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[Mike Sokol]

If the distro had a misplaced/accidental ground -neutral bond and there was a missing ground-neutral bond in the panel, that could explain the mystery neutral current but it would be unusual to find 2 problems working in concert.

So another crazy idea rears its ugly head. Was the backline gear plugged into your distro system, or did they grab their own stage power for the guitar and bass amps? If they grabbed stage power and there was ground loop current via grounded DI boxes (but not necessarily ground loop hum) then you could easily get a few XLR shields adding a couple amperes each into your distro ground. I've personally measured 5 amperes of ground loop current in an XLR shield. Now, as Steve mentioned, you would also need a secondary Ground-Neutral bonding point somewhere in your measurement loop for the current to cross into the neutral, but I've seen stranger things before. So could it be back-fed ground loop current from stage amps? Remember, it's not 120-volts, it's mid-amperage current that might not be obvious unless you go looking for it.

[Jeffery Foster]

Thank you to everyone who contributes information to this thread! A couple of points;

1. This condition occurred from virtual sound check (no live backline) all the way thru the night. This act only used two DIs if I remember correctly.

2.  I did test with the amp racks powered down as well.  The current decreased to nothing as one would expect.

3. I used my mag light to illuminate the reading, because I too couldn't believe my eyes.  I'll put it at a 1% chance that didn't see the decimal point.

4. I might make it a point to go back to this venue if I'm ever near that city and walk thru things with the electrician.

I appreciate everyone's input!

[Mike Sokol]

4. I might make it a point to go back to this venue if I'm ever near that city and walk thru things with the electrician.

What city/state was this in and what's the name of the venue?

[Jeffery Foster]

Oh man do I have a bizarre update on this. Gotta get thru this show first...

[Jeffery Foster]

New venue, same issue. Gotta be something on my end.

I'm testing everything as we speak. The neutral current is exactly the same as the two hot legs metered together. Right now 15Arms per leg, approx 25Arms on neutral. 0.1 on ground.

[Tom Bourke]

Does your clamp meter have a frequency counter?  In a 60 Hz 3 phase system with lots of Triplen Harmonics you can read 180 Hz on the neutral.  Not sure how that equates to single phase but it is at least some thing to look at.

[Guy Holt]

Does your clamp meter have a frequency counter?  In a 60 Hz 3 phase system with lots of Triplen Harmonics you can read 180 Hz on the neutral.  Not sure how that equates to single phase but it is at least some thing to look at.

It doesn’t. It is a common misconception that the current returning on the neutral is 180Hz because it consists of 180Hz  3rd Harmonics. In a power quality workshop I developed for IATSE Local 481, we look at the harmonic currents contributed to the neutral return by each phase leg by turning on, one at a time, a non-linear fluorescent light on each phase leg.

As you will notice from the power point slide above from that workshop, when the distorted current drawn by a nonlinear load on one phase leg is returned to the neutral conductor it is still at 60 Hz (top left PQM reading.)  As we add the second  leg the frequency (cycles/second) increases from 60, to 120 (middle PQM reading.) And, when we add the third leg the frequency increases to 180hz (lower right PQM reading.) That is how we end up with 180hz current on the neutral conductor of a three-phase distribution system.  The resulting frequency is 180hz because each phase leg contributes a pulse of current (consisting of many harmonics) not because all harmonic currents cancel out except the 3rd.  With non-linear loads on only two legs of a three-phase system, the highest frequency the current on the neutral can be is 120Hz.

Guy Holt, Gaffer
ScreenLight & Grip
www.screenlightandgrip.com

[Tom Bourke]

It doesn’t. It is a common misconception that the current returning on the neutral is 180Hz because it consists of 180Hz  3rd Harmonics.

While the current may not really be at 180 Hz the harmonics can fool the frequency counter in the clamp on meter.  I would be curios if in this case he reads some thing other than 60 Hz, indicating some strong harmonics.  It would also be cool to start poking around with a differential probe on an Oscilloscope.  I assume the OP does not have one of those. I would just use the counter function and systematically power different things up.  See what the readings are with various things on.

