Strange resistance reading

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I work in the rock quarry industry as an electrician. Through MSHA we are required to perform an annual ground continuity test. Those not aware of what this is, it is when you take a ground test lead (I use 2500' #12 THHN) and connect one end to the grounding electrode at the service and run it to a motor and take an ohm reading between the motor housing and the test lead. After this is done take the test result and subtract the test lead resistance to get your true result. The question I have is there are some results that are a less ohm reading then the test lead itself. can anyone respond with a educated answer? Thank you in advance for your responses.
 

petersonra

Senior Member
Location
Northern illinois
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engineer
I work in the rock quarry industry as an electrician. Through MSHA we are required to perform an annual ground continuity test. Those not aware of what this is, it is when you take a ground test lead (I use 2500' #12 THHN) and connect one end to the grounding electrode at the service and run it to a motor and take an ohm reading between the motor housing and the test lead. After this is done take the test result and subtract the test lead resistance to get your true result. The question I have is there are some results that are a less ohm reading then the test lead itself. can anyone respond with a educated answer? Thank you in advance for your responses.

I would expect that it would always be less. Whatever your EGC is out to the motor is in parallel with the 2500 foot wire. I am not sure what the point of subtracting the test lead resistance is.
 

GoldDigger

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I would expect that it would always be less. Whatever your EGC is out to the motor is in parallel with the 2500 foot wire. I am not sure what the point of subtracting the test lead resistance is.
The OP is measuring the resistance of a wire loop. One side of the loop (from bond point to meter) is the test lead. The other side of the loop is the parallel combination of the EGC and any local path to ground at the motor.
Subtracting the resistance of the test lead is useful because it defines the resistance of the fault clearing path.

When you have a large loop like that I would worry about induced AC voltages messing up the ohmmeter readings.
 

iceworm

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I work in the rock quarry industry as an electrician. Through MSHA we are required to perform an annual ground continuity test. Those not aware of what this is, it is when you take a ground test lead (I use 2500' #12 THHN) and connect one end to the grounding electrode at the service and run it to a motor and take an ohm reading between the motor housing and the test lead. After this is done take the test result and subtract the test lead resistance to get your true result. The question I have is there are some results that are a less ohm reading then the test lead itself. can anyone respond with a educated answer? Thank you in advance for your responses.
I can't. From what you described that doesn't seem possible.
. . . . . . . . . . . . . xxxx' motor EBC - Rebc
S GE___________________________________Motor__
....... | ........................................................................|
........|.........................................................................|
....... |...................................................................... meter connected between to motor
....... | ....................................................................... |..... frame and motor end of #12
....... | .............. R #12 ~ 5 ohms ............................... |
....... |________________________________________|
....................... 2500" #12 connected to service grounding electrode

One exception: if there is a DC bias on the motor EBC, that can throw off the meter.

edit to add: As gold said - probably AC as well

ice
 

don_resqcapt19

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retired electrician
This application note about the difficulties of taking ground resistance readings might be a good read. I'm sure Fluke has similar literature, but I don't have links handy.

http://literature.cdn.keysight.com/litweb/pdf/5990-7323EN.pdf
The test required by MSHA is not a ground resistance test...it is a test of the EGC itself. Some MSHA flexible power cables even have an additional "ground check" conductor within the cable for the purpose of checking the actual equipment grounding conductor.
 

PetrosA

Senior Member
The test required by MSHA is not a ground resistance test...it is a test of the EGC itself. Some MSHA flexible power cables even have an additional "ground check" conductor within the cable for the purpose of checking the actual equipment grounding conductor.

Understood. I may have been unclear with my comment. What the application note addresses seems to be exactly what the OP is referring to - the difficulties of taking EGC resistance measurements due to voltage being present in the loop and how this affects a normal DMM's resistance reading of that loop. I've never tried this function on my U1272A (the Agilent/Keysight model that has the "smartohm" ability) but as I understand it, it works by performing multiple resistance readings at different burden voltages to eliminate the offset introduced by AC or DC voltage that may already be present in the loop.

Following a few links to other threads on this subject, it seems others have had issues in the past with how to take these measurements as well. One thread on electriciantalk.com describes a situation where the 500' test wire showed 0.7 Ohm resistance while the loop measured 0.6 Ohm with a Fluke 87 V. Obviously a similar error to the OP's was taking place. In another thread here on MH Brian John linked to some low resistance microohm meters from Megger which use up to 10A or 200A burden voltages to take the reading, but these are $2500-$5000 units. I don't know what the exact needs of the OP are, but it definitely sounds like this is not a job for a regular DMM and that some kind of advanced resistance measurement ability is required. I suspect the main difference between my U1272A and the $2500+ Meggers is how much voltage in the loop they will be able to read through. The Megger units also have test result logging and certain models can be set for go/no go testing.

