Measuring resistance

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Cow

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Eastern Oregon
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Electrician
I'm trying to get a project finished up involving a new fuel system with a cardlock controller. The cardlock controller instructions state specifically that my neutral-ground resistance measured at the cardlock with power on, must be less than 1 ohm or it will cause issues with the memory and transaction data they need to store.

Here's where I'm starting to question my methods. I have a brand new Fluke 289 that has been used possibly 5 times. My resistance reading is .258-.4xx ohms using the 50 ohm, low ohm setting. I'm good, right?

The fuel system tech puts his fairly new Fluke 117 in there and gets 5.5-8ish ohms, depending on which time we were testing. He also had another Ideal meter that I didn't catch the model of, that had similar readings within a couple ohms.

I'm trying to understand if that error comes from the different accuracy levels of each meter or something else I'm not thinking of? The fuel tech is adamant there is a resistance issue, of course I think just the opposite. I should mention that swapping leads, made almost zero difference. Terminals are nice, clean and tight. The test probes/clips were put in the same spot everytime, just to rule out some variables.

http://en-us.fluke.com/products/digital-multimeters/fluke-117-digital-multimeter.html#techspecs

http://en-us.fluke.com/products/digital-multimeters/fluke-289-digital-multimeter.html#techspecs

I'd appreciate it if someone could take a peek at both meters specs and let me know if the accuracy for each meter is enough to cause that big of a discrepancy? I can't tell by the specs if accuracy is a percentage of the meters range or a percentage of the tested result? At this point, I'm about to send my 289 in to have a fresh calibration done, just to cover my bases and assure the customer this installation is correct. I may just call Fluke tomorrow and ask them about the accuracy as well as possible calibration, but I figured some of you guys on here could get me pointed in the right direction.

Thanks.
 
For run of the mill multimeters, the specifications of the two do not seem all that different.
But for low resistance measurements it is more important to have a means to null out the effect of lead and contact resistance.


A true four lead low resistance ohmmeter will have two current leads that force a known current through the resistance being tested and two voltage leads that attach on the load side of the current leads and which do not themselves carry significant current.


All that the contact resistance and lead resistance on the current leads does is raise the voltage that the tester needs to apply to get the set current.
And since the voltage leads carry no current their lead and contact resistance do not affect the voltage reading.
Result: you get the resistance under test without worrying about any deficiencies in your connection to the tested resistance.

No low ohm resistance tester can be guaranteed to work well when there is any voltage difference between the two ends of the resistor under test. That is almost certain to screw up any measurements made with power on between EGC and neutral unless the device filters out 60Hz and stray DC in some way.
 
GoldDigger said:
No low ohm resistance tester can be guaranteed to work well when there is any voltage difference between the two ends of the resistor under test. That is almost certain to screw up any measurements made with power on between EGC and neutral unless the device filters out 60Hz and stray DC in some way.
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Well, that puts me in tough spot. The fuel tech made it clear the test had to be made with the system powered up to meet the manufacturers guidelines. It sounds like I need to verify that myself by calling the manufacturer. I've never had to do a neutral-ground test on an electronic piece of equipment, so this cardlock kind of has me thrown for a loop.

Thanks Golddigger.
 
Cow said:
Well, that puts me in tough spot. The fuel tech made it clear the test had to be made with the system powered up to meet the manufacturers guidelines. It sounds like I need to verify that myself by calling the manufacturer. I've never had to do a neutral-ground test on an electronic piece of equipment, so this cardlock kind of has me thrown for a loop.

Thanks Golddigger.
Click to expand...

An impedance measuring set that uses a high audio range test voltage current source fed through a series capacitor to block out both DC and 60z could do the job. It would almost have to be a four lead type to keep the series capacitance in the voltage leads from affecting the low ohm measurement.
 
What Gdigger said in in #2.

Ohmeter (esp 117 on ohms) useless if there is a potential difference between neutral and ground.

Another way besides a specialized piece of equipment is to measure the ac and dc voltage between neutral and ground. (a scopemeter trace is better). Easiest to do the following if that number is zero, which it may not be.

Then, take a 12V or 18V power tool battery able to drive 10A or so short circuit with a dc ammeter in line and put it across the neutral-ground points and read the neutral to ground voltage, setup like a 4 terminal resistance measurement. Calculate R. 99% of the time you can neglect the ac voltage, but to be very accurate you need to take any ground-neutral leakage current that gives a ground-neutral voltage with no battery connected.
 
ptonsparky said:
The real problem you have is which testing equipment and what procedure will the MFG accept. (Their tech's of course.)
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I would question the Service Tech about how many times he/she has performed this test on the same equipment at other locations useing the same test equipment and what results were obtained.