[Guy Holt]

While the current may not really be at 180 Hz the harmonics can fool the frequency counter in the clamp on meter.  I would be curios if in this case he reads some thing other than 60 Hz, indicating some strong harmonics.

I doubt it. Under balanced load conditions as is the case here harmonic currents in the neutral are minimal (in out of balance conditions they can be significant.) That’s not to say that triplen harmonic currents don’t exist in single phase systems as they do in three phase systems – they do.

In the same power quality workshop mentioned above, I also compare the amount of triplen harmonics in three phase and single phase systems.  To demonstrate the source of elevated neutral return currents in three phase systems, I use a non-linear load that is used by the hundreds on motion picture stages for CGI production: the Kino Flo Image 85.

As you can see in the Power Point slide from the workshop above, Image 85s are rich in harmonics with a THD of 77% and a large triplen component (3rd, 9th, & 15th.) In the workshop I demonstrate how and why these non-linear lighting fixtures can lead to elevated neutral returns in 3-phase systems by watching what happens on a power quality meter as fixtures on separate legs of the service are turned on one at a time.

As you can see in the power quality readings above, absent phases B & C, the Image 85 on phase A returned 9A to the neutral (left picture.) When the Image 85 on phase B is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 12.71A (center picture.) And, finally, when the Image 85 on phase C is switched on, and it returns another 9A to the neutral conductor, the current on the neutral climbs to 15.86A (right picture.)  Even though the three phases are perfectly balanced (9A on each) the current on the neutral is 176% of any one of the individual phase legs. Clearly, there is some cancellation between the phase legs going on (otherwise the neutral would be carrying 27A), but it is not complete cancellation. Why?

When we dump return current from each leg into the “stew pot” of the common neutral, out of phase current cancels. The Fundamentals (A1,B1,C1 ) cancel each other out. The positive sequence harmonics (4th,7th, etc.) cancel each other out. The negative sequence harmonics (2nd, 5th etc.) cancel each other out. But, the zero sequence harmonics (3rd, 9th, 15th, etc.) do not cancel each other out.  Instead they add. Why?

If, for a moment, we consider only the 3rd harmonic (180 Hz) of each phase as they return on the neutral, you will notice in the illustration below that in each positive half-cycle of any of the fundamental waveforms, there are exactly two positive half-cycles and one negative half-cycle of 3rd harmonic. 

The net result, as illustrated above, is that the 3rd-harmonic waveforms of three 120 degree phase-shifted fundamental-frequency waveforms are actually in phase with each other and so stack on one another rather than cancel out as the fundamentals, positive, and negative sequence harmonics do. The phase shift figure of 120 degrees generally assumed in three-phase AC systems applies only to the fundamental frequencies, not to their harmonic multiples. A closer look at the harmonic currents making up the neutral return of our demonstration setup reveals that, though made up primarily of the third harmonics from each phase stacking one on another, the high neutral current also consists of the, 9th, and 15th harmonics from each phase stacking one on another as well.

As is evident in the illustration above, it is an altogether different situation in single-phase systems. Since the phase angle between legs is 180 degrees rather than 120 degrees, 3rd harmonic currents are also out of phase and will for the most part cancel just as the Fundamentals do. This is evident in the power quality meter reading below of the neutral of a single-phase system for first just one Image 85 on one leg (left) and then a 2nd Image 85 on the other leg which is 180 degrees out of phase (right.) As you can see here, when the two legs of a single-phase system are perfectly balanced (9 Amps on each) the triplen currents generated by a non-linear load will nearly cancel out – but not completely.

It is worth noting that the total harmonic distortion (THD) of the single-phase neutral current (98.3%) is as high as the three-phase neutral current (100%) – it just consists of higher order harmonic currents (other than the 3rd) at lower amplitudes.

Guy Holt, Gaffer
ScreeenLight & Grip
www.screenlightandgrip.com

[Greg_Cameron]

Just a thought... Is there any possibility your distro was designed to also service a 3 phase system as well? I'm wondering if the internal wiring is 3 phase capable to where the Z phase is jumped to the X or Y phase causing more juice to always pull more on one leg if you're evenly loading it.

[ProSoundWeb Community]