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[Mike Sokol]

I'm doing a Tune-Up session at a church near Wash DC today, and there's 60 Hz hum everywhere, especially in the video feed to the monitors in the lobby. After service I'm going to dive into this and try for a fix. Of course there's also hum in the recording feed, hum in the monitors, hum in the main speakers, etc.... I'll update later with what I find out, but this is in an industrial park so I'm sure it's tied into a 3-phase power panel. Should be an interesting troubleshooting session.

[Mike Sokol]

Should be an interesting troubleshooting session.

Here's the first hint, a sub-panel in the media room that feeds everything in the church. I can't get a peek inside right now, but it appears to be a Delta High Leg transformer with 480-volts on the H leg but which somehow makes 120-volts for the room power on the other two legs. Is there really such a thing? This is going to get crazy soon.

Oh, the last inspection sticker was from 2000, so this panel hasn't been opened in 14 years. Yikes...!

[Mike Sokol]

Are there any video EE types here who can give a basic primer on how video hum bars work? I understand the concepts of the hum bar rolling frequency caused by the 29.97 video refresh rate beating against 60 Hz power line hum, but I'm not really qualified to discuss details and teach on the subject. So I need a quick brain injection of hum bar details. 

[Kevin Graf]

Not very technical, but for the video problem start at page 166.

An Overview of Audio System
"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

**********************************
As for the "high leg delta" one of several Ham threads by Jim Brown and others.

High Leg Delta - TopBand

    This is just the real world we live in. The power mains try to use the Ham gear grounds as a ground. If I read the neutral to ground bond at my house, I typically have 1 amp or more showing from the mains ground (the primary return is always bonded to the transformer secondary center tap at the pole) trying to ground to my tower grounds and radials.

    The voltage driving that current isn't high, it is caused by the neutral voltage drop back to the substation, but it is a real voltage and current that exists.

No, it is caused by triplen harmonics combining on the neutral (and ground) of 3-phase systems to add rather than cancel. That's the source of power line "buzz" -- 180, 360, 540, 720 Hz. While we don't have 3-phase power in our homes, the power on the street is nearly always 3-phase, and often a very nasty variation of 3-phase called "high leg delta." It's 240V delta, with all three phases fed to light industrial customers, and one of the legs center-tapped to feed single-phase users. Those 3-phase users get no neutral and no ground, so their harmonic current finds its path to ground through OUR neutral. THAT'S the source of our "buzz." And yes, it's not unusual to see an amp or more of that stuff on our neutral. High leg delta is all over mixed residential/light business/industrial neighborhoods of nearly all cities

73, Jim Brown K9YC 28Jan2014
http://lists.contesting.com/_topband/2014-01/msg00306.html

**************************************************************
That's not entirely correct... Triplen harmonics are created by non-linear
loads, but the voltage developed on the neutral as a result of them is a result
of voltage drop between the source of the "problem" and the return which will
be one or more utility transformers. The farther you are from the transformer
the worse the issue will be (higher voltage developed). Triplen harmonics
primarily result in additional heating in the neutral and transformer due to
higher than normal currents (since they don't cancel). Triplen harmonics are
also becoming less of an issue as many electronic loads now incorporate power
factor correction. This used to be a very big deal in data-centers due to older
computer power supplies (which are rectifier loads and very non-linear), but
most new equipment isn't a problem.

Note that "high leg delta", when used to deliver 120/240 single phase power
will *always* have the center tap grounded and  that includes in industrial,
3-phase customer sites. If the center tap isn't grounded then you would have to
have a corner-grounded system, but there is always, by code, a ground bond at
the site. The only impedance-grounded system I'm aware of is the old 4800 volt
primary system but in that case all of the step-down transformers on the poles
also provide transformer isolation between the primary and secondary (since
it's not possible to use autotransformers in that application).

Most of the newer (since the 70s) power distribution used by utilities is a wye
on the primary, and the neutral of that wye is grounded at each pole and the
substation. All customer (both single a three phase) services have their
ground/neutral bonded to that common ground/neutral used by the primary. And
note that from the utility's perspective, on the poles, the neutral and ground
are essentially the same thing (at least in terms of the way the system is
wired).

And our home and ham station grounds, when properly bonded to the service
entrance, WILL share current with the utility system. I see about 7 amps at
home that "comes from" the utility (but I have a ground ring, lots of rods,
probably 500+ feet of copper in the ground and a well casing). It's not a
problem, but it's there.

