Meter reads voltage anyways...

bryan77 · Dec 4, 2007

I have a question which is probably just theory related but couldnt find anything in search and didnt know what to look for online.

We have a 4kv to 480v transformers that feed some 480v switchgear that was recently installed. Of course there is a tiebreaker between the 2. We open the shutter and check voltage with the Fluke 87 to see if we are in phase. We get all the correct reading except A to A which reads 280v, out of phase we say.

The Engineer says it is a meter issue, so we get and Analog simpson meter and now read 0v A to A.

I found something about the inout impedance of the fluke and being a solid state device, but cant find something that explains it simply.

Can anyone help with an explanation or link.

Thanks a lot as I am new the the high voltage stuff.

OkiePC · Dec 4, 2007

Does the Fluke show a normal frequency indication at that voltage?

bryan77 · Dec 4, 2007

What do you mean?

Oh yeah, sorry about the **** poor spelling and grammer in the above post. I was rushed to get the kids ready to go byes..

OkiePC · Dec 4, 2007

This is a total WAG, but the only time my Fluke meters have seriously deceived me was in measuring ac voltage about 120VAC on a wire that was disconnected at both ends.

My Story: I needed to replace a 3phase 460vac motor without shutting down the whole machine. So, I locked it out, and disconnected the coil and the motor leads from the starter, taped the ends and tucked them into the panduit, so I would be safe since we had just been lectured on LOTO.

So I proceeded to de-terminate the blower motor while the machine was restarted and running. When I pulled the wire nuts off the motor leads, one of them arc-ed to ground.

Hmmm. So I put my leads on it and read about 120vac to ground. It was a little unstable, but hovered between 105 and 120VAC for about 3-5 seconds. I went back and double checked the main panel,and all looked well.

I thought, "It must be shorted to L1 somewhere in the wireway". They had used multiconductor cables for the whole machine, and there were a couple of dozen of them running overhead for about 30 feet in an open aluminum wireway. This wireway had been knocked over by careless forklift drivers no less than four times, and propped back up, so it was possible a cable was damaged, but how could it withstand being shorted to a motor lead?

I went to check voltage at the motor lead again, and while I read very low millivolts, the four drives started and my meter went up to about 118 again. The other two motor leads showed about the same voltage, and I then realized it was just noise. So, I held my breath, squinted and shorted them all to ground with one hand in my back pocket. The 10 amp fuse didn't blow becasue the product was still flowing, and now the meter showed zero.

So, it turned out to be caused by four VFDs in the same cable, tightly bundled with two other starter operated motors. I guess the frequency pattern could not be properly filtered by the meter, because later, before I hooked up the new motor, my meter showed a frequency that was "all over the place"...It was trying to "lock-in" on the carrier frequency noise of four AC drives and could not do it.

I followed up to find that all 6 motor leads were brought over in a 29 conductor cable about 50 feet long!

I know there should be no noise on your transformer, but if there were noise, you (EDIT) MIGHT see a frequency of something other that 60Hz.

Does the 87 have a frequency mode? Some of the higher end meters show it simultaneously when on AC volts. I guess I haven't used a Fluke 87 yet.

Paul

OkiePC · Dec 4, 2007

Ahhh, I think the answer in in your first post. . .meter impedance. . .

Googling Fluke 87 specs

10megohm input impedance.

Does that mean your switch had approximately 0.0000208Amps of current (or apparent current) flowing through it?

IR=E?

I*10,000,000=208?

Or am I oversimplifying this?

timbo_uk · Dec 4, 2007

Yes, up until now MOST fluke meters only had a high input impeadance (mainly for electronic circuit measurements) so as not to affect voltages in high impeadance circuits.

As you have found out, this is no good for measuring open circuit cables. These can act in a similar way to a TV/FM ariel (I can never spell that one!). This induces voltages onto them, and if you are only looking for a 0-10v signal it can throw you off on a tangent.

