DC at a 120VAC Receptacle

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SG-1

SG-1

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Ware Shoals, South Carolina
What magnitude of DC would you consider to be excessive for a 120 VAC 15A or 20A general use receptacle in an industrial plant ?

At what magnitude would you begin a search for the source ?

Why do I ask ? Just found nearly 5VDC with a LoZ meter. I will continue to monitor & see if it fluctuates over time.
 
130419-1936 EDT

SG-1:

My experiment:

Measurement tool is Fluke 27 with the input terminals shunted by 0.47 ufd capacitor, a 1 megohm resistor in series from the hot outlet terminal to the meter + terminal, a direct connection from the neutral terminal to the - meter terminal, and looking at an outlet on my work bench. Without the filter I can not get a reading because the TED power line communication data pulse stream every 1 second causes auto-ranging to jump around.

With filtering the reading fluctuates from 0.000 to 0.006 V. TED is not the cause of this signal.

At the same outlet a 75 W incandescent with a series diode was connected and the DC increased to 0.16 V.

What is your normal work time shift?

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GAR, my normal time is 4PM to 12:30AM.

Measurement tool is an older Fluke 27 with a Fluke SV225 plugged into the input. By older I mean the meter has no CAT rating.

Just measured 13.3 VDC a few minutes ago.

I am going to find a heat gun to plug in.
 
A 14 Ampere load reduced the DC from 13.5 to 12.5.

Now for the really weird part. My meter cannot see polarity. No matter which way I turn the leads it sees + DC. :blink:
It functions normally testing a battery. I can reverse polarity & see a (-) reading. :happyyes:
 
Have access to an Oscope? More likely to help explain.
 
130419-2354 EDT

SG-1:

If you insert a low pass filter, like what I described, you should see the polarity change when you switch the meter leads. I don't know why you don't presently, but it probably relates to saturating the meter input with the very large AC waveform compared to the DC magnitude. The time constant of 1 megohm and 0.47 ufd is 0.47 seconds.

You have no need to work with the meter in a low impedance mode.

Somewhere there is phase controlled equipment where the plus and minus turn-on phase angles are not quite the same and thus a DC current is created, or half waving is occurring.

The source of the DC generated current is going to flow thru all the loads and the supply transformer that are connected to that transformer. How much flows where is dependent upon wiring and all the various load impedances.

If you did not have the changing DC voltage, then I might think the source of the current had an open diode or SCR. On the other hand possibly the variation results from various loads being turned on and off.

How to find the source?

This will be isolated to the loads on the secondary of the supply transformer. Suppose this is a 50 kVA 120-0-120 split phase source. Any electronic controlled load with input diodes or SCRs, that is a fairly large power load, is a likely source. Maybe there are many sources. I picked on 50 kVA as an example because that is what supplies my home. But the voltage I mentioned in my previous post would be due to wiring impedance. Since the DC voltage can only occur on the secondary side of a transformer you don't have to look beyond the loads on that secondary.

How can we sense a small DC current superimposed on a large AC current? No good answer. I would try to use a Hall probe, with DC capability, where the maximum AC current does not saturate the Hall device. Use a low pass filter and look for a change in the DC component. My Fluke Y8100 probe on the 200 A range will resolve 100/10000 A (10 mA) on my Fluke 27.

I did not quite beat your 12:30 .

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That is interesting. I initially discovered a DC component on my Test Equipment. It is feed by a dedicated 30KVA Delta-wye transformer. The transformer actually feeds 5 Test Tables. The magnitude was only a couple of volts DC at 120 AC.

After I finished checking some timers I then decided to check a nearby wall outlet for DC . That is where I was really surprised. I am sure the steel shop is generating the DC. They have Finn Powers, Lasers, break presses, & benders... & are going to add more soon.

I noticed 3 levels of DC volts, 2.5VDC, 8VDC, & 14VDC.

I will not be directly involved in the search. If there even is a search. That will fall to ATS. Any information that will help is appreciated.
I need to make some more measurements to see how much DC is present in the Test area. That may determine if a search is deemed necessary.
 
130429-0907 EDT

SG-1:

Your transformer is relatively small, and should be under 100 A per leg. Thus, even with peaking current, crest factor, I would not expect a 200 A RMS rated Hall probe to be saturated by the currents you encounter. and to produce the DC voltages you are reading will require more than 10 mA of DC current. Standard instrumentation should allow you to find the source(s).

For your DC voltage tests do use some sort of filter, 1 megohm and 0.47 ufd, like I described. This should prevent the meter from being saturated.

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I would think that you?d have to turn it an official experiment with a few good deductive hypotenuses.

It?s a piece of equipment that cycles, it?s on a control wire that interfaces for controls and equipment.

One might even get lucky if one could completely turn off each machine and any interfacing equipment if at all possible. I would suggest that all drawn systems of air, hydraulic, pneumatic, gases be depressurized in respects to the machine. One might see this mystery spike when it is reloaded and comes to life.

One could also split the search into a few more aspects. 1st is to test all isolated runs of low voltage. 2nd is check wires of 120V with low voltage in conduits. 3rd back board panel of the low voltage controller is not grounded or should not be grounded, IE the isolation bushings are shoot. 4th a transformer is failing. 5th is look for wrong type of insulation values on mixed equipment.

