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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Mike Sokol on February 10, 2016, 11:25:13 AM

Title: 2-phase hum?
Post by: Mike Sokol on February 10, 2016, 11:25:13 AM
I just read this article posted on PSW about chasing hum in a church sound system. http://www.prosoundweb.com/article/accidental_electrician_eliminating_dreaded_sound_system_hum_buzz/

While much of it was a reasonable tutorial on troubleshooting hum, one particular paragraph seems to be incorrect.

"So the original system installer (OI) had specified that the circuits needed to be all on the same phase. My guess is that some well-meaning electrician had thought it would be smart to group the circuits together during one of the church expansion projects. I returned the circuits to their original spacing.

Having all the power circuits on an identical phase is important for minimizing the possibility of an inter-chassis current in a ground loop.

Power supplies leak small amounts of AC to ground. If there happens to be enough leakage from gear in the ground loop, and its of differing phases, an inter-chassis current will flow through the loop great enough to induce a hum.

By establishing a common system power phase I minimized the ground loop hum potential."

Now I've heard this idea that connecting a sound system across two different phase legs can cause ground loop hum, and it appears to be common industry "knowledge", at least with some of the sound tech's I've talked to over the years. However, I think it's an "old wives tale" that's simply incorrect.

I've seen where connecting a sound system so that it bridges two different sub-panels causing a lot of ground loop hum. But that's because the sub-panel grounds are at two different voltage potentials and the equalization current can be huge (many amperes).

So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?
Title: Re: 2-phase hum?
Post by: Mac Kerr on February 10, 2016, 11:43:56 AM
So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?

My personal experience is mostly with 3 power, and I make an effort to balance the load across the phases to minimize neutral current, but I have not had any notable experiences of hum caused by multi phase power.

Mac
Title: Re: 2-phase hum?
Post by: Tom Bourke on February 10, 2016, 12:12:19 PM
If a properly wired split phase or 3 phase system causes hum then the problem is in the connected gear.  The only exception, as already noted, is spanning several panels or services in a building.
Title: Re: 2-phase hum?
Post by: John Roberts {JR} on February 10, 2016, 12:15:39 PM
"Ground loop" (?) hum potential.  If a loop exists that converts a magnetic flux into voltage, it seems drawing  power from both polarities (same phase) could in theory create opposing magnetic fluxes, so logically could reduce ground loop hum not increase it.

"Ground Loops" are used as a catch all phrase for ground potential differences.

JR 
Title: Re: 2-phase hum?
Post by: Chris Hindle on February 10, 2016, 12:44:16 PM

So what's been your observations? Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?

Like Mac, Whether 3 phase, 2 legs of 3, or split 220/110 never a problem for me. I also try to balance the loads.
Chris.
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 10, 2016, 12:57:09 PM
Here's where ground loop hum comes from...  ;D

Gear that has the "pin-1" problem will hum if you have current coming in the (pin-1) shield. My bench testing confirms that there can easily be around 1 amp of current per volt of ground potential difference. Since I've found that even 100 mA of ground loop current can make many self-powered speakers hum significantly, that suggests it only takes 1/10 of a volt ground difference to make a sound system hum.

I like to use a clamp-ammeter right around the XLR cables to look for this current. As you can see from the diagram, since the return current is outside of the jaws of the clamp-meter, there's no need to split out the twisted pair from the shield. Just clamp around the cable and you'll see if there's any ground loop current in the shield.

Since there's really no way that connecting across gear across two different phases can cause this current path, I don't see how it can contribute to hum. And you can also see that normal (sub-mA) power supply leakage currents can't be anywhere near the 100mA hum threshold I've noticed on the bench. 
Title: Re: 2-phase hum?
Post by: John Roberts {JR} on February 10, 2016, 01:52:34 PM
Perhaps I'm being overly pedantic about the terminology but "ground loops" are a real thing, actual physical conductive path loops typically inside products, even on circuit boards that convert magnetic energy present to voltage like a one turn transformer winding (a real issue inside power amps with huge transformer magnetic fields).