This is one reason why I am trying to get my boss to approve me a new meter

http://www.fluke.co.uk/comx/show_product.aspx?locale=uken&pid=37737

This also features a LO-Z voltage reading scale, this is a special low impeadance input that will "short out" and stray induced voltages whilst not affecting a real voltage.

Also the logger function looks to be a great idea, now I am on my own for breakdowns etc.

Lancie1 · Dec 4, 2007

Okie,

I have been frightened by this phenomonon many times. Generally if you have a disconnected wire in a bundle (or just close to) some wires that are live, the disconnected wire will pick up "induced" voltage from the other wires. Usually, the old Simpson meters had enough load resistance to dissipate this induced low-current high voltage.

Induced voltage is what allows an AC induction motor to turn, and a transformer to transform. Basically, Somebody-or-other's Law says that current flowing in a wire will induce a magnetic field around the wire, so that if any other wire cuts this field, it will also obtain an induced voltage (and current if it forms a circuit loop).

Without induced voltage, we would be in a pickle, but sometimes it can make your rear sphincter muscle draw up painfully.

krk · Dec 5, 2007

Fluke SV225 Stray Voltage Adapter

John Soltesz · Dec 5, 2007

Sometimes you can't beat a solenoid type of tester (wiggy) or test light if you just want to know it is dead. Over 30 years ago I worked as a diesel/electric locomotive electrician. The control system was 72vdc floating from carbody and 95% of the control troubleshooting was done with a test light. The first thing you would do is check for a control ground. You never used a meter, there were only Simpson 260 types available, the high impeadence would pick the leakage from the dirt on the batteries. But a light bulb wouldn't. Simple but worked every time. New guys would take out a pocket meter and go nuts. We would tell them just clean the batteries first and the ground would go away.

Electrically_Bound · Dec 5, 2007

I have used a Fluke 87 for years and they are good at picking up "ghost voltages" when measuring ac. Where I worked at, we had a lot of wires with very long runs, anywhere from a few hundred feet to almost a mile. It was VERY common to read 40 to 60VAC with a wires open at both ends. Whereas if you measured the voltage with a Simpson 260, you would read 0v because it had a much lower input impedance. I guess "wiggys" have their place, but I have never been fond of them.

The bad thing about reading "ghost voltages" is that it is hard to convince the the uneducated that a dead circuit is truly dead. They are trying to follow safe work practices by measuring the voltage after opening a switch and locking it out, but then see that "ghost voltage" and believe the circuit is not dead.

The thing that I despise the most is these little voltage checkers that are given out as freebies by some vendors. I'm not sure what principle these things work on, they do not contain a power source, but just about anything will light them up. You can actually make them read voltage by moving your fingers rapidly upand down them.

plcsoftvsd · Dec 5, 2007

This happened at our site also, we had a PLC which had 220vac output which went through a 19core cable for 2000 metres to another remote panel, and we used to get stray voltage ( about 60 volts) even on unenergised cores at the remote panel.

We got rid of the problem by using 24vdc supply instead of 220vac.

John Soltesz · Dec 5, 2007

Electrically_Bound said:
I have used a Fluke 87 for years and they are good at picking up "ghost voltages" when measuring ac. Where I worked at, we had a lot of wires with very long runs, anywhere from a few hundred feet to almost a mile. It was VERY common to read 40 to 60VAC with a wires open at both ends. Whereas if you measured the voltage with a Simpson 260, you would read 0v because it had a much lower input impedance. I guess "wiggys" have their place, but I have never been fond of them.

The bad thing about reading "ghost voltages" is that it is hard to convince the the uneducated that a dead circuit is truly dead. They are trying to follow safe work practices by measuring the voltage after opening a switch and locking it out, but then see that "ghost voltage" and believe the circuit is not dead.

The thing that I despise the most is these little voltage checkers that are given out as freebies by some vendors. I'm not sure what principle these things work on, they do not contain a power source, but just about anything will light them up. You can actually make them read voltage by moving your fingers rapidly upand down them.

