ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: 2 volts too much?  (Read 862 times)

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3055
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: 2 volts too much?
« Reply #10 on: November 14, 2017, 04:34:08 pm »

That 0 volts is because you've now contaminated the ground with the voltage fluctuations on the neutral due to load variations. And THAT'S what causes ground loop hums.
And that's what can also cause GLID (Ground Loop Intermodulation Distortion). None of you here believe that's a real thing, but I'm going to set up a demonstration to prove it.  ;)

Jonathan Johnson

  • Hero Member
  • *****
  • Online Online
  • Posts: 2628
  • Southwest Washington (state, not DC)
Re: 2 volts too much?
« Reply #11 on: November 20, 2017, 01:49:51 am »

I drew this graphic up a while ago to help me better understand the various voltage drops in a typical power distro system, and what certain measurements could possibly mean. I try to go into every voltage or current test situation with at least a guess of what measurement to expect. If the voltage measured doesn't line up with my expectations, then there's a hint as to what's wrong. For instance, as you load a typical home branch circuit with more current draw and approach 20 amperes, you'll likely come close to a 5% voltage drop. So 120 volts drops by 6 volts to 114 volts. But only 3 of those volts are on the hot/line conductor. The neutral conductor does an equal and opposite "rise" in voltage, so it will create a voltage difference if you measure between it and the grounding conductor. This basic knowledge is essential to troubleshooting any grounding system. That's because all wires have resistance. And resistance always results in a voltage drop if there's any current draw. It's a fundamental law of electricity.

Let me see if I can simplify it.

Measuring between hot and neutral, you are measuring against a loaded neutral. The load on the neutral causes a voltage drop.

Measuring between hot and ground, you are measuring against an unloaded ground. There is no voltage drop on the ground.

Voltage drop is a function of wire resistance and load. Where there is no load, wire resistance is mostly irrelevant for a high impedance voltmeter. The voltage drop on the hot wire is irrelevant in this comparative analysis; it doesn't make a difference in the math because we are not measuring the voltage differential between the beginning and end of the hot wire. (But that is what we are effectively doing with the neutral wire.)

The difference between the hot-neutral voltage and the hot-ground voltage should equal the neutral-ground voltage reading. The neutral-ground reading is equal to the voltage drop on the neutral.
« Last Edit: November 20, 2017, 01:57:48 am by Jonathan Johnson »
Logged
Stop confusing the issue with facts and logic!

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3055
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: 2 volts too much?
« Reply #12 on: November 20, 2017, 08:17:37 am »

The difference between the hot-neutral voltage and the hot-ground voltage should equal the neutral-ground voltage reading. The neutral-ground reading is equal to the voltage drop on the neutral.

Exactly. And that's why you don't want to double-bond the ground wires to the neutral at the subpanel, or bootleg ground any receptacle. Because if there's significant current in the neutral, which causes a voltage drop across it's length, then the ground wire will be sharing this current and participate in the voltage drop. You can easily see this if you run a long test wire between two receptacle grounds. If both grounds are properly bonded only at the service panel, and properly isolated from neutral, and not accidentally bonded to building steel, then you should measure very close (within a few mV) to 0 volts between them, no matter what the load. However, if one of them has been improperly connected to neutral or building steel, then you can watch the ground voltage difference between them move up and down as you load one circuit or the other.