As others have shared these are generally product design (pin 1 problems) issues, while large fault currents flowing in and between sundry grounds can cause problems.

UL specifies that equipment safety grounds can sink tens of amps with only modest voltage rise. OTOH tens of amps in mic cable shields will likely release some smoke (as you know and have seen). The magnetic field to generate that much current in a wire loop would be significant. I expect that much ground current is coming from other sources.

JR
Title: Re: 2-phase hum?
Post by: Stephen Swaffer on February 10, 2016, 02:15:15 PM
Our church sound system is installed on a 3 phase system-3 phases sharing a common neutral (probably would do it different today-but have seen no reason to change it).  I have never run into an issue and quite honestly, really don't concern myself too much with what phase we are on as we don't use subs and really not pushing the circuits.  The only issue I have ever fought has been with a ground loop caused by a projector connected to another panel (most subpanels in this building are not wired correctly).

Leakage to ground from different phases would tend to cancel-leakage from the same phase would add thus making the problem worse.  Maybe you just keep changing wires around willy nilly until it sounds better?   
Title: Re: 2-phase hum?
Post by: David Buckley on February 10, 2016, 02:37:55 PM
Has anyone ever proved the allegation that ground loop hum can be caused by connecting a sound system so that it bridges 2 different legs of 3-phase power?
This is utter bollocks, so no-one ever will.  Ground loop hum is caused by, well, ground, more specifically, potential difference across what is supposed to be a single ground.  The hots and the neutral can come from anywhere, but if the ground isn't common, well...

The other noteworthy thing is that the first act that happens to power of any phase in a sound system is it gets converted to DC.
Title: Re: 2-phase hum?
Post by: Stephen Kirby on February 11, 2016, 02:05:04 PM
My first thought is that everyone running a small California plug distro would immediately have such a problem.  But the neutral to both legs as well as the ground to both legs is at the same potential in the distro since each is on a single wire.  Any neutral to ground current will be common to both legs.  Using only one side of the distro (phase, if you want to misuse that term) kind of cripples why you have it in the first place.
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 11, 2016, 03:46:53 PM
Using only one side of the distro (phase, if you want to misuse that term) kind of cripples why you have it in the first place.

Not to throw gas on the fire, but what should we call the two sides of the distro, if not phase?
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 11, 2016, 03:59:11 PM
I just read this article posted on PSW about chasing hum in a church sound system. http://www.prosoundweb.com/article/accidental_electrician_eliminating_dreaded_sound_system_hum_buzz/

So I sent an email with our concerns to the editor of PSW, and he's pulled the article due to inaccuracy. That's one reason I like e-zines rather than paper magazines. We can easily correct mistakes.

Now it looks like I get to write a primer on hum and buzz issues. Don't worry, I won't say "phase" except when referring to 3-phase power.  ;D
Title: Re: 2-phase hum?
Post by: Jonathan Johnson on February 11, 2016, 04:51:40 PM
Not to throw gas on the fire, but what should we call the two sides of the distro, if not phase?

Poles.

It could be a 120/240V single-phase, double-pole system with a center-tapped secondary with the center tap bonded to ground. You have two hot/live/line poles, and one neutral pole.

It could be a 120/208V three-phase, triple-pole system with a common-neutral secondary (common bonded to ground). You have three hot/live/line poles and one neutral pole.

In wiring in the United States, the hot/live/line poles are often referred to as line conductors. You may notice that wiring devices are often labeled L1, L2, (L3,) N, G. That's three "LINE" conductors (which are live), one NEUTRAL conductor, and one GROUND conductor. Sometimes instead of L1, L2, (L3,) N, G you have X, Y, (Z,) W, G -- X, Y, and Z are the line conductors, W is the neutral conductor (W for WHITE), and G is the ground (G for "green" probably). Of course, L3/Z are only used with 3-phase power.
Title: Re: 2-phase hum?
Post by: John Roberts {JR} on February 11, 2016, 04:54:43 PM
Poles.
+1 polarity.... 

as if phase isn't complicated and misused enough in audio, power distribution gives it even more nuanced meanings.

