Voltage drop in underground cable

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JWelectric

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Can You offer some help here ?


The simple fix would be to replace the underground circuit based on the information given.

The original poster made three statements
1- Low voltage reading from the hot to neutral
2- 2- low voltage reading from hot to EGC
Placing any type of load in series with this already voltage drop will not make the voltage be higher but instead it would make the voltage lower. The load of the light will drop the remaining voltage.
3- I then tested the voltage at the breaker and had 120 volts
Now we know that the problem is with the conductors somewhere between the breaker and the receptacle so what would you suggest to look at for the problem?
 

LLigetfa

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Placing any type of load in series with this already voltage drop will not make the voltage be higher but instead it would make the voltage lower. The load of the light will drop the remaining voltage.
True, but the voltage readings are suspect. The OP should repeat the test at the breaker with a load on the circuit to verify there is no voltage drop at the breaker. A light bulb as in a work light plugged into the GFI outlet would provide a load that is safe to use. Unlike a motor, that can burn out from under-voltage, I've yet to see a light bulb do so. I fail to see what hazard there is in loading the circuit with a light bulb.

I would not trust a reading with a high impedance voltmeter on a circuit with no load.
 

Jadnashua

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If the voltage is normal at the supply and low at the end of the wire, the wire is the likely problem. Course, he could reattach the GFCI and plug a lamp into it to see what's there, but if the breaker was bad, the vast majority of the time, you'd read the same voltage there and at the end of the wire, which is not the case.
 

JWelectric

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True, but the voltage readings are suspect. The OP should repeat the test at the breaker with a load on the circuit to verify there is no voltage drop at the breaker. A light bulb as in a work light plugged into the GFI outlet would provide a load that is safe to use. Unlike a motor, that can burn out from under-voltage, I've yet to see a light bulb do so. I fail to see what hazard there is in loading the circuit with a light bulb.

I would not trust a reading with a high impedance voltmeter on a circuit with no load.

In the first post of this thread the statement was made; œI removed the gfi receptical and put my tester on the wires coming out of the ground and got a reading of 62 volts
If he is getting a reading of 62 volts without the receptacle being installed why would he need to reinstall it to plug something into the receptacle to see if the voltage is the same?

It doesn't matter if it is an analog or DVM type of meter reinstalling the receptacle and plugging in a load will not make the voltage somehow peak out to an average of 120.
This same meter got a reading of; œtested from the breaker to neutral and got 120 volts and this was out any load on the breaker.
Unless it is your contention that this meter and circuit is somehow loaded at the breaker in order to get these readings then the simple answer is replace the underground circuit.

All of this hogwash of high impedance voltmeter and needing a load to check voltage leaves me asking questions.
Why did they ever come out with such an item and why would anyone ever buy one if it entail all this extra work?
Could someone explain to me just how one could be used to trouble shoot a motor control center if every one of those control circuits will require a load to check for voltage?

controlwiring.jpg


Recon this feller needs to get a light bulb?
 
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LLigetfa

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So now you want to divert attention from your knee-jerk over-reaction to using a lightbulb in a test by ridiculing? Then you go off topic about how to troubleshoot a motor control centre? How you!

I never said it wasn't the underground wire. It most likely is. All I said was the test procedure is flawed. Before I dug up the yard to replace the wire, I would verify the test procedure.

I'm out of here.
 

JWelectric

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So now you want to divert attention from your knee-jerk over-reaction to using a lightbulb in a test by ridiculing? Then you go off topic about how to troubleshoot a motor control centre? How you!

I never said it wasn't the underground wire. It most likely is. All I said was the test procedure is flawed. Before I dug up the yard to replace the wire, I would verify the test procedure.

I'm out of here.

