I'm troubleshooting a old branch circuit that goes out to a water pump that is approximately 300ft underground. I short the wires on one end of this 10/2 w/grnd UF cable and I can't see it with my Fluke at the other. Is it possible that my Ohm meter function wont read at those kinds of distances with only the 9v battery potential? I dont own a megger...
How far will a Fluke Ohm meter read?
- Thread starter Mule
- Start date
- Status
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
A shorted far end of 300 ft is just a few ohms, any meter should be able to tell the difference between that and the wires not shorted. If you cant, you have a broken wire.
The 9V battery has nothing to do with it, and a megger would add no insight in continuity terms.
The 9V battery has nothing to do with it, and a megger would add no insight in continuity terms.
Just making sure, I predicted the cable is open and I dont have a clue where its at. I've already told the customer that we are looking at replacing the cable unless we can find the open connection. Just grab'n straws trying to think of something I've missed....
You can try a toner if you don't get any sound then your wire may be no good.
tell the customer that you want to Run it in pipe this time it will last longer.:grin:
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
Take a look at this thread:
http://forums.mikeholt.com/showthread.php?t=104391
I saw a Fluke that could measure the distance to the fault or the amount of cable left on a spool. You would need to know the exact path of the cable for this to work.
http://www.flukenetworks.com/fnet/en-us/products/TS90/Overview.htm?PID=50081&categorycode=BCT
http://forums.mikeholt.com/showthread.php?t=104391
I saw a Fluke that could measure the distance to the fault or the amount of cable left on a spool. You would need to know the exact path of the cable for this to work.
http://www.flukenetworks.com/fnet/en-us/products/TS90/Overview.htm?PID=50081&categorycode=BCT
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
Consider getting one. Things like this are so much easier with it.
Imagine walking back out there with a megger, disconnecting the load and hooking it up, and watching the results - when you hook it up, try to charge it to 500v and see that it can only load it to 3v, you have conclusive evidence that the cable is toast. As you're repairing the cable, you can test at the break and determine conclusively that in this direction you're fine, but in the other direction there's more trouble. No guesswork.
I walked into a house that was tripping an arc fault. I meggered the wires, and found a reading of 32.1MΩ. Found a problem, fixed it, and only improved it to 48.4 MΩ. I knew to keep looking, and found another problem. After that fix it grew to 1484MΩ, so I knew that the second fix was a substantial contributor to the problem.
It tells you so much (accurately) about how things are behaving, it's priceless, IMO.
- Location
- Massachusetts
George, I may be thinking wrong here but it seems to me using a high voltage to see if conductors that should be closed are open is exactly the opposite of what you would want to do.
I think the standard ohmmeter test would be a better choice.
It seems to me a high voltage low current test could jump across an open that would show up with low voltage test or the high current of the pump.
But I am not sure of my thinking here as a newbie with a Mega.
I think the standard ohmmeter test would be a better choice.
It seems to me a high voltage low current test could jump across an open that would show up with low voltage test or the high current of the pump.
But I am not sure of my thinking here as a newbie with a Mega.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
I am thinking more in terms of separating those conductors and meggering them to see what they have to say. Meggering while connected would not tell us much.
- Location
- Massachusetts
I am thinking more in terms of separating those conductors and meggering them to see what they have to say. Meggering while connected would not tell us much.
I do not see how the Mega will reliably find an open used in the way you seem to be describing.
It seems to be using the Mega for the opposite task it is designed for.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
It's an underground UF run. Chances are good that if it is indeed open somewhere between points A and B, then it's in contact with the earth.
You're right, I would start my process by
Edit to add - A simple ohmmeter and voltage test should give us a yes or no answer, but the megger speaks volumes about the maybe's.
You're right, I would start my process by
- Turning on the breaker and attempting to read voltage at the load.
- If this turned up a low voltage at the load, I would walk back, shut the breaker off, remove and cap the conductors from the breaker, grab the megger and walk back to the load.
- Disconnect the load.
- Megger the conductors to earth and to the EGC.
- Evaluate what it's telling me.
- Try a short on the cable with the ohmmeter for fun to kill time while I evaluate a course of action for the problem, and to confirm the results of the megger reading.
Edit to add - A simple ohmmeter and voltage test should give us a yes or no answer, but the megger speaks volumes about the maybe's.
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I am thinking more in terms of separating those conductors and meggering them to see what they have to say. Meggering while connected would not tell us much.
I like your way of thinking on this. To clarify iwire's confusion the test with conductors shorted at other end and a regular ohm meter is a continuity test, testing for continuity with the megger is pointless, testing to ground with the megger is an insulation test. He likely knows this, but the way it was presented was possibly a little confusing.
ptonsparky
Tom
- Occupation
- EC - retired
I agree with George. Get a Megger. I lived for years without one but wouldn't go back with out kicking and screaming.
As to the OP it will depend on which Fluke you have.
Rarely ,once, have I come across a buried open conductor that was not also going to ground or earth. We found that one by walking the path and paying attention to what had changed. Damned near tripped over it.
As to the OP it will depend on which Fluke you have.