I hope you aren't thinking of the little pocket tester like a Fluke 1AC-11 VoltAlert instead of the old rugged solenoid type.
These are made by SquareD- Wiggy, and Ideal- Vol-Test. Big difference between them. There are even updated CAT III versions. Each has it's place.

bernie_carlton · Dec 5, 2007

Isn't induction great? We were preparing a machine with multiple VFD's. The control panel was to be located, in the final installation, about 60 feet away from the actual machine. So on our shop floor the intervening cable was neatly piled up in coils. When one VFD was powered up another, which had absolutely no power at the time, would occasionally come to life, LED numbers showing, then go back off. Obviously enough energy was being coupled from the coiled up motor leads into those from the unpowered VFD and feeding back just enough to bring the unpowered VFD to life for a moment. That had some scratching their heads for a moment.

Ron Beaufort · Dec 5, 2007

just as a word of caution for our beginner readers ...

the “induced” voltage issues being discussed in this thread are DIFFERENT from the “leakage current” issues that are more commonly talked about on the forum ... leakage current has to do with the small (but important) trickle of current that can flow through a solid state “switch” (for example: a TRIAC) in a PLC output module ...

from Electrically Bound:

The bad thing about reading "ghost voltages" is that it is hard to convince the uneducated that a dead circuit is truly dead. They are trying to follow safe work practices by measuring the voltage after opening a switch and locking it out, but then see that "ghost voltage" and believe the circuit is not dead.

now that is correct - and I am NOT saying anything to contradict it ... but ... sometimes beginners read these posts and misinterpret what is being said ...

here’s my point ... suppose that we’re using a sensitive digital meter (such as a Fluke) to check for voltage to a field output device (for example a contactor coil) which is controlled by a PLC with a TRIAC-type output module ... the PLC’s logic tells the output to turn OFF ... the PLC’s output is forced OFF ... the coil in the field is DE-energized and definitely OFF by inspection ... we measure the voltage across the coil and get something like 1.8 volts ... we disconnect the “line” wire from the coil’s terminals and then measure the voltage again - from the disconnected “line” wire to the “return” wire still attached to the coil ... this time we read 120 volts ...

now then ...

suppose that a beginner has misinterpreted the discussion in this thread and thinks to himself: “Ah, this must be that “induced” GHOST VOLTAGE effect that I’ve read about. The sensitive meter is telling me that I have voltage present - but the circuit is really DEAD and safe for me to work on.”

that’s WRONG! ...

once again, the posts in this thread are talking about “induced” voltages ... “leakage current” is a different animal ... if the beginner in our example above happens to touch that loose “line” wire, he’s probably going to get the beejeebers shocked out of him ...

now some people will say that the leakage current is only a trickle - and therefore not enough to really “hurt” you ... but most of us will agree that the surprise of a sudden shock in the wrong place - at the wrong time - can certainly make you hurt yourself ...

summing up:

(1) “induced” voltages are generally caused by signals which are “coupled” from a live wire to a disconnected wire either by inductance or by capacitance ... in MOST cases these “induced” voltages will not cause a safety problem ...

(2) “leakage current” is generally caused by the inability of a solid state device to completely stop the flow of current - even when turned OFF ... in MANY cases this “leakage current” can indeed cause a safety problem ...

the biggest problem is in convincing the uninitiated (in other words the “unshocked”) that there is indeed a difference between these two similar effects ...

we now rejoin our previous thread on “induced” voltages - already in progress ...

keithkyll · Dec 5, 2007

Ron Beaufort said:
...
(2) “leakage current” is generally caused by the inability of a solid state device to completely stop the flow of current - even when turned OFF ... in MANY cases this “leakage current” can indeed cause a safety problem ...

With Triacs and Triac-based SSR's, the largest portion of the leakage current is from the internal 'Snubber'. The snubber works similar to the diode across a coil when using DC. It supresses back EMF. Typically, it is a 100 ohm resistor in seies with a .1 uF cap. This causes 4 ma or so of leakage.
There is some leakage from the semiconductor device itself, but less significant than the leakage caused by the snubber.

Meter reads voltage anyways...

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