Bill Wichers 29Jan2014
http://lists.contesting.com/_topband/2014-01/msg00316.html

[Kevin Graf]

A Pro Audio E-mail group thread on High Leg Delta:

High Leg Delta
[ProAud] Grounding practices inside an unbalanced...
Jim Brown jim at audiosystemsgroup.com
Thu Jul 8 10:52:13 PDT 2010
________________________________________
On Mon, 5 Jul 2010 19:02:17 -0400, Tom Maguire wrote:

>My first choice would be to use isolated RCA jacks and bypass them to a mecca style copper strip with .01 monolithic caps and the shortest leads you can muster. Then use the shortest wires you can to connect both signal and ground.

This a direct violation of AES48, and creates a pin 1 problem.

Paul Frindle's approach is right on.  All of the rest of the stuff you propose for RFI is completely un-necessary if you do what he outlines.

On Tue, 06 Jul 2010 01:15:14 +0100, Paul Frindle wrote:

>But the instead of isolating the headphone plug and allowing in all sorts of potential RF, the aim should be to stop the induced current causing a potential difference across the internal circuits, their inputs and outputs. The way to do that is to ensure they are as far as possible referred to the same place and the same potential - and to reduce the possibility of inducing into other connected gear - the same line ground potential as well.

Right on again.

On Wed, 7 Jul 2010 00:24:35 -0700, Christopher Deckard wrote:

> here's a pic of an amp i have on the bench:
>www.audioworksandengineering.com/secret/thegroundedamp.jpg

>all the blue wires are ground wires.  they're "star" grounded to the chassis right above the caps.  the rca jacks are isolated, with a blue wire going to the grounds on them.  the neg speaker terminals are isolated from chassis, connected via blue wires to the star node.interestingly, the U ground (or earth ground) on the IEC connector is not connected.  i looked for a dangling wire, but couldn't find one.

>the drive boards have two ground connections -- one for the low current side (close to the inputs), and one at the high current side (close to the outputs).

What a mess! Give this abortion the bucket treatment! (Fill a bucket with water. Put the thing in twice. Take it out once.)

> I wonder if the origin of the buzz is high induced current in the "ground plane" from the power transformer?

BUZZ is nearly always the result of leakage current from the AC power (mains) system. It can be identified as such because there is very little 50 or 60 Hz, but mostly harmonics. Those harmonics occur because mains current flows in pulses that recharge the power supply input filter cap, which causes IR drop to consist of harmonics of 50/60 Hz, causes the mains voltage to be a distorted sine wave (more IR drop at the top of the waveform). LOTS of equipment all over the building produces this leakage current, and more equipment OUTSIDE the building produces this leakage current.

In 3-phase systems, triplen harmonics ADD in the neutral rather than cancelling. This is well known. What is NOT well understood is that they also add in the ground! How does this affect us? In most of the world, the power system is a 3-phase system, and earth currents from that 3-phase systems are flowing in places where we don't expect them. An especially nasty power system connection, called High Leg Delta, uses 240V Delta 3-phase to feed single-phase power to residences and 3-phase power to businesses on the same street. That system uses a center-tapped transformer (at the street) to feed single phase, so WE get two legs of that 3-phase power plus a neutral. That neutral carries all the ground current, much of it harmonics, from the 3-phase system!

Neil Muncy first taught me about the mess that High Leg Delta could make in an audio studio. He was troubleshooting a buzz problem in an urban studio that he diagnosed as ground currents from that High Leg Delta system going to ground via a pipe that ran under the studio, almost directly under the guitar booth. The current created a huge magnetic field that got into everything.

I live in a somewhat remote area of the Santa Cruz Mountains, and have single phase 120/240 power, but it's fed from a High Leg Delta system. There's a lot of buzz current on the neutral that feeds me. I've had to be quite careful about grounding of the power system to avoid buzz problems.

Getting back to the buzz in the power amp, I'd bet that somehow, the current charging that filter cap had a path that includes the signal return for a gain stage. In other words, lousy circuit board (or chassis) layout -- what Henry Ott calls the invisible schematic hiding behind the ground symbol. That cap SHOULD have a return that goes straight back to the rectifier network with minimal inductance and resistance, not to some random point on the 0V trace.

Jim Brown

________________________________________
Paul Frindle paul at pafrindle.free-online.co.uk
Thu Jul 8 17:10:06 PDT 2010
________________________________________
Jim Brown wrote:
> In 3-phase systems, triplen harmonics ADD in the neutral rather than  cancelling. This is well known. What is NOT well understood is that they also add in the ground! How does this affect us? In most of the world, the power system is a 3-phase system, and earth currents from that 3-phase  systems are flowing in places where we don't expect them. An especially  nasty power system connection, called High Leg Delta, uses 240V Delta 3- phase to feed single-phase power to residences and 3-phase power to businesses on the same street. That system uses a center-tapped transformer (at the street) to feed single phase, so WE get two legs of that 3-phase power plus a neutral. That neutral carries all the ground current, much of it harmonics, from the 3-phase system!
>
>   

What is also not realised is that 3 phase cable also has a net circular field proportional to the current passing - which cannot be cancelled by the neutral. This can be so strong (in the case of heavy machinery like feeding large motors) that it heats the switch box enclosures with eddy currents and saturated steel where the cable passes into and out of it.
It can also vibrate ferrous objects a metre or more away from the cables. I have actually seen this happening when I worked in industrial control systems. Amazing to witness.. Of course, all this is a major overall loss in nation wide AC grid systems.....