The company that manufacturer & services this equipment should have a very specific proceedure for performing this test and even the equipment necessary to perform said test.
 
A properly designed cardlock controller would not care what the resistance between the EGC and the grounded conductor is.

What is the voltage between the neutral and the EGC at the equipment with the equipment powered up? A more common spec is to place a limit on that voltage, but again in properly designed equipment it doesn't make any difference unless the voltage between the EGC and the grounded conductor is so high that the equipment is not getting enough voltage to work (when you measure the voltage between the neutral and the EGC, you are really measuring the voltage drop on the neutral)
 
I have seen two different pieces of electronic equipment that had errors with as little as 1.5 volts from neutral to ground. The manufacturer's literature for one of them had a voltage limit stated in the manual.
 
Golddigger and Junkhound, let me contact the mfr and see what they say about which testing method they are expecting to achieve this required reading. I have a hard time believing I will have to jump through a bunch of hoops, batteries, high end meters, when it appears the fuel tech has been doing it for years with just a simple meter. As well as other electricians on previous cardlock installs.

ptonsparky said:
Cow,

Do you have a Fluke 1507 insulation tester? The Ohm range on that seems to be pretty good.

The real problem you have is which testing equipment and what procedure will the MFG accept. (Their tech's of course.)
Click to expand...

I do have a 1507, I will have to give it a shot the next time I'm out there doing testing.

growler said:
I would question the Service Tech about how many times he/she has performed this test on the same equipment at other locations useing the same test equipment and what results were obtained.

The company that manufacturer & services this equipment should have a very specific proceedure for performing this test and even the equipment necessary to perform said test.
Click to expand...

I agree Growler. I know this tech has been doing this for a while, I'm sure he's done many of these exact same installs. He made the claim these exact meters are what he's used on previous installs to verify the installing electrician has been under 1 ohm. I believe that's why he feels so strongly his readings are correct and mine are not.

don_resqcapt19 said:
A properly designed cardlock controller would not care what the resistance between the EGC and the grounded conductor is.

What is the voltage between the neutral and the EGC at the equipment with the equipment powered up? A more common spec is to place a limit on that voltage, but again in properly designed equipment it doesn't make any difference unless the voltage between the EGC and the grounded conductor is so high that the equipment is not getting enough voltage to work (when you measure the voltage between the neutral and the EGC, you are really measuring the voltage drop on the neutral)
Click to expand...

I can take that measurement next time I'm out, I only took a simple voltage measurement between L-N just at initial power up. I question the reason this N-G measurement is so critical also. I can't help but think there is something I'm missing about the cardlocks circuitry that makes it so sensitive to this.

Thanks for the replies everyone.
 
dkidd said:
I have seen two different pieces of electronic equipment that had errors with as little as 1.5 volts from neutral to ground. The manufacturer's literature for one of them had a voltage limit stated in the manual.
Click to expand...
That is a common statement, but I stand by what I said...a properly designed piece of electrical equipment will not care about the voltage between the grounded conductor and the EGC.
 
160203-1512 EST

GoldDigger and junkhound have provided you with a good and simple approach. The important point is to use a 4 terminal method of resistance measurement.

To run an experiment on my bench I did the following.
1. Measured the DC voltage between neutral and EGC at the point where the measurement is to be made. Result 0.9 mV. Should be no problem.
2. Connected a variable DC source to an ammeter and a 5 ohm power resistor.
3. This series combination was connected between Neutral and EGC of a duplex receptacle in a box with two duplexes.
4. Current was adjusted to 1 A.
5. DC voltage was measured between Neutral and EGC from the adjacent duplex to get a 4 terminal measurement that excluded the contact resistance drops in the first duplex.
6. Result 0.46 V or 0.46 ohms for the neutral-EGC loop resistance. This is higher than I might have expected.

With a Fluke 27, two plugs, #16 wire to banana plugs the reading was 0.6 ohms. Reading was 2.7 ohms with Fluke test leads because I could not make good contact. Test lead to test lead with lots of pressure reads about 0.2 ohms. A dead short between banana plugs at meter terminals is essentially 0.

Cow this DC 4 terminal test may not be easy for you. That will depend upon the kind of parts you have available. However, it is probably the best way to perform the measurement.

.
 
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