Ohm reading from ungrounded conductor to grounding conductor

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Quick question...... When I measure resistance at an outlet between the ungrounded conductor (hot) and the grounding conductor (green), I get 1.2 ohms. What is the path to ground here? I am assuming that I am reading the through the utility companies transformer coil?

There is no short because no OCPD are tripping and every outlet reads the same.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Quick question...... When I measure resistance at an outlet between the ungrounded conductor (hot) and the grounding conductor (green), I get 1.2 ohms. What is the path to ground here? I am assuming that I am reading the through the utility companies transformer coil?

There is no short because no OCPD are tripping and every outlet reads the same.

The EG and the Grounded conductor should be bonded together at the Service Entrance. 1.2 ohms from hot to EG would indicate to me that you have a line to neutral load connected some place. It could possibly be the secondary of the POCO transformer but it would be odd that it is connected and not energized. Ohm meters don't normally like line voltage. IDK what the secondary resistance of a transformer would be.

Explain how you are checking L-EG resistance with power available?
 

Ingenieur

Senior Member
Location
Earth
The EG and the Grounded conductor should be bonded together at the Service Entrance. 1.2 ohms from hot to EG would indicate to me that you have a line to neutral load connected some place. It could possibly be the secondary of the POCO transformer but it would be odd that it is connected and not energized. Ohm meters don't normally like line voltage. IDK what the secondary resistance of a transformer would be.

Re-edit
He measured line-gnd (ungrounded cond to gnd)
the xfmr Z might be 0.01-0.03 ohms give or take
 
Last edited:

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Not sure but if 1.2 is accurate big v drop issues

How so?

If he is checking a standard residential receptacle with 120 volts present, a reading of 1.2 on the meter may mean 1.2 x 100 volts, or 120 volts.

You have to be careful with some meters and make sure you know how they work. I had one that worked like the above and every now and then I would get fouled up. Like seeing 1.2 and thinking it's 1.2 volts. Which it would be if the meter was set on volts X 1.

:slaphead:
 
Thanks for your response guys. I am using a Fluke 324. I am reading ohms between the line (hot) and equipment grounding conductor. Its on a hot circuit. I am getting a reading of 1.2 ohms. When I open the main breaker. Resistance is OL (infinity).

Check any outlet at your house. I am pretty sure you will find the same. Very low resistance between the ungrounded conductor and grounding conductor on a live circuit.

Today is the first day I have ever checked resistance on a live circuit. I burned up a Fluke 787 ($800) about 7 years ago doing it. So I usually only check resistance with the breaker open. I was pretty surprised to see such low resistance between the hot and the ground. But I have to be reading through the transformer coil, its the only thing that makes sense.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Thanks for your response guys. I am using a Fluke 324. I am reading ohms between the line (hot) and equipment grounding conductor. Its on a hot circuit. I am getting a reading of 1.2 ohms. When I open the main breaker. Resistance is OL (infinity).

Check any outlet at your house. I am pretty sure you will find the same. Very low resistance between the ungrounded conductor and grounding conductor on a live circuit.

Today is the first day I have ever checked resistance on a live circuit. I burned up a Fluke 787 ($800) about 7 years ago doing it. So I usually only check resistance with the breaker open. I was pretty surprised to see such low resistance between the hot and the ground. But I have to be reading through the transformer coil, its the only thing that makes sense.

Your advice is to risk burning up meters?
 
No, I was assuming that anyone that read my post in entirety, would see that I have burned up a $800 meter doing this in the past and would consider that before they stuck their meter in a live circuit with it set on resistance. Common sense applies.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I doubt the OP is seeing an accurate reading. I just looked at the (crappy) user manual for the 324 and there doesn't seem to be anything special about it that would allow for the reading of resistance on live circuits.

