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Author Topic: Grounding to the neutral bar?  (Read 20657 times)

Steve M Smith

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Re: Grounding to the neutral bar?
« Reply #10 on: September 19, 2013, 01:34:25 pm »

we can't on a public forum condone what you're doing, no matter how many times you've done it before without apparent issue.

And no matter how many time we have done it ourselves!


Steve.
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Mike Sokol

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Re: Grounding to the neutral bar?
« Reply #11 on: September 19, 2013, 01:59:13 pm »

And no matter how many time we have done it ourselves!

Steve.

Everybody chill out a bit. Yes, I also did crazy power connections in the 70's, but I actually went and got my Master's Electricians License in 1978 just so I could flash my card and have access to the power panels. And nowadays liability issues are much greater. If  you can talk the club into installing a simple 50-amp/240-volt 4-wire stove outlet near the panel, you should be able to run most anything you could want from your own distro. And yes, go ahead and meter anything an electrician installs for you, especially in older converted industrial building with 3-phase power. There's something called High-leg delta wiring which is VERY dangerous to connect into. Here's a primer which we can discuss later: http://en.wikipedia.org/wiki/High-leg_delta

As far as looking inside a service panel for a separate or combined Ground-Neutral bus bar, I'm going to say that's beyond the scope of this forum for now. The real problem isn't showing you how everything is SUPPOSED to be wired, but rather how it's ACTUALLY wired. The wiring code has changed a lot over the years, especially in building constructed pre-1970s. So much so that even with my 40-years experience in electrical power I'm sometimes confused trying to figure out exactly what some electrician did to the wiring 30 years ago that could cause a problem today.

At some point I'm going to write an article about panel and sub-panel grounding vs. bonding that should help you all understand this better. But in the meantime, don't tie into any panels unless you're licensed to do so, and let's stay safe out there.... 
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Jamin Lynch

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Re: Grounding to the neutral bar?
« Reply #12 on: September 19, 2013, 01:59:26 pm »

And no matter how many time we have done it ourselves!


Steve.

Whoa! Let's back up. I'm not asking anybody to condone or approve of anything.

Of coarse if a breaker panel looks unsafe, I'll not mess with it. If I burn the place down, I loose my equipment along with it. Everybody looses.

I'm merely asking when inserting my own set of breakers into a panel to connect a distro, is it generally OK to attach the ground wire to the neutral bar when connecting the distro to a main panel? I believe the ground bar and neutral bar are usually bonded together. If it's a sub panel it will most likely have a ground bar and I assume attaching the neutral wire to the neutral bar and the ground wire to the ground bar is probably correct. If there is no ground bar in a sub panel then what would be best practice? Is there something other than "don't use it" that would be a safe option?
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Steve M Smith

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Re: Grounding to the neutral bar?
« Reply #13 on: September 19, 2013, 03:32:04 pm »

I wasn't suggesting we condone anything but I'm sure many of us have done things like this which we shouldn't have done.


Steve.
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Greg_Cameron

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Re: Grounding to the neutral bar?
« Reply #14 on: September 19, 2013, 03:50:19 pm »

I'm merely asking when inserting my own set of breakers into a panel to connect a distro, is it generally OK to attach the ground wire to the neutral bar when connecting the distro to a main panel? I believe the ground bar and neutral bar are usually bonded together. If it's a sub panel it will most likely have a ground bar and I assume attaching the neutral wire to the neutral bar and the ground wire to the ground bar is probably correct. If there is no ground bar in a sub panel then what would be best practice? Is there something other than "don't use it" that would be a safe option?

Current code stipulates that the neutral bus and ground bus are one in the same at a service entrance or separately derived service since they are bonded. All sub panels downstream from the service entrance are to have neutral and ground un-bonded on separate buses. That topology has been in the NEC since right before the 70s I believe. Any electrical installations prior to that are a crap shoot. Technically, if a sub-panel to be tie into by qualified personnel doesn't have the neutral and ground buses separate, feeder should be run to one that's wired correctly or all the way back to the service entrance if need be to ensure a distro is compliant. Plenty of feeder should be kept on hand for such occasions.
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Jonathan Johnson

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Re: Grounding to the neutral bar?
« Reply #15 on: September 19, 2013, 04:15:30 pm »

Please review this white paper by Middle Atlantic. It should answer many questions.

http://www.middleatlantic.com/pdf/PowerPaper.pdf
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Mike Sokol

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Re: Grounding to the neutral bar?
« Reply #16 on: September 19, 2013, 04:55:00 pm »

Please review this white paper by Middle Atlantic. It should answer many questions.

http://www.middleatlantic.com/pdf/PowerPaper.pdf

That's a great paper. Thanks for posting it, Jonathan...
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Steve M Smith

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Re: Grounding to the neutral bar?
« Reply #17 on: September 19, 2013, 05:17:19 pm »

Can someone explain the isolated ground to me?  I don't see why or how it's any different to a normal ground.


