Stumped?

[HotConductor]

I was working with some utility guys troubleshooting some underground SE conductors pulled through PVC. I rang them out and easily found the culprit, one bad conductor. This was at night in the dark so I couldn't see what they did but the lineman said his meter read 50v with the conductor disconnected from the transformer. He said it was probably grounded. He said if it was a good conductor it would read maybe 1-5 volts. He was using a TEGAM VOLTMAN 125 meter. Can someone explain how this works?

 

[brian john]

HMMMMM

Dis you use your meter also?
Were other conductors in the conduit energized?
Did he ground the conductor and the reading go away?

If a meter gives an unexpected reading check the meter user, check the meter, check the testing practices. SOMETHING is not right.

Meter has phantom voltage suppression

From Tegman


125
Voltage: 750 Volts AC and DC
Accuracy: ? (0.2% rdg + 1 Volt) DC at 50/60 Hz
Resolution: 1 Volt
Frequency Range: 45 to 2000 Hz
Phantom Voltage Suppression
1 year warranty
Made in USA


http://www.tegam.com/product.asp?modelNumber=125

 

[HotConductor]

Was there capacitance from a primary overhead?

 

[brian john]

The phantom voltage suppression should address this.

 

[LarryFine]

He said if it was a good conductor it would read maybe 1-5 volts. He was using a TEGAM VOLTMAN 125 meter. Can someone explain how this works?

He probably had the other test lead on one of the line terminals. so the reading was from hot to (what should have been) a floating wire. Yes, it is a phantom-voltage reading, and his meter sounds like it has a low-impedance (not zero-impedance) input.

 

[HotConductor]

He probably had the other test lead on one of the line terminals. so the reading was from hot to (what should have been) a floating wire. Yes, it is a phantom-voltage reading, and his meter sounds like it has a low-impedance (not zero-impedance) input.

Larry, I think you're dead on. All the conductors were disconnected from the pot but he probably tested from one hot lug on the xfmr to each of the hots. One of the nice features of his meter was it had 36" leads.

 

[brian john]

Larry did you see the specs for this meter?

 

[LarryFine]

Larry did you see the specs for this meter?

Yes. Why? The phantom-voltage-suppression feature? I'm sure that's why it read a few volts instead of near line voltage.

If it were capable of zeroing any induced voltage, it would allow to much current when used on energized terminals.


Added: I use my K-60 to test against a live breaker terminal before energizing new or uncertain circuits.

 

[HotConductor]

Yes. Why? The phantom-voltage-suppression feature? I'm sure that's why it read a few volts instead of near line voltage.

If it were capable of zeroing any induced voltage, it would allow to much current when used on energized terminals.


Added: I use my K-60 to test against a live breaker terminal before energizing new or uncertain circuits.

"I use my K-60 to test against a live breaker terminal before energizing new or uncertain circuits."

Larry, please explain?

 

[LarryFine]

"I use my K-60 to test against a live breaker terminal before energizing new or uncertain circuits."

Larry, please explain?

Okie-dokie! Before I energize a new circuit, or one that's been repaired or modified, I check to see if there is a short circuit by touching one lead of my solenoid-type tester to the hot wire of the circuit in question, and the other lead on the load terminal of a breaker that is on.

Imagine you're troubleshooting a circuit that has a short circuit somewhere. Instead of repeatedly resetting the breaker, you connect a rubber socket and a bulb between the breaker and the load wire; i.e., black wire onto breaker terminal, white wire wirenutted to load wire.

This can also work if the problem is an intermittent open, like with back-stabbed receptacles. Have one person monitor the bulb while the other wiggles and taps devices until the bulb flickers. Wire in a receptacle instead and plug a cord in with a lamp if you're alone.

 

[TEGAM]

How the TEGAM 125 works

How the TEGAM 125 works

I saw the discussion regarding testing coductors and thought everyone would like a little background on the TEGAM 125. In the interest of full disclosure and credibility please be aware that I represent TEGAM.

The 125 was designed in conjunction with utility companies to specifically address this problem. Phantom voltages are capacitively coupled into adjacent conductors from live ones. Since they are capacitively coupled instead of directly connected, they are inherently current limited. This will produce a significant voltage reading on a standard multi-meter that has a high input impedance. However, the TEGAM 125 contains circuitry that presents a low input impedance and loads down the circuit to produce the correct voltage reading. If the circuit is actually live, the 125's input impedance changes to higher level to operate as a standard voltmeter.

The recommeded practice with any meter is to test a known live circuit to verify the meter's valid operation before testing an unknown one. Imagine that you have a broken lead on a test probe and you test an unknown circuit. Your meter would read 0V!

Be Certain,
TEGAM

 

[HotConductor]

I saw the discussion regarding testing coductors and thought everyone would like a little background on the TEGAM 125. In the interest of full disclosure and credibility please be aware that I represent TEGAM.

The 125 was designed in conjunction with utility companies to specifically address this problem. Phantom voltages are capacitively coupled into adjacent conductors from live ones. Since they are capacitively coupled instead of directly connected, they are inherently current limited. This will produce a significant voltage reading on a standard multi-meter that has a high input impedance. However, the TEGAM 125 contains circuitry that presents a low input impedance and loads down the circuit to produce the correct voltage reading. If the circuit is actually live, the 125's input impedance changes to higher level to operate as a standard voltmeter.

