

Zach was referring to the ungrounded conductor.

I was typing in a hurry, you are correct. However, there should be a low, low (less than 1.2 ohm) ohm reading between the the UNgrounded conductor and the Equipment Grounding conductor when the transformer is de energized, otherwise you don't have a low impedance path to clear a fault when the transformer is energized.

 

[kwired]

It is a newer protected meter, and for whatever reason 1.2 is what it displays when connected to 120 volts while set for resistance measurement.

I think all cat III meters will withstand 1000 volts in this situation, they may vary what is displayed though.

 

[gar]

160404-0904 EDT

Playing with my Fluke 27 on ohms some of the results were:

This is an autoranging meter. The lowest ohms range is 300 ohms resolved to 0.1 ohm, and the highest is 30 megohms resolved to 0.01 megohm. With a 10 M test resistance (another meter or scope probe) the resistance reads about 10 M and there is about 0.75 V across the resistance (voltage reading on a 10 M input impedance DVM when used as the test resistance).

My decade resistance box has a minimum resistance of about 0.1 ohm when set to 0 ohms (60 year old contacts). My Fluke with the leads used reads between 0.1 and 0.2 ohms when shorted. The meter itself reads 0 with a good short.

Some readings:

R Box .... F27 R ..... V across R

0 ........... 0.2 ........ 0.0001 .... 300 range
100 ....... 100.3 ..... 0.047
300.0 .... 300.4 ..... 0.134

301.0 .... 0.301 k ... 0.035 ...... 3 k range

10 k ...... 10.03 k ... 0.113 ...... 30 k range

10 M ...... 10.01 M .. 0.756 ...... 30 M range

I don't plan to test with 120 V 60 Hz input, but some lower voltages are:

AC Volts ..... F27 R

0.94 ........... 34 ....... 300 range
2.6 ........... 140 ....... 300 range
4.6 ........... 125 ....... 300 range
10 .............. 50 ....... 300 range


Internally in the Fluke there is a resistance and possibly other components that determine the DC current used to measure resistance. This at least pseudo current source and the associated resistance changes with the range. When an external voltage is applied there will be power dissipation and abnormal voltages produced in these components. No idea what Fluke has done to protect these components from damage from any externally applied voltage.

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