Steve.
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Jonathan Johnson

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Re: Grounding to the neutral bar?
« Reply #18 on: September 19, 2013, 05:36:11 pm »

Can someone explain the isolated ground to me?  I don't see why or how it's any different to a normal ground.


Steve.

The standard ground may be subject to ground currents due to different voltage potentials on grounded components -- including structural components such as beams & studs, or piping systems such as gas and water -- throughout the premises. This ground current can induce noise in electronic components connected to this grounding system. Consider that the standard ground may have several connection points throughout the system. The further apart the components, the greater the likelihood of ground currents.

The isolated ground is a single-point ground (hub-and-spoke configuration). As the Middle Atlantic white paper I linked to earlier shows, all grounding of audio components -- including the rack -- is only through the isolated ground back to the panel. The rack is electrically insulated from structural components of the building (and if it cannot be, the components must be insulated from the rack). Ideally, the chassis ground of all audio components would be connected through a single, common isolated grounding conductor. When you have audio components (racks) in different parts of the premises, they may be powered by different conduits, thereby providing different isolated ground conductors to the different racks. In this case, it would be ideal to have the shield of balanced signal lines between the racks connected at one end only, to further prevent ground loops. Even though both racks' isolated grounds terminate at a common point (the subpanel or the main panel), there is still potential for ground loop current (if signal lines are grounded at both ends). There's even potential for ground loop current between two components installed adjacent in the same rack, connected to the same power supply, and connected with a signal line grounded at both ends. However, the minimized physical distance reduces that potential to immeasurable levels.

(In all matter, there is always voltage and there is always current. The question isn't how to eliminate it, but rather how to minimize its effect.)
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Mike Sokol

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Re: Grounding to the neutral bar?
« Reply #19 on: September 19, 2013, 06:44:51 pm »

The standard ground may be subject to ground currents due to different voltage potentials on grounded components -- including structural components such as beams & studs, or piping systems such as gas and water -- throughout the premises. This ground current can induce noise in electronic components connected to this grounding system. Consider that the standard ground may have several connection points throughout the system. The further apart the components, the greater the likelihood of ground currents.

I once measured the "ground" voltage between two sides of a large warehouse (1,000,000 square feet or so) and found there was a 5 volt difference, even with all those I-beams interconnecting everything. The currents involved must have been HUGE, but was never noticed plugging in single piece of gear. The problem reared it's head when we ran CCTV coax from the railroad dock to the guard shack some 500 ft apart. Of course, we grounded both the cameras and the video monitors at each outlet, but soon found the image unusable from all the hum bars in the video. Some research and a few video baluns allowed us to terminate the coax shield on just one end, and the hum bars went away.

My current research in ground loop hum finds that a standard 100 ft run of XLR cable will pass about 1 amp of current for every 1 volt of ground differential (yup, that about 1 ohm). And many pieces of sound gear will generate significant hum with less than 100 mA of ground loop current (the pin-1 problem). It's not high order math to see that even 1 volt of difference between outlet grounds will make a lot of gear hum like crazy.

When I see conduit being used as the safety ground in a building I know there's going to be trouble since by definition it will be bonded to building steel. Plus another issue I see a lot is the sub-panel itself bolted to the building steel. That's also a guarantee of ground loop trouble down the road.

One really easy way to determine in advance if you're going to have audio ground loop issues is the simply run a 100 ft (or longer) extension cord from the far outlet close to the one you're testing. Then poke a voltmeter between the safety ground in your "local" outlet and the safety ground in the extension cord coming from the "remote" outlet. It should measure very close to zero volts, maybe 1/10th of a volt max. If you're measuring any significant voltage, then your safety ground is probably double bonded to building steel somewhere.

Now, load the local outlet with something that draws maybe 8 or 10 amps of current. A small space heater or PAR light is good. You should NOT see the voltage measurement between the remote and local grounds change at all. If it goes up at all, could be 1 to 5 volts at times) then you know that the safety ground bus and and neutral bus have been double bonded somewhere, usually in a sub panel.  As mentioned earlier, that's not to code in post 1970's buildings.

Of course, you're not going to do this for a one-off gig, so for those cases I carry an array of audio isolation transformers and DI boxes with ground lift switches. If you do have your own studio or live stage in need of proper power, then getting a good electrician to help you sort out the grounds can be a big help.

So, what makes an isolated ground reduce hum is a single point ground with a lack of contamination from other ground and neutral wires feeding it. By itself, that Orange outlet does nothing to isolate you from anything except the box it's mounted in.

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