But the worst case in an audio studio I have ever seen was in Baden Baden in Germany in the early 80's where I was called to diagnose an SSL that was humming after a TV studio replaced their old console with it. I found that the AC field in the sound control room was massive - so massive in fact that when they put up the TV studio lighting the metalwork in the control room actually vibrated making acoustically audible buzzing and rattling and making the TV monitor picture wiggle amusingly - I have never seen anything like it outside of a factory full of 100HP+ motors! Of course all this was making the mix busses buzz away in the SSL.

After much discussion it transpired that the mains substation for the whole complex was in the basement directly under the audio control room!! Amazing and astonished, I exclaimed, "you couldn't make this crap up - obviously that's the cause of the trouble". We were eventually taken downstairs and sure enough there was collection of 3 massive transformers each large enough feed a medium sized town, humming away at 150Hz like you might expect. But when we got hold of a magnetometer to prove the problem (yes they could rustle stuff like this up in an instant from their tech dept!), incredibly I found that there was virtually no field from these transformers at all - ALL the magnetic field was from the HV 3 phase cable that ran into the substation. You didn't need a magnetometer (or the telephone pick up coil I used to carry around), just holding a screwdriver near it was enough to feel the vibration. And of course when 100s of KWs of thyristor controlled lighting ran up in the studio the waveform of the current and it's field was even more damaging - and rattled stuff even more.

Now I remember having this discussion with them, (during one of the tense morning meetings we had at the start of every day - they would stand one side of the console and us the other, as though in battle) when they claimed quite 'logically' that it couldn't be the cable because (obviously) whatever current went down the live wires must return via the same route - so they must cancel!! They didn't understand 3 phase theory at all - of course they didn't - why would they? I'm not sure I convinced them either - but we had to crack on and fix it anyway despite the disagreement - because there was no chance they would ever get the cable moved or additionally shielded based on advice from an unqualified scruff like me...

So we flew over a team of assembly people from the factory and modified all the busses in situ to make them quasi-balanced, by hand, by using the interleaved ground buss tracks as bucking references. We worked all hours for about a week with me making up the circuit modification by the 'seat of my pants'. Luckily it worked - the only noise after we had finished wasn't from the busses and monitors any more, but from the rattling metal in the control room which had always been there...

It's amazing what you can pull out of the bag when you're up against it like this..
________________________________________
Jim Brown jim at audiosystemsgroup.com
Thu Jul 15 11:16:14 PDT 2010
________________________________________
On Sat, 10 Jul 2010 20:08:35 -0400, Bob Katz wrote:

>I had the toughest problem convincing the chief engineer that I could not use any dynamic microphones as the induced hum/buzz was literally electromagnetic field coupling into the microphone's audio transformers.... coming from the lighting grid and lighting power in the studio and perhaps the lighting power transformer in the basement.

An important correction. It was the MAGNETIC field. The field produced by passing current through a wire is a MAGNETIC field. An ELECTROMAGNETIC field is a radio wave, which consists of a magnetic field and an electric field at right angles to each other.

Many years ago, I had a gig both maintaining and operating a live music reinforcement system in the plaza of the First National Bank building in the middle of the Chicago loop. There were multiple challenges. First, all the TV and FM transmitters licensed to Chicago were within a mile or so. Second, the stage was directly over the main transformer vault for the 57-story building.

The system (specified by others long before I was hired) included mostly Neumann mics, probably to avoid hum coupled into dynamic mics. Rectified video sounds like buzz, and both the Langevin mix desk and the U87s were somewhat susceptible to the VHF RFI. The headphone amp was particularly susceptible. It was a challenging gig.

FWIW, dynamic mics with hum-bucking coils, like the MD421, D224, RE16 and RE20, ARE pretty immune to strong magnetic fields like this.

Jim Brown

________________________________________

[Kevin Graf]

Another Jim Brown post:

High Leg Delta

BUZZ is nearly always the result of leakage current from the AC power
(mains) system. It can be identified as such because there is very little
50 or 60 Hz, but mostly harmonics. Those harmonics occur because mains
current flows in pulses that recharge the power supply input filter cap,
which causes IR drop to consist of harmonics of 50/60 Hz, causes the mains
voltage to be a distorted sine wave (more IR drop at the top of the
waveform). LOTS of equipment all over the building produces this leakage
current, and more equipment OUTSIDE the building produces this leakage
current.

