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#66729 06/16/06 08:57 AM
Joined: Jan 2006
Posts: 60
R
Member
No matter the source, whether capacative coupling, leakage across a device or induction. The electrician in the field needs to know if the voltage is real or "phantom". This is one of the reasons why I always carry a "solenoid" type of voltage tester. Mine is of the "Ideal" variety. They are a good test for this condition.

I have recently seen it where an unplugged meter base, with suite feeders attached and circuit breaker at the suite open, showed 120 volts to ground on the load side of the meter jaws when tested with a real good quality Fluke. I Did not have my Ideal tester with me so we used a 100 watt bulb, the voltage collapsed to 0 as soon as any load was injected.

[This message has been edited by Rick Kelly (edited 06-16-2006).]

#66730 06/16/06 03:37 PM
Joined: Nov 2002
Posts: 174
K
Member
I'm thinking that the voltage may induced into the 'disconnected' line from the grounded conductor. When you open the breaker, there is no longer any current flowing in the remainder of the circuit to induce voltage into it.

With the breaker closed, whatever else is running on that circuit will create a current flow in the parallel conductor.

If this were DC, we could use a compass to tell which way the mystery current is flowing.

#66731 06/16/06 04:17 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
I have seen up to 90 volts of 'phantom' voltage with my Fluke 87 on open conductors that share a raceway with other 277 circuits.

It is never just millivolts, the meter left unconnected shows millivolt readings. [Linked Image]

I agree with the above posters that for general line volt troubleshooting leave the DMM in the truck and use any solenoid type tester. They don't lie, assuming you check them against a known live circuit.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#66732 06/18/06 10:29 AM
Joined: Dec 2005
Posts: 44
T
TNSunny Offline OP
Member
Thanks for the info and advice! I guess I'll be heading out to buy me a wiggy.

Kevin


Kevin
#66733 06/18/06 05:29 PM
Joined: Dec 2003
Posts: 886
H
Member
I should have been more specific and said the voltage was capacitive coupled between the conductors and switch contacts rather than induced.

At any rate, your first choice when testing electrical circuits should be a Wiggy or other similar tester designed for the purpose. No real reason to use anything else like a VOM unless you need to do in depth troubleshooting. Then you need to keep in mind the high input impedance and the errors it may cause.

-Hal

#66734 06/18/06 05:38 PM
Joined: Nov 2005
Posts: 827
Likes: 1
J
Member
I don't think I could ever leave my high quality but high-z meter back in the office in favor of one that gets me less. You can always go high to low. You can save alot of bucks with a couple feet of 1000V flexible test lead, a couple insulated gator clips, a little heat shrink, and a 27k, 2W common resistor. That would put just under 5mA load on a 120V circuit. If that's less current than GFCI specs say will kill you, it should be a good safety test when used with a hi-z meter. Just keep it coiled up in your Fluke pouch. If you get a non-zero voltage that concerns you, just clip the resitor on and measure again. Problem solved.
Joe

#66735 06/18/06 05:58 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Joe I am not being a wise guy at all here but can you explain what kind of common line voltage troubleshooting requires such precise measurements?


Give me a solenoid tester with continuity function and I can troubleshoot most any typical lighting or power problem.

Many times (and I am guilty of it as well) I think folks just want to look 'cool' with more meter than is necessary. [Add Tim the tool man grunt here]

Of course there are times when you need to bring out the big guns.

But troubleshooting a lighting outlet with a DMM is IMO like using a sledge hammer to drive a finish nail. [Linked Image]

JMO, Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#66736 06/19/06 01:50 AM
Joined: Nov 2005
Posts: 827
Likes: 1
J
Member
Bob,
A few months ago, a gentleman suggested just using a Wiggy Hot to Gnd to check a GFCI. I thought that a solenoid type tester might draw too much current to indicate proper function of a GFCI. We could search to find my old results, but I seem to recall that the current was about 8 times the trip spec. If this is the case, a solenoid type tester might well place a greater burden on the so called "phantom", or "residual", or "voodoo that you do so well" voltage, compared to a human. I know that my body will never see more voltage across it than my meter will at the same two points. Not so for a Wiggy.

Whatever TNSunny's needs might be, he already owns a high quality Fluke DMM. To invest in another tester just to put a greater load on a circuit isn't the way to go IMHO. I use just about every function on my 187 so a Wiggy wouldn't cut it for me. I might well suggest buying a Wiggy over a Fluke to someone, depending on the planned usage. Why pay the extra bucks if you don't need the extra function?

As a specific answer to your question, motor starts come to mind. What were the minimum voltages present on my phase loss monitor when the lead pump started? Do I have enough lower margin dialed in or will we just be doing repetitive pre-lubes from here on out. That UPS is non-synced. What is my input voltage and frequency? If I kill my alternate, am I free running at 60 Hz? Is my battery float at 133V and Equalize at 140V?

Finally, I like to play, "Sledgehammer" while driving finishing nails with a claw or ball peen hammer. "Solsbury Hill" goes better with troubleshooting except for the "boom, boom, boom" part. "Shock the Monkey" should probably be kept out of the rotation.
Joe

#66737 06/19/06 02:30 AM
Joined: Jul 2002
Posts: 8,443
Likes: 3
Member
I find it rather hard to believe that seasoned Electricians are even discussing this.
I'm with Bob (Iwire) on this, look, if you get a dodgy reading on a DMM, you are obviously using the wrong test gear.
For the majority of my testing, I use a set of Duspol Solenoid testers, and I've never had a problem with "Ghost or Phantom" Voltages.
All you need is a tester that puts a bit of load on the circuit.
The Duspol (pictured below) draws 120mA with the solenoid energised, more than enough to get rid of nasty misleading Ghost voltages.

[Linked Image]

#66738 06/19/06 04:32 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Joe it is definitely just a personal choice issue. [Linked Image]

Look at the OPs question here, there are no motor loads, there is nothing tricky.

It is just a simple branch circuit. [Linked Image]

I feel more comfortable dragging around a $30 almost indestructible meter than having a $300 fragile meter out on the job when it's not needed. (Theft, breakage etc.)

Quote
You can save alot of bucks with a couple feet of 1000V flexible test lead, a couple insulated gator clips, a little heat shrink, and a 27k, 2W common resistor.

I don't see that as easer or better than using the right meter for the job at hand.

AS far as GFCI testing the button on the GFCI is more than up to the task.

I am not surprised a guy with a handle of 'testing engineer' would want to use overkill equipment. [Linked Image] [Linked Image]

You could break out a Yokogawa Scope and find out even more info. [Linked Image]

[Linked Image from yokogawa.com]

But when all you need to know is;

Is it on?

Is it Off?

Is there continuity between X and Y?

It seems a little over the top.


JMO, Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
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