JR
Title: Re: 2-phase hum?
Post by: Jonathan Johnson on February 11, 2016, 05:03:53 PM
"So the original system installer (OI) had specified that the circuits needed to be all on the same phase. My guess is that some well-meaning electrician had thought it would be smart to group the circuits together during one of the church expansion projects. I returned the circuits to their original spacing.

Having all the power circuits on an identical phase is important for minimizing the possibility of an inter-chassis current in a ground loop.

This portion you quoted indicates that the troubleshooter changed the configuration of the POWER wires to fix a GROUND LOOP problem. Did they change anything with the ground conductors?

That's a bit like switching brands of gasoline to fix a flat tire.
Title: Re: 2-phase hum?
Post by: Jerome Malsack on February 11, 2016, 05:47:00 PM
All true on the polarity and will it make a difference.  However Mike if you put a computer UPS system in front of the digital mixer you have the added complexity. 
Especially when the S16 and Behringer X32.  If the S16 is connected to the stage power and not back to the UPS then what will this do when the power is pulled down enough to cause the UPS to switch onto the battery ?  Now the S16 is at a different power source than the X32.  Add in the Ethernet cable is shielded ? 
Title: Re: 2-phase hum?
Post by: John Roberts {JR} on February 11, 2016, 06:37:20 PM
All true on the polarity and will it make a difference.  However Mike if you put a computer UPS system in front of the digital mixer you have the added complexity. 
Especially when the S16 and Behringer X32.  If the S16 is connected to the stage power and not back to the UPS then what will this do when the power is pulled down enough to cause the UPS to switch onto the battery ?  Now the S16 is at a different power source than the X32.  Add in the Ethernet cable is shielded ?

Which brings us back around to equipment design... The more bits we try to send down network cables the easier it is to confuse it. I like the concept of running equipment off the same power drop because they all see the exact same ground corruption, but there is no theory to support this, just practice.

JR   
Title: Re: 2-phase hum?
Post by: Rob Spence on February 11, 2016, 06:55:49 PM
Which brings us back around to equipment design... The more bits we try to send down network cables the easier it is to confuse it. I like the concept of running equipment off the same power drop because they all see the exact same ground corruption, but there is no theory to support this, just practice.

JR

I like digital mixers that use UTP cable to connect to the stage. Well designed category cable interfaces that are static resistant mean there is no ground connection through the snake cable to contribute to problems.

I agree with others that is simply good practice to bring power to foh so all are on the same ground. However,  much of the entry level gear is used without following this practice and having a ground in the connection to the stage box creates problems.



Sent from my iPad using Tapatalk HD
Title: Re: 2-phase hum?
Post by: Stephen Swaffer on February 11, 2016, 11:17:13 PM
Poles.

It could be a 120/240V single-phase, double-pole system with a center-tapped secondary with the center tap bonded to ground. You have two hot/live/line poles, and one neutral pole.

It could be a 120/208V three-phase, triple-pole system with a common-neutral secondary (common bonded to ground). You have three hot/live/line poles and one neutral pole.


I would agree that this terminology seems to be correct and fit with code usage.

Practically speaking, in most cases, using "Phase A & B or Phases A, B & C" is the common language.  Most people when the ask for a "coke" are content with whatever cola is available-we use dozens of common nicknames on a daily basis-how many if us actually answer to our given names (unless its mom's voice)?  When troubleshooting industrial controls we use the term "contact" to refer to a physical contact or a logical bit without explanation, the potential ramifications are understood. I feel like, if people are working at a level that it really matters, they need to take the time to understand. Perhaps a "white paper" explaining the situation is in order-but it you go asking a the average electrician what "pole" this circuit is connected to, he is going to be trying to figure which structural pole the conduit containing that circuit is attached to.
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 12, 2016, 08:11:20 AM
+1 polarity.... 

But I see this term mis-used in the RV industry all the time when swapped Hot and Neutral wires are referred to as "reverse polarity". Of course, AC power in the US swaps polarity 120 times a second (60 Hz). Sadly, many so called RV "experts" think it causes an RV hot-skin voltage, which isn't true.