I've fought ground loop hum that would come and go, depending on loads like kitchen appliances being turned on in a church. For example, when the big coffee urn starts pulling 15 amps, there's likely a 4 or 5 volt drop in its branch circuit with 120volts from Hot to Neutral dropping to maybe 115 volts. If the ground in the sub-panel or branch circuit is contaminated with a connection to the neutral, now the receptacle you plugged your mixer into at the back of the room (and connected to the kitchen branch circuit), can have its ground wire potential shift by 2 volts or more as the loads change. Since your stage power is probably on another circuit and maybe even a different sub-panel, you can get a ground-loop hum that comes and goes as the coffee pot thermostat cycles on and off. I had this exact scenario happen once which drove me crazy for hours until I figured it out.
« Last Edit: November 20, 2017, 08:23:56 am by Mike Sokol »
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Online Online
  • Posts: 1855
Re: 2 volts too much?
« Reply #13 on: November 20, 2017, 12:55:02 pm »

If both grounds are properly bonded only at the service panel, and properly isolated from neutral, and not accidentally bonded to building steel, then you should measure very close (within a few mV) to 0 volts between them, no matter what the load. However, if one of them has been improperly connected to neutral or building steel, then you can watch the ground voltage difference between them move up and down as you load one circuit or the other.


If you are dealing with a steel frame bulding and a diligent inspector, you will almost never find a ground that is only connected at the panel.  J-boxes are generally attached to steel and code requires them to be bonded to any grounding conductor in them.  As a matter of technicality, usually most red iron in a building is part of the grounding electrode and as such it is an acceptable ground.  If there is current in the grounding wire/building that is causing a voltage gradient that is causing trouble, then by definition that is objectionable current and a code violation.

Theory, code and real world are often at odds, though!

The neutral of course, is another story.
« Last Edit: November 20, 2017, 10:04:31 pm by Stephen Swaffer »
Logged
Steve Swaffer

Jonathan Johnson

  • Hero Member
  • *****
  • Online Online
  • Posts: 2628
  • Southwest Washington (state, not DC)
Re: 2 volts too much?
« Reply #14 on: November 20, 2017, 03:17:48 pm »

If you are dealing with a steel frame bulding and a diligent inspector, you will almost never find a ground that is only connected at the panel.  J-boxes are generally attached to steel and code requires them to be bonded to any grounding conductor in them.  As a matter of tecnicailty, usually most red iron in a building is part of the grounding electrode and as such it is an acceptable ground.  If there is current in the grounding wire/building that is causing a voltage gradient that is causing trouble, then by definition that is objectionable current and a code violation.

Theory, code and real world are often at odds, though!

The neutral of course, is another story.

I don't think multiple bonds of grounding wires (think of a mesh) is a problem. After all, the shields on the cable between the mixer and the amplifiers are typically grounded on both ends to the equipment chassis, which is in turn connected to the ground wire of their respective power cords, which may be on different circuits and the ground wires are connected back in the panel. That's at least two potential ground paths. And that's quite normal, and not a problem as long as there's no current or voltage potential on the ground wire.

The problem is when ground and neutral are bonded at multiple points. That causes currents to flow on the ground wire. If the current path happens to include the shield of a link between two pieces of audio equipment, you have ground current induced hum. The neutral and ground should be bonded at one point only.
« Last Edit: November 20, 2017, 03:20:14 pm by Jonathan Johnson »
Logged
Stop confusing the issue with facts and logic!

Mark Wilkinson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 520
Re: 2 volts too much?
« Reply #15 on: November 20, 2017, 07:00:21 pm »

.................

The problem is when ground and neutral are bonded at multiple points. That causes currents to flow on the ground wire. If the current path happens to include the shield of a link between two pieces of audio equipment, you have ground current induced hum. The neutral and ground should be bonded at one point only.

Thank you guys, between Mike and you, this has become very clear.
Logged

Bill McIntosh

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 355
  • Louisville KY
Re: 2 volts too much?
« Reply #16 on: November 20, 2017, 08:24:47 pm »

Thank you guys, between Mike and you, this has become very clear.

Thanks from me too. 

Also I want to clarify that the tour tech did not make a fuss, just showed the meter readings to our electrical contractor.  They discussed, tour tech was happy and the show went as scheduled with no audible issues.
Logged
Opinions are like belly buttons.  Everybody has one, but not all are suitable for public display.
Pages: 1 [2]  All   Go Up
 


Page created in 0.09 seconds with 17 queries.