If you would be so kind before you leave please explain just what you mean by; “All I said was the test procedure is flawed.â€

Just how is the test procedure flawed? As a licensed electrician with over 44 years of experience I would have done nothing different except I wouldn’t have removed the receptacle, I would have done my voltage readings with the receptacle in tack using this meter,

categoryratings.jpg


He tested the voltage at the receptacle and got 62 volts. Then he checked the voltage at the breaker and got 120 volts. What is flawed in his procedure?

By the way there is no difference in checking voltage in a residential setting than checking voltage in a motor control center or in your car for that matter.
In my entire electrical experience the only place I have ever heard of plugging in a light to test voltages is on DIY type web sites where the knowledge of the posters are very limited and most will swear by something they read that another limited knowledge person has posted.
 
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LLigetfa

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OK JW, since you asked nicely...

My background is in electronics where the impedance of a voltmeter can more commonly influence the result. Voltage across a series of resistors gets divided based on the resistance values. The voltmeter is one of those resistors.

In one of your examples, you were measuring the voltage across an open switch wired to a light bulb. With the switch open, you would see essentially the full voltage despite the bulb itself being a resistor in series with the neutral. When the switch was closed you said, the voltage would read zero which is very true. You could simulate similar results by closing the switch and lifting the return conductor and measuring the voltage between the threaded base of the bulb and ground. That way, the resistance of the bulb would be in series with the hot instead but still, the voltage would show very near to that of the panel. This is because the voltage divided by the very high impedance of the meter has little influence.

The OP's measurement obviously included an in-line resistance or else there would not have been a voltage drop. The inline resistance however of the test I outlined above would not have indicated that much of a drop despite there being an in-line resistance. The difference is in current flow. Adding a bulb at the GFI introduces another series resistor (the first being the fault) and induces some current for accuracy.

I could much easier explain this in person on a workbench with a series of bulbs and a voltmeter.
 

DonL

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Could someone explain to me just how one could be used to trouble shoot a motor control center if every one of those control circuits will require a load to check for voltage?

Recon this feller needs to get a light bulb?


A motor control center wiring is tested using a continuity tester before power is applied.

And they do use Lightbulbs, a bunch of them for testing the SCR Motor drivers, Before it is connected to a motor or when a motor is not available. They also use giant Resistor banks for load testing.

Light bulbs give a visual indication if all is working properly without smoking the scrs, Just in case there is a wiring or component fault.

I used to work on Oil Drilling SCR motor controls, But we are working on a pump motor here.

But the same theory applies.

Maybe a meter with 100+ foot test leads would make for a better test.

Sure would not want to mention shorting the conductors together for testing continuity if a light bulb is not allowed.
 

LLigetfa

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Maybe another example would be easier to visualize. Take an old fashion string of Christmas tree lights... the kind where the bulbs are in series... one burned out bulb and none of them light. Pull out the last bulb in the series. Check the voltage sequentially at each good bulb and they should all read close to full voltage because there is no current to divide the voltage. Of course YMMV depending on the impedance of your voltmeter.
 

Jadnashua

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If the pump didn't run, he measured 120vac with no load, then it went down to 62vac with a load, that would indicate one thing, but if it reads 62vac without a load and it reads full voltage on the other end of the wire, no load is required to further test this...there's a problem in the circuit somewhere. For all practical purposes, a modern DMM is effectively an open circuit. As part of a voltage divider circuit, it can become critical in an electronic circuit, but seldom is a factor in a power circuit. Now, if he also got 62vac at the breaker, that would be telling - indicating the breaker was bad.
 

DonL

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If the pump didn't run, he measured 120vac with no load, then it went down to 62vac with a load, that would indicate one thing, but if it reads 62vac without a load and it reads full voltage on the other end of the wire, no load is required to further test this...there's a problem in the circuit somewhere. For all practical purposes, a modern DMM is effectively an open circuit. As part of a voltage divider circuit, it can become critical in an electronic circuit, but seldom is a factor in a power circuit. Now, if he also got 62vac at the breaker, that would be telling - indicating the breaker was bad.

Very True.