So what do we call swapped Hot and Neutral wires?
Title: Re: 2-phase hum?
Post by: Chris Hindle on February 12, 2016, 08:22:18 AM
Not to throw gas on the fire, but what should we call the two sides of the distro, if not phase?
I call it a Leg. Red Leg, or Black Leg.
Chris.
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 12, 2016, 08:24:11 AM
My first thought is that everyone running a small California plug distro would immediately have such a problem.  But the neutral to both legs as well as the ground to both legs is at the same potential in the distro since each is on a single wire.  Any neutral to ground current will be common to both legs.  Using only one side of the distro (phase, if you want to misuse that term) kind of cripples why you have it in the first place.

Since we understand that ground loop hum in a system is typically caused by ground loop currents in the shields, we can predict the types of situations that can cause these currents to form in the first place. And as I've demonstrated many times, it's pretty simple to measure these shield currents with a clamp-on ammeter. 
Title: Re: 2-phase hum?
Post by: Chris Hindle on February 12, 2016, 08:26:52 AM
So what do we call swapped Hot and Neutral wires?

"Potentially Lethal"

Chris.
Title: Re: 2-phase hum?
Post by: Stephen Swaffer on February 12, 2016, 06:36:21 PM
But I see this term mis-used in the RV industry all the time when swapped Hot and Neutral wires are referred to as "reverse polarity". Of course, AC power in the US swaps polarity 120 times a second (60 Hz). Sadly, many so called RV "experts" think it causes an RV hot-skin voltage, which isn't true.

So what do we call swapped Hot and Neutral wires?

Well, if you follow NEC usage you call it reversed polarity. 410.50 talks about "polarization of lumiaires"

"200.11 Polarity of Connections  No grounded conductor shall be attached to any terminal or lead so as to reverse the designated polarity."

Who opened this can o'worms?
Title: Re: 2-phase hum?
Post by: John Roberts {JR} on February 12, 2016, 07:30:22 PM
Well, if you follow NEC usage you call it reversed polarity. 410.50 talks about "polarization of lumiaires"

"200.11 Polarity of Connections  No grounded conductor shall be attached to any terminal or lead so as to reverse the designated polarity."

Who opened this can o'worms?
I hate the rather confusing use of english words to vaguely suggest some real word. I think (hope?) "grounded" conductor means neutral.  The real problem is where the "ungrounded" (hot, line, whatever) conductor gets connected.

Neutral (grounded) should be low voltage so relatively harmless. >:(

JR
Title: Re: 2-phase hum?
Post by: Stephen Swaffer on February 12, 2016, 07:42:18 PM
True, but assuming you have the same number of wires as terminals and attach one wire to each terminal, if the grounded conductor is on the wrong terminal, then it impossible to put the ungrounded conductor on the correct terminal.
Title: Re: 2-phase hum?
Post by: Mike Sokol on February 12, 2016, 08:39:47 PM
Who opened this can o'worms?

Ummmmmmm...... :)
Title: Re: 2-phase hum?
Post by: David Buckley on February 12, 2016, 09:17:12 PM
Ummmmmmm...... :)

Typical.  Misuse of a silent "h".
Title: Re: 2-phase hum?
Post by: Lyle Williams on February 12, 2016, 10:57:29 PM
Which brings us back around to equipment design... The more bits we try to send down network cables the easier it is to confuse it. I like the concept of running equipment off the same power drop because they all see the exact same ground corruption, but there is no theory to support this, just practice.

JR

People get bent out of shape insisting on the highest quality source of power.  They must have the most pristine 115v, or 12v, or whatever.

But voltage is all relative.  You need a fabulous source of "0v" to begin with.

Selling "zero volt supplies" does sound like a scam, doesn't it?

Title: Re: 2-phase hum?
Post by: Mike Sokol on February 12, 2016, 11:53:33 PM
Typical.  Misuse of a silent "h".

hmmmmmmmmmmmm.....?  :)