Ultimately it sounds like the OP needs to make a few phone calls to test equipment manufacturers and get some options on what testers will meet his needs.
 

don_resqcapt19

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Location
Illinois
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retired electrician
Peter,
I'm sorry...I stopped reading the document in the link when I read the term "ground resistance". To me that is the term used when you are checking the resistance of a grounding electrode.

Now that I have read the complete link, I understand why you posted it.
 

gar

Senior Member
Location
Ann Arbor, Michigan
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150107-2043 EST

A fluke 27 or 87 reads a very small voltage across 1 ohm in ohms position and has poor resolution. These meters use a low constant current source for resistance measurement.

Number 12 copper wire at 20 C has a resistance of about 1.6 ohms.

I would try an experiment using a 12 V car battery as a DC source to make the test. If the loop resistance is less than 0.5 ohms add a fixed series resistance of 0.5 ohms to the battery circuit.

Measure DC current to the loop, and the voltage across the loop. If resistance is added do not make the voltage measurement to include that added resistance. Calculate the total loop resistance. Subtract the test lead rresistance.

Reverse the battery polarity and repeat the measurement. Average the two resistance measurements.

An even better way. Run two test lead wires. One wire is the #12 and the second is a smaller wire. Could be #30 or even smaller, but use #18 or #16. This is a non current carrying test lead to allow direct measurement of the voltage drop across the EGC wire. With a 10 V source and a 10 megohm meter the current thru the voltage test lead is at most 1 microamp. This makes the resistance a 4 terminal resistance measurement.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
150107-2157 EST

Did not proofread before submitting. Number 12 copper wire has a resistance of about 1.6 ohms/1000 ft.

A few other comments could be clearer, but I don't plan to edit any further.

.
 

Tony S

Senior Member
M&Q statutory testing (UK)

M&Q statutory testing (UK)

Speaking as a retired M&Q (Mines and Quarries) electrician (UK).

Earth return testing would be an annual statutory test.

This is the English version. Under M&Q statutory regulations the maximum allowable earth return path for a circuit is 50% of the line conductor resistance. Above that immediate remedial action is required. Our company standard was a much lower % but didn?t have the ?statutory? requirement for action.*

The test was based on the initial commissioning test for line resistance. Shutting the plant down was out of the question for subsequent testing. Therefore a wander lead would be used to test between the motor earth/ground terminal and the plant MET.
To conduct the test a ?Ductor? high current mili ohm meter was used in conjunction with the 70mm2 wander test lead. 70mm2 I believe is approximately 00 AWG.
A Ductor can inject currents of up to 600A DC hence the large test leads.

Normally through parallel return paths (IE the plant steelwork and multiple cables) readings of 0.001% were common place.
The low readings obtained by the OP should be the normal expected.

*Sorry I can?t say precisely what the company % was because the moment statutory testing was mentioned I vanished like the spirits at cock crow. I was asked if I would undertake the testing for a year, I offered to resign. We had about 2500 drives on the works, the thought of doing the same thing day in day out??? Arrrgggg!!!!
 
I greatly appreciate everyone's input. Peter, I purchased the AGILENT U1273AX meter with the smart ohm feature. Very sharp meter. a lot of test settings and the temperature reading on the display is a nice touch. Next is the blue tooth adaptor. I received it yesterday and tried it out today. What a difference. On a few of my test locations I compared the readings from the Agilent meter to a Fluke 787 Processor meter. A lot more desireable readings. Thank you for the suggestion.
 

Tony S

Senior Member
I work in the rock quarry industry as an electrician. Through MSHA we are required to perform an annual ground continuity test. Those not aware of what this is, it is when you take a ground test lead (I use 2500' #12 THHN) and connect one end to the grounding electrode at the service and run it to a motor and take an ohm reading between the motor housing and the test lead. After this is done take the test result and subtract the test lead resistance to get your true result. The question I have is there are some results that are a less ohm reading then the test lead itself. can anyone respond with a educated answer? Thank you in advance for your responses.

Could I ask what the requirements are for the ground continuity test? I?ve said the UK requirements, there seems a very great difference in our standards. So great I?m somewhat shocked.
 

PetrosA

Senior Member
Could I ask what the requirements are for the ground continuity test? I?ve said the UK requirements, there seems a very great difference in our standards. So great I?m somewhat shocked.

I'm confused as well that there don't seem to be any clearly defined parameters for this testing here. I did some quick searching the other night before I responded to Don regarding my poorly worded first post, and there didn't seem to be any definitive guidelines or standards mentioned anywhere. Even the MSHA has only this to say in the matter (from what I was able to find...):

56/57.12028 Testing Grounding Systems
This intent of this standard is to ensure that continuity and resistance tests of grounding systems are conducted on a specific schedule. These tests will alert the mine operator if a problem exists in the grounding system which may not allow the circuit protective devices to quickly operate when faults occur. With the exception of fixed installations, numerous fatalities and injuries have occurred due to high resistance or lack of continuity in equipment ground systems. These accident could have been prevented by proper testing and maintenance of grounding systems.

