EC&M question

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electricalperson

electricalperson

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Location
massachusetts
in this months ec&m, they have an article about troubleshooting well pumps

at the end hes talking about locating the fault in the underground cable without digging up the entire thing. he says if you have a short between 2 wires, you can use a heavy load with a voltage in series and use a clamp on ammeter to locate the fault.

another method he uses: he measures ohms at the house then measures ohms at the wellhead. if ohms at the house is double than the ohms at the wellhead he knows the fault is one third the distance away from the wellhead.

can anybody explain what he means to me?
 
I haven't read the article, but I seem to remember reading in a very old book about the Varley Bridge method to just use ohm measurements and math to pinpoint faults. Plug that into your Google and see if that's what the article was about.
 
the wheatstone bridge came up when i googled that. do you have one of those or even know what they are? i never used or seen one before. i actually never even heard of it
 
electricalperson said:
the wheatstone bridge came up when i googled that. do you have one of those or even know what they are? i never used or seen one before. i actually never even heard of it
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You've got me curious now. When you posted your question, I had a flashback to that earlier reading. I've got to remember what book I was reading that in, or go wake up some 90 year old electrician from his bed at the nursing home. I remember the picture from the book, and it had you using a "6 volt truck battery".
 
mdshunk said:
You've got me curious now. When you posted your question, I had a flashback to that earlier reading. I've got to remember what book I was reading that in, or go wake up some 90 year old electrician from his bed at the nursing home. I remember the picture from the book, and it had you using a "6 volt truck battery".
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i have a very old electrical book too. ill check and look what it says about it. the book was printed in 1920
 
I'll have to twist my mind around the first method, and read the article but the second one would be hit & miss at best. I have never found a nice neat short with underground power cables. Not one where you could reliabley say this is a two to one ratio. Dirt can be almost concrete like around a bad spot and if in sand, glass.

I did suggest that method to a pivot company trying to find a fault in the antenna cable that circled a 160 acre field. They came within 30 feet or less. I believe they now have a tdr for those locates.
 
Marc,
I've designed in a Wheatstone Bridge in a strain gauge apparatus for measuring Wheel Chair Balance specs. It help establish criteria for designing a wheel chair that had known 'tip over' specs. Imagine a wheel chair bound person flipping their chair... very disasterous.

So, In an open field, locating a break in the wire (with a shunt to earth ground), the cables might have to be laid across a lot of ground; very un-wieldly, difficult.

I like the idea of the "Loaded Short Method", tracing with a clamp-on Amp meter.

Just a comment.
 
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LarryFine said:
But, how do you do that ...



:confused:​
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the person who wrote the article just emailed me back. he said you measure ohms across 2 shorted conductors and using the DC resistance tables in the NEC you can calculate the distance to the short. that sounds like it would work. im going to try that nex time i have a short in an underground wire.

now that im thinking about it. wont this require a DLRO?
 
he also said you can put the load in series with the shorted conductors, put a voltage and measure amps and use ohms law to get the ohms and calculate the distance with that. i should make a box that has a receptacle in series with a cord and plug in a drop light or something like that
 
electricalperson said:
the person who wrote the article ... said you measure ohms across 2 shorted conductors and using the DC resistance tables in the NEC you can calculate the distance to the short.
Click to expand...
With resistances that low, I'd think the resistance of each of your connections, as well as that of the offending short itself, would introduce great error.
 
mdshunk said:
I haven't read the article, but I seem to remember reading in a very old book about the Varley Bridge method to just use ohm measurements and math to pinpoint faults. Plug that into your Google and see if that's what the article was about.
Click to expand...

was the old book you have called localisations of faults in electric mains? i got one and it was printed in 1920 i forgot i had it and it talks about varley bridge method
 
090101-2120 EST

To make low resistance measurements you want to treat the resistor under test as a four terminal resistor.

Consider a straight piece of wire. Connect each end to a current source. Inboard of the current connections but near the ends connect a high input impedance voltmeter. Now you have a four terminal resistor and the wire resistance can be measured without including contact resistance errors at each end where the current is injected. The impedance of the meter only has to be high in relationship to the resistance of the wire being measured.

Shunts for current measurement are four terminal resistors.

10 amps thru a 0.001 ohm resistor produces 10 MV across the resistor.

0000 copper wire has a resistance of 0.049 ohms per 1000 ft. So a short at 500 ft in a cable would produce an 0.49 V drop with a 10 A current.

For the resistance method to accurately determine the location of a short it is necessary that the resistance of the short be very small compared to the wire resistance.

.
 
090101-2330 EST

electricalperson:

This is in answer to your PM to me.

Yes I have a Wheatstone bridge. It is part of my General Radio 1650-A LRC bridge.

You can make your own. A Wheatstone bridge consists of four resistors in a bridge configuration. Draw a square and on each side place a resistor. Between opposite corners place a voltage source. Now you have two voltage dividers across the voltage source. Place a voltmeter across the other two corners. This meter will read zero when the bridge is balanced. The bridge balance point has nothing to do with the excitation voltage other than the bridge sensitivity.

If you know the value of three of the resistors you can calculate the value of the fourth. If you know the ratio of two of the resistors, but not their precise values, and know the value of the third resistor, then you can determine the value of the fourth.

For the equations and a circuit diagram see:
http://en.wikipedia.org/wiki/Wheatstone_bridge

For much greater information see:
"Basic Electrical Measurements", by Melville B. Stout, Prentice-Hall, 1950.

I have no idea if Stout's book is available outside of some university libraries.

One patent that I have was a direct result of Stout's course and his emphasis on bridge circuits.

.
 
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