It is a classic example that the voltmeter will tell you that it is bad, but will not necessarily tell you if it is good.
 

JWelectric

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OK JW, since you asked nicely Icould much easier explain this in person on a workbench with a series of bulbs and a voltmeter.

Without a workbench a simple explanation would be that the total of the voltage drops in a series circuit will equal the applied voltage.

So in this case where the resistance seems to be somewhere in the underground wiring, with the circuit open (the receptacle is removed) the difference in potential at the opening in the circuit would show the applied voltage with or without a load.
 

JWelectric

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A motor control center wiring is tested using a continuity tester before power is applied.

And they do use Lightbulbs, a bunch of them for testing the SCR Motor drivers, Before it is connected to a motor or when a motor is not available. They also use giant Resistor banks for load testing.

Light bulbs give a visual indication if all is working properly without smoking the scrs, Just in case there is a wiring or component fault.

I used to work on Oil Drilling SCR motor controls, But we are working on a pump motor here.

But the same theory applies.

Maybe a meter with 100+ foot test leads would make for a better test.

Sure would not want to mention shorting the conductors together for testing continuity if a light bulb is not allowed.

fullppe.jpg


Now we know why this happened, he didn't use a light bulb
 
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JWelectric

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A motor control center wiring is tested using a continuity tester before power is applied.
Not if the problem starts while the center is in operation. At this stage the control circuits will be tested live using a volt meter.

And they do use Lightbulbs, a bunch of them for testing the SCR Motor drivers, Before it is connected to a motor or when a motor is not available. They also use giant Resistor banks for load testing.
I have seen and installed indicator lights but have never used a light for testing in a motor control center. The use of inductive banks is mostly used when an alternate power source is being connected for backup power. The resistor banks are to ensure that the backup power supply can carry the full load when starting. I have such inductive banks that I use for generators. I have three 1KW heating coil banks that I use.

Light bulbs give a visual indication if all is working properly without smoking the scrs, Just in case there is a wiring or component fault.
Unless the light bulbs equal the current draw of the load being served then they could not determine if the load is too large for the SCR or any other current limiting device. Bulbs installed on the control front cover is to indicate that a circuit is energized not for testing.

I used to work on Oil Drilling SCR motor controls, But we are working on a pump motor here. But the same theory applies.
It wouldn’t matter what the motor controller was controlling be it an oil rig or the conveyer system at Federal Express the testing producers are the same. The theory of current flow does not change just because it is an oil rig. As an instructor of NFPA 70E I have heard all sorts of horror stories that transpire while doing voltage testing in the field such as the one you mention here. I have a CD full of pictures that I use in the classroom to show just how dangerous this type of method is and the results of such testing procedures.

Maybe a meter with 100+ foot test leads would make for a better test. Sure would not want to mention shorting the conductors together for testing continuity if a light bulb is not allowed.
Shorting the wires together would be perfectly alright if there was no power on the system. If there is power on the system checking continuity would be just about as dangerous as pulling the pin on a grenade then sticking it your pocket with or without a light bulb.
Checking for continuity with a light bulb in series will give you an inaccurate reading of the resistance of the circuit as the resistance of the light bulb would be included. If checking continuity of the circuit while reading parallel with the bulb would show a good circuit as the reading would be through the bulb not the conductors.
 

silverfox52009

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Gentlemen I would like to thank you all for your input I walk the path of the wire and found what appeared to be a gopher hole I dug up the wire at that point and found a small nip in the wire. I cut that section out and reconnected the ends and have 120 volts at gfi receptical. Thx again for your suggestions.
 

BobL43

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Gentlemen I would like to thank you all for your input I walk the path of the wire and found what appeared to be a gopher hole I dug up the wire at that point and found a small nip in the wire. I cut that section out and reconnected the ends and have 120 volts at gfi receptical. Thx again for your suggestions.
Sounds unsafe. Tell us more how you made the repair and if it is only temporary. Thanks
 
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