Grounding systems typically include the following:

equipment grounding conductors - the conductors used to connect the metal frames or enclosures of electrical equipment to the grounding electrode conductor;

grounding electrode conductor - the conductors connecting the grounding electrode to the equipment grounding conductor; and

grounding electrodes - usually driven rods connected to each other by suitable means, buried metal, or other effective methods located at the source, to provide a low resistance earth connection.

Operators shall conduct the following tests:

Equipment grounding conductors - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subjected to vibration, flexing or corrosive environments;

Grounding electrode conductor - continuity and resistance must be tested immediately after installation, repair, or modification, and annually if conductors are subjected to vibration, flexing or corrosive environments; and

Grounding electrodes - resistance must be tested immediately after installation, repair, or modification, and annually thereafter.

Conductors in fixed installations, such as rigid conduit, armored cable, raceways, cable trays, etc., that are not subjected to vibrations, flexing or corrosive environments may be examined annually by visual observation to check for damage in lieu of the annual resistance test. When operators elect to conduct this visual examination as a method of compliance with 30 CFR 56/57.12028, MSHA will require that a record be maintained of the most recent annual visual examination.

The grounding conductors in trailing cables, power cables, and cords that supply power to tools and portable or mobile equipment must be tested as prescribed in the regulation. This requirement does not apply to double insulated tools or circuits protected by ground-fault-circuit interrupters that trip a 5 milli-amperes or less.

Testing of equipment grounding conductors and grounding electrode conductors is not required if a fail-safe ground wire monitor is used to continuously monitor the grounding circuit and which will cause the circuit protective devices to operate when the grounding conductor continuity is broken.

A record of the most recent resistance tests conducted must be kept and made available to the Secretary or his authorized representative upon request. When a record of testing is required by the standard, MSHA intends that the test results be recorded in resistance value in ohms.

That's copied from HERE

I'm seeing nothing there to indicate what values are required or specifications as to how the test is actually conducted. I guess if you wanted to you could use a $3.99 Chinese tester from your local discount auto parts store as long as the numbers look good on paper. I think EU countries are ahead of us when it comes to defining testing standards and methods as much of the typical equipment for certifying an installation in the EU isn't even available in the US. We're happy with an inspector walking around with a $10 GFI tester vs. a contractor over there needing to use a $1000-$2000 installation tester. Crazy.
 

PetrosA

Senior Member
Next Generation,

Congratulations on the new meter. I hope it does what you need it to do. If you're going to invest in a Bluetooth module, you should know that there are two generations of them available. The first generation module is the U1177A which has a range of ~10 meters and will work with your android or iOS device. The second generation module is the U1117A with a range of ~100 meters when used with the U1115A Remote Logging Display or ~10 meters when used with your android or iOS device. There's a considerable difference in price, but depending on your needs, the second gen option might come in really handy. 100 meters of range can really save you a lot of walking during troubleshooting...

P.S. to add, Both generations of modules work with the U1115A remote display but range for the first gen module remains the same.
 

Tony S

Senior Member
I'm confused as well that there don't seem to be any clearly defined parameters for this testing here. I did some quick searching the other night before I responded to Don regarding my poorly worded first post, and there didn't seem to be any definitive guidelines or standards mentioned anywhere. Even the MSHA has only this to say in the matter (from what I was able to find...):



That's copied from HERE

I'm seeing nothing there to indicate what values are required or specifications as to how the test is actually conducted. I guess if you wanted to you could use a $3.99 Chinese tester from your local discount auto parts store as long as the numbers look good on paper. I think EU countries are ahead of us when it comes to defining testing standards and methods as much of the typical equipment for certifying an installation in the EU isn't even available in the US. We're happy with an inspector walking around with a $10 GFI tester vs. a contractor over there needing to use a $1000-$2000 installation tester. Crazy.

Peter, Thank you for the ?clarification? ;-)

I?m now at a total loss, as you say there seems to be no definitions or standards.
Thanks to the link you kindly provided it looks like I?ve got some reading to do. I?ve got to do something since retirement.

A MFT (Multi Function Tester) that almost all UK electricians carry will set you back anything up to ?900.
The trouble with them is the manufactures have tried to cram every test function into a small box. The site I help run in the UK constantly has new threads ?anyone know what error code xxxx is?. They?re so unreliable that a friend of mine caries three of them on his van.

That?s ?900 I?ve not spent.
 
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