The recommeded practice with any meter is to test a known live circuit to verify the meter's valid operation before testing an unknown one. Imagine that you have a broken lead on a test probe and you test an unknown circuit. Your meter would read 0V!

Be Certain,
TEGAM

Welcome to the forum TEGAM. I just purchased this meter from Hanover Sales.

This might seem a little irrelevant but the other thing I liked about the meter was there are no settings, just on or off(with auto off). When testing continuity it reads "CON" and it automatically knows dc or ac which makes it pretty dummy proof.

 

[LarryFine]

Welcome to the zoo! :smile:

In the interest of full disclosure and credibility please be aware that I represent TEGAM.

Does that make you more or less credible?

If the circuit is actually live, the 125's input impedance changes to higher level to operate as a standard voltmeter.

How does that operate? Current-limiting, or something more complex?

The recommeded practice with any meter is to test a known live circuit to verify the meter's valid operation before testing an unknown one.

We usually do that, then check the live one again, especially with a non-contact tester.

 

[TEGAM]

Welcome to the forum TEGAM. I just purchased this meter from Hanover Sales.

This might seem a little irrelevant but the other thing I liked about the meter was there are no settings, just on or off(with auto off). When testing continuity it reads "CON" and it automatically knows dc or ac which makes it pretty dummy proof.

That was actually an intentional part of the design. It had been observed that people were injuring themselves with standard multi-function meters by inadvertantly leaving them in the ohms setting and connecting them to significant potentials. KABOOM! The leads also do not detach to prevent an accident where one end of a probe is on a lethal voltage and the other end is swinging in the breeze.
Thanks for your business.
TEGAM

 

[TEGAM]

Welcome to the zoo! :smile:

Does that make you more or less credible?

How does that operate? Current-limiting, or something more complex?

We usually do that, then check the live one again, especially with a non-contact tester.

Probably less credible because I have a profit motive. I'll try to keep the salesman in the can. Feel free to beat on me if I don't.

The way both the model 122 and 125 work is:
There is a positive temperature coefficient thermistor across the leads that is about 5 KOhms when cold. This is sufficient to load down capacitively coupled voltages due to their limited current capacity. However, a real connection to voltage sources enough current to heat the thermistor which then increases its resistance to keep from overheating the thermistor. The meter always reads the voltage across the leads regardless what the thermistor is doing.
The continuity test is also automatically engaged when the voltage across the leads is less than 1V.

 

[wptski]

The continuity test is also automatically engaged when the voltage across the leads is less than 1V.

That's like the Fluke T+Pro too! I like that as you can test for voltage and if none is present, it will tell you if you have a cellphone charger, etc. plugged in somewhere on the circuit. Check N-G continuity also. Of course this all relies on the circuit being wired correctly in the first place!

Larry Fine:

In your Post #10, the first sentence, checking from the hot to hot from another circuit, etc. If you had a cellphone charger plugged in on the circuit that's turned OFF at the breaker, it'll give you a false reading that the hot is grounded. Lots of modern electronics are still really ON all the time to keep the memory alive, clocks, etc.

 

[LarryFine]

Larry Fine:

In your Post #10, the first sentence, checking from the hot to hot from another circuit, etc. If you had a cellphone charger plugged in on the circuit that's turned OFF at the breaker, it'll give you a false reading that the hot is grounded. Lots of modern electronics are still really ON all the time to keep the memory alive, clocks, etc.

True enough, but then I could look for that to be the case, unplug the forgorren load, and then re-test.

I'd rather stop and check for a short and be wrong, than go ahead and energize the unknown and be wrong.


Added: It's like the non-contact tester argument that they sometimes indicate hot falsely. Big deal!

 

[TEGAM]

Added: It's like the non-contact tester argument that they sometimes indicate hot falsely. Big deal![/QUOTE]

One of the things I don't like about the noncontact sensors (and we have built them in the past) is that they don't always discriminate well between conductors in a bundle. You end up having to pull apart several conductors to sort out which ones are hot. Sometimes you will get the false reading you mentioned but you can also miss an energized conductor if it is shielded. That's the scary one. That isn't too common in structural wiring but it is in equipment and controls.

 

[LarryFine]

One of the things I don't like about the noncontact sensors ... is that they don't always discriminate well between conductors in a bundle.

No argument there. Once I detect energized conductors, unless I'm looking for complete de-energization, I presume every one is hot until I test them individually, which is when I break out the K-60.

Added: The point: I'd rather presume wires to be hot and be wrong than presume them to be dead and be wrong. The former just wastes a few minutes, the latter can prove deadly.

 

[HotConductor]

He probably had the other test lead on one of the line terminals. so the reading was from hot to (what should have been) a floating wire. Yes, it is a phantom-voltage reading, and his meter sounds like it has a low-impedance (not zero-impedance) input.

What would be a normal reading I can expect to get from known good underground conductors? I tested 2- #8 in pvc today with a 120v source and I read 17v. That seems high to me for good conductors. The first test was a 500vdc insulation test and that was good. I tested one against the other and one against the other and ground tied together.

Last week I tested some known bad direct buried conductors and I read 277v from a 277v source. That seemed almost impossible to me. The only thing I could think was thay were grounded right near the light pole they went to. The light pole had a driven rod bonded to the egc.