You certainly do work on some unusual equipment!
 
gar said:
130429-0907 EDT

SG-1:

Your transformer is relatively small, and should be under 100 A per leg. Thus, even with peaking current, crest factor, I would not expect a 200 A RMS rated Hall probe to be saturated by the currents you encounter. and to produce the DC voltages you are reading will require more than 10 mA of DC current. Standard instrumentation should allow you to find the source(s).

For your DC voltage tests do use some sort of filter, 1 megohm and 0.47 ufd, like I described. This should prevent the meter from being saturated.

.
Click to expand...

Since the DC cannot pass through transformers, then I am looking at two different sources. The before mentioned 30KVA transformer just supplies the test equipment. It has a 240VAC L-L secondary. I would be surprised to find 30A of load there under normal conditions.

The general purpose receptacles are supplied by some other transformer, location unknown, but probably near our switchgear. How the receptacles are connected to the machines in the steel shop is an interesting question.

I will have to scavenge for an AC capacitor. It may be easier to change meters, but I still intend to try this. I used my test equipment earlier last week to reproduce a drafting error that connected the AC & DC together in a control circuit. The Fluke 27 could not see the DC polarity then either. I did not have time to consider why then. I had to find the link & remove it.
 
130421-0743 EDT

SG-1:

Steve many different capacitors can be used and 0.47 ufd is not critical. With the 1 megohm series input resistance voltage won't be a problem. The capacitor I used is a Sprague 730P (Metallized Polypropylene) 250 V DC.

Any paper, Mylar, Polypropylene, mica, ceramic, or other non-electrolytic capacitor can be used. The Fluke 27 is just fine for your measurements. At 120 V AC 60 Hz input the AC voltage across the 0.47 ufd capacitor is about 120*6000/1,000,000 = 0.72 V RMS. With my components I measured 0.69 V,

The 6000 I estimated from my Shure reactance slide rule.

Correct on steady state DC not passing thru a transformer. The closer in a circuit you get to the source of the DC the greater will be the DC voltage reading assuming the DC source is constant.

.
 
130421-0824 EDT

SG-1:

With the 1 megohm, 0.47 ufd, 120 V, and 60 Hz I decided to switch the Fluke 27 to DC mV. The meter was saturated. Added a 12.5 ufd Polypropylene (motor capacitor) in parallel with the 0.47 ufd. Now Fluke DC mV was not saturated. My normal bench load of a couple computers and what ever else in the house and my neighbor's house only produced about 1.4 mV of DC at the bench outlet.

With a 10 megohm input impedance meter the 1 megohm series resistance only produces a 10% error in the measured DC voltage. Thus, increasing capacitance is the better choice than going higher than 1 megohm in resistance in the filter.

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GAR, Found a .22 ufd & will try it later today. Actually, I have two of them. My problem is that I have plethora of electrolitics & very few ac caps, never having a need for them before. I need to get an assortment.
 
GAR, if the DC turns out to be the upper or lower half of a sine wave, or chopped up in some fashon, like a square wave or saw tooth, it will come across a transformer winding.

It is time to go take this things picture. If it will rear it's ugly head today. :bye:


Thanks, Jumper.
 
130421-2013 EDT

SG-1:

If you have an AC waveform added to a DC component at the input to a transformer, and disregard transient DC items (these really have an AC component), then output from the transformer is the AC component of the input with the DC component stripped away. A old vacuum tube power output circuit, or a transistor power output circuit where an output transformer is used are examples.

When a DC current is present thru a transformer coil, then the magnetic field is unbalanced, and the hysteresis curve is shifted and there is an unbalanced (+ to -) AC magnetizing current to compensate for and subtract from the DC component.

A push pull circuit automatically does the compensation because the two DC currents create DC magnetic fields that are equal and opposite. Same in a full wave center tapped rectifier circuit. A half wave rectifier load on a transformer creates an unbalanced DC current in the transformer secondary. Effectively I did that with my experiment with the incandescent bulb and half wave diode in series.

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When measuring using the filter with the Fluke 27 the DC across the capacitor is zero.

Without the filter. Leads plugged into the recepticle.
A Fluke 337 True RMS reads .1 to .2 VDC.
The Fluke 27 is measuring 13VDC.

Would this indicate a half wave condition ?
 
130422-2131 EDT

SG-1:

Your reading indicates an instrumentation error type of problem when the filter is not present. My Fluke 27 won't settle down to produce a reading in autorange mode. Switching to fixed range I get .xxx (unstable), 1.71, 0.2, 000 for the four fixed ranges.

The measurement of 0.000 with the filter at the meter input indicates that there is no DC component on your AC waveform. Thus, no half waving of anything. But verify that the filter is not a problem by measuring a 1.5 V battery with and without the filter. The reading with the filter should be about 10/11 = 0.909 times the reading without the filter.

And yes my fluke reads positive on the erroneous readings.

A different brand of meter or different model may produce different results. My Beckman 4410 does not have anywhere the problem that Fluke has.

I believe the Fluke true RMS when set for DC does not use the RMS converter. Sort of guaranteed because the reading was so low. But clearly a different front end is used on DC in the 337 compared to the 27.

.
 
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