In 3-phase systems, triplen harmonics ADD in the neutral rather than
cancelling. This is well known. What is NOT well understood is that they
also add in the ground! How does this affect us? In most of the world, the
power system is a 3-phase system, and earth currents from that 3-phase
systems are flowing in places where we don't expect them. An especially
nasty power system connection, called High Leg Delta, uses 240V Delta 3-
phase to feed single-phase power to residences and 3-phase power to
businesses on the same street. That system uses a center-tapped transformer
(at the street) to feed single phase, so WE get two legs of that 3-phase
power plus a neutral. That neutral carries all the ground current, much of
it harmonics, from the 3-phase system!

Neil Muncy first taught me about the mess that High Leg Delta could make in
an audio studio. He was troubleshooting a buzz problem in an urban studio
that he diagnosed as ground currents from that High Leg Delta system going
to ground via a pipe that ran under the studio, almost directly under the
guitar booth. The current created a huge magnetic field that got into
everything.

I live in a somewhat remote area of the Santa Cruz Mountains, and have
single phase 120/240 power, but it's fed from a High Leg Delta system.
There's a lot of buzz current on the neutral that feeds me. I've had to be
quite careful about grounding of the power system to avoid buzz problems.

[Stephen Swaffer]

Mike,

The manufacturers tag on that panel looks to me like a standard 480 volt 3 phase panel with a neutral.  I would expect to be able to get either 480 or 277 volts from it.  I would take the sharpie mark to mean either the panel number (High voltage number 2???) or it is possible it is a 480 volt version  of the 240 volt with a high leg panels I am used to.  In that case H2 to neutral would be 480 volts.  That would make me very cautious as usually even when working wih 480 VAC systems you never see more than 277 volts to ground-480 volts to ground could bite really bad.

If it IS supplying 120 volt loads, is it simply a repurposed panel that is a 208/120 V panel?  Markings don't always get fixed, unfortunately.

[Mike Sokol]

Mike,

The manufacturers tag on that panel looks to me like a standard 480 volt 3 phase panel with a neutral.  I would expect to be able to get either 480 or 277 volts from it.  I would take the sharpie mark to mean either the panel number (High voltage number 2???) or it is possible it is a 480 volt version  of the 240 volt with a high leg panels I am used to.  In that case H2 to neutral would be 480 volts.  That would make me very cautious as usually even when working wih 480 VAC systems you never see more than 277 volts to ground-480 volts to ground could bite really bad.

If it IS supplying 120 volt loads, is it simply a repurposed panel that is a 208/120 V panel?  Markings don't always get fixed, unfortunately.

I didn't have time to open the panel since there were so many other non-hum problems to solve, and to be honest I was a bit worried about opening up something so old and causing an arc-flash. I didn't have any of my PPE gear with me, and it's way too easy for one of those antique circuit breakers to do something nasty if you jiggle them around. I just have no clue as to what other loads may be in that box so it's best to let sleeping dogs lie, at least for now.

I did stop a bunch of the audio hum simply by flipping the ground lift switches on their WW IMP-2 DI boxes. Yes, they had the proper DI boxes, but no understanding of what the ground lift switch was for. Also, the gain structure was really goofed with lots of abnormal boosts and cuts in various stages and nearly ALL of the 1/3 octave equalizer bands pulled down to -12 or so. Of course, every boost followed by a cut only makes hum and hiss problems worse. So after a few hours of resetting everything from scratch the hum level is sufficiently low for these guys.

However, I didn't make any headway with the video hum except to see that it's on one of the video feeds coming into their little Edirol switcher, not between the video distro and the monitor in the lobby. Since there's a rats-nest of wires behind the desk it's going to take some serious untangling to figure out just where everything is connected. So that's a troubleshooting job for another day.

But all the posts above do confirm my suspicions about high-leg delta panels and A-V interference. I'm seeing a lot of churches built into older industrial parks, and many of them have HLD power. More to study...

[John Roberts {JR}]

Are there any video EE types here who can give a basic primer on how video hum bars work? I understand the concepts of the hum bar rolling frequency caused by the 29.97 video refresh rate beating against 60 Hz power line hum, but I'm not really qualified to discuss details and teach on the subject. So I need a quick brain injection of hum bar details.

AFAIK the hum bars are a visible artifact of video signal signal contamination just like audible hum is contamination of the audio signal. Get the 60 Hz out of the signal, just like audio...(transformer, lead dress, perhaps a neutral current flowing in a ground for unbalanced interfaces).

Good Luck..

JR

[Taylor Hall]

I gotta say, as rudimentary as my knowledge is about electricity, this section of the forum continues to fascinate me even though 90% of the terminology flies over my head. This is one area of simple ignorance I have a growing interest to rectify.

[ProSoundWeb Community]