I think he may be lucky he didn't blow this one up, too.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Basic meter use safety:

However, be sure to never measure the resistance of any electrically “live” object or circuit. In other words, do not attempt to measure the resistance of a battery or any other source of substantial voltage using a multimeter set to the resistance (“ohms”) function. Failing to heed this warning will likely result in meter damage and even personal injury.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160403-1440 EDT

zach_zachary1:

Something is fishy. Any ordinary device called an ohmmeter would be destroyed with 120 V 60 Hz or DC applied to its terminals. Can I design a device to mreasure DC resistance and not be destroyed by qpplying 120 V to the terminals? Possibly, but I have not applied any thought on how to accomplish that.

What is the DC resistance from an outlet back thru the secondary of the power company transformer measured with the primary unenergized? Quite low, probably mostly in the branch circuit wire. To this you have in parallel all of the shunt load from other circuits, yours and your neighbors. These are not low resistances, but they contribute to reducing the resistance that would be read. My guess is that the secondary DC resistance of a small residential transformer is less than 0.05 ohms. Current at 120 V and 0.05 ohms is 2400 A. #4 copper wire is 0.25 ohms per 1000 ft and #12 is 1.6 ohms per 1000 ft. A breaker resistance is fairly small.

Read your Fluke manual and see if there is any designed in protection. I don't believe there is any way the meter is reading the DC circuit resistance. Often times with Fluke probes it is difficult to measure fractional ohms. The meter itself will read 0.0 ohms with a short between two banana plugs. With good probes shorted together the lowest I get is 0.1 ohm on my Fluke 27. With not so good probes I get 3 ohms.

We really need to know how your Fluke meter works in its ohms position.

.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
160403-1440 EDT

zach_zachary1:

Something is fishy. Any ordinary device called an ohmmeter would be destroyed with 120 V 60 Hz or DC applied to its terminals. Can I design a device to mreasure DC resistance and not be destroyed by qpplying 120 V to the terminals? Possibly, but I have not applied any thought on how to accomplish that.

What is the DC resistance from an outlet back thru the secondary of the power company transformer measured with the primary unenergized? Quite low, probably mostly in the branch circuit wire. To this you have in parallel all of the shunt load from other circuits, yours and your neighbors. These are not low resistances, but they contribute to reducing the resistance that would be read. My guess is that the secondary DC resistance of a small residential transformer is less than 0.05 ohms. Current at 120 V and 0.05 ohms is 2400 A. #4 copper wire is 0.25 ohms per 1000 ft and #12 is 1.6 ohms per 1000 ft. A breaker resistance is fairly small.

Read your Fluke manual and see if there is any designed in protection. I don't believe there is any way the meter is reading the DC circuit resistance. Often times with Fluke probes it is difficult to measure fractional ohms. The meter itself will read 0.0 ohms with a short between two banana plugs. With good probes shorted together the lowest I get is 0.1 ohm on my Fluke 27. With not so good probes I get 3 ohms.

We really need to know how your Fluke meter works in its ohms position.

.

My Flukes are older and don't go into self destruct mode but they do give a warning similar to WTH YOU DOING!!
 

Ingenieur

Senior Member
Location
Earth
How so?

If he is checking a standard residential receptacle with 120 volts present, a reading of 1.2 on the meter may mean 1.2 x 100 volts, or 120 volts.

You have to be careful with some meters and make sure you know how they work. I had one that worked like the above and every now and then I would get fouled up. Like seeing 1.2 and thinking it's 1.2 volts. Which it would be if the meter was set on volts X 1.

:slaphead:

he said ohms on line-gnd
so he was reading 1/2 of the xfmr
and it sounds like he knows how to read the meter
so if 1.2 ohm is accurate and 20 A load drop is 24 volt on 120 or 20%
large

I have doubts about the reading
assume 25kva 120/240 xfmr at 1.8% Z
X base = 240^2/25000 = 2.3 ohm
0.018 x 2.3 = 0.0415 ohm
1/2 winding 0.0208

1.2 xfmr + service cnd + branch is too much
 
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