Rarely ,once, have I come across a buried open conductor that was not also going to ground or earth. We found that one by walking the path and paying attention to what had changed. Damned near tripped over it.
I would like to have a megger as the condition of the insulation is equally important as the Ohm meter is to continuity......In this case, the original 10/2 UF cable in truth is too small. I havent looked at the HP rating of the new pump, but they did not consult with me prior to their purchase and its a 120v motor (ouch) AND we'll need to install heat tracing as well. So it's looking like I'll be figuring VD and installing PVC pipe and a larger sized URD cable most likely. Right now I'm guessing at the 300ft as it could be further when measured....These folks live on top of a hill and have only 20lbs of water pressure as the previous pump has not worked in years....not good
- Location
- Massachusetts
I was not confused in the least.
What I was saying is from the beginning the op was looking for an open.
The best way to verify the conductors are open would be a simply continuity test.
There is no point in testing the insulation of conductors that are open. It is simply a wasted step.
This in no way means I am not glad I finally got a meaga and it has helped me find a fault.
- Location
- Massachusetts
I disagree and would not bother trying to test for an 'open' based on the assumption that the open has closed the connection to 'earth'
I agree, step one.
To me grabing the Mega at this time is a waste.
IMPO it is time for a standard continuity test.
Edit to add - A simple ohmmeter and voltage test should give us a yes or no answer, but the megger speaks volumes about the maybe's.
I am not getting the volumes, I am getting one page
that tells me the condition of the insulation and nothing about the condition of the conductors.Any guess about the conductors conditions would be just that .... a guess.
Now, if the call was for a pump that every so often is tripping the breaker the mega would be my first choice to look for an intermittent ground fault due to insulation break down.
I failed to mention that currently I have multipe cables at the panelboard that could be going to the pump, all of which are 10/2 UF (New home owner)Three of which are not hooked up and dont read at the other end when shorted, and the other three are live and we have no voltage at the other end. I stopped troubleshooting, and told the customer that it doesnt look good unless we have more information. So nothing appears to be grounded or shorted, but does appear to be open.....
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
100918-0848 EST
For this problem it is probably pointless to determine where the open is because other factors imply the cable needs to be replaced with a larger cable.
However, besides TDR, one could measure the cable capacitance and get an estimate of the distance to a break. To do this nothing should be connected at either end of the cable. You would want to know the normal cable capacitance per foot, or if you could assume a single break point, then measurements from the two ends and a ratio would indicate the break. If there are more than two wires in the cable, then different pairs would need to be tested.
.
For this problem it is probably pointless to determine where the open is because other factors imply the cable needs to be replaced with a larger cable.
However, besides TDR, one could measure the cable capacitance and get an estimate of the distance to a break. To do this nothing should be connected at either end of the cable. You would want to know the normal cable capacitance per foot, or if you could assume a single break point, then measurements from the two ends and a ratio would indicate the break. If there are more than two wires in the cable, then different pairs would need to be tested.
.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Hospital Master Electrician
I should start with saying that I trust that Bob would likely find any problem faster than I would, so I am interested in this difference in our thinking to help make me better too.
Wouldn't you rather have your T-87's digital number readout?
Didn't your voltage test verify the 'open' without thinking about ohms? Why are we looking for an open?
I would not expect to find the 5% chance that the wire is 100% wide "open", not touching anything else. So I guess what I'm saying is, I would not be looking for an open in the first place. I would be attempting to ascertain why I've got 120v on one end and 3v on the other. That condition generated the phone call.
With a good conductor surrounding my insulation (dirt around my UF), then if it passes the insulation test, I've learned two things: the insulation is great, it has a specific resistance, and if this curveball is thrown my way...
...then I have a specific cable to find, now. It has a specific resistance (unless the insulation is beyond my meter's ability to read) and from my first test I know it will hold a tone for a wire hound so I can go find the right cable and/or breaker to energize.
Then why you starting trouble?
Let's take this a step further - Let's say I ripped that T-87 out of your hands and handed you my vol-con. It rings when there is roughly a 1000Ω or less of resistance, give or take. Would you feel comfortable with the results of a test that provided you no numbers, just a ring if the reading was within a range? Would you stop investigating if it didn't ring?
Wouldn't you rather have your T-87's digital number readout?
Didn't your voltage test verify the 'open' without thinking about ohms? Why are we looking for an open?
Why not? In my experience, when there is no breaker tripping on an underground UF run, and no (or extremely low) voltage at the other end, there's a 95% chance that the voltage is being bled out into the earth at some point along the run, at too low a current to trip the breaker.
I would not expect to find the 5% chance that the wire is 100% wide "open", not touching anything else. So I guess what I'm saying is, I would not be looking for an open in the first place. I would be attempting to ascertain why I've got 120v on one end and 3v on the other. That condition generated the phone call.
With a good conductor surrounding my insulation (dirt around my UF), then if it passes the insulation test, I've learned two things: the insulation is great, it has a specific resistance, and if this curveball is thrown my way...
...then I have a specific cable to find, now. It has a specific resistance (unless the insulation is beyond my meter's ability to read) and from my first test I know it will hold a tone for a wire hound so I can go find the right cable and/or breaker to energize.
- Status