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Author Topic: Power from multiple sources  (Read 10211 times)

Timothy J. Trace

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Power from multiple sources
« on: May 18, 2012, 11:44:32 am »

I've just learned that electrical power for this Saturday's gig will be provided from several sources.  It's a favor to a friend, and the budget can't sustain the rental of a right-sized entertainment generator.

Here's what I'll have to work with:

1> One 110VAC 20A branch, on a GFI, with 5 downstream duplexes.
2> Two jobsite generators (it's proving difficult to get the kW rating from the promoter)
3> A battery pack/inverter

The gig is one performance in a series of smallish outdoor summer concerts given by a town in our metro area.  Judging the venue, a metal gazebo facing a ~3/4-acre grassy hillside, I'd say no more than 300 people will attend.

My sub amps are 4x PLX2402 at 4-ohms bridged, and fortunately I shouldn't have to get into them very hard at this gig.  Likewise, back line and lighting are light eaters: The client is completely on IEM with amp simulators, and my stage lighting is comprised of non-moving LED instruments.

I'm considering using the 20A branch circuit to power the backline, lighting, full-range amps and FOH.  I'd like to split the sub amps, two to each generator, with audio ground lifts where necessary.  The battery pack can charge my phone  ::)

Now for a couple of quick questions:

1> Any thoughts about splitting the available power in a different manner?
2> Do I need tie the branch circuit and the generators into a common electrical ground? Can this be done without freaking out the GFIs?


Thank you for taking the time to help me out  :)

Tim ==

.
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Geoff Doane

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Re: Power from multiple sources
« Reply #1 on: May 18, 2012, 01:30:10 pm »



2> Do I need tie the branch circuit and the generators into a common electrical ground? Can this be done without freaking out the GFIs?


If I was in your situation, I would do my best to tie all the grounds together, but only the grounds.  If the neutrals are already bonded to ground at the generators, leave them that way.  Don't attempt to create common neutrals anywhere else.  You want a common ground because you will have a common audio ground, and you don't want the audio ground to have any AC current flowing in it.

The GFIs don't really have anything to do with the grounds.  They simply look for an imbalance between the line and neutral.  If they don't match within 5-10 mA, they assume current is leaking out somewhere, or to someone, and trip.

There might be some value in having an isolation transformer in the feed(s) to the sub amps, but hopefully the CMRR of the amp inputs will be good enough once everything is on a common ground.

Your power split would seem to be making the best of a bad situation.  Hopefully you don't have too long a run from the shore power that you will have excessive losses at the end of the line.

GTD

P.S.  And if you haven't already, read up on some of the things that can go wrong with electrical systems here:

http://forums.prosoundweb.com/index.php/topic,137760.0.html

and get yourself one of those non-contact voltage testers.  I've been having great fun with the $20 Klein model I picked up last week.
« Last Edit: May 18, 2012, 01:33:39 pm by Geoff Doane »
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Jay Barracato

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Re: Power from multiple sources
« Reply #2 on: May 18, 2012, 02:18:19 pm »

I've just learned that electrical power for this Saturday's gig will be provided from several sources.  It's a favor to a friend, and the budget can't sustain the rental of a right-sized entertainment generator.

Here's what I'll have to work with:

1> One 110VAC 20A branch, on a GFI, with 5 downstream duplexes.
2> Two jobsite generators (it's proving difficult to get the kW rating from the promoter)
3> A battery pack/inverter

The gig is one performance in a series of smallish outdoor summer concerts given by a town in our metro area.  Judging the venue, a metal gazebo facing a ~3/4-acre grassy hillside, I'd say no more than 300 people will attend.

My sub amps are 4x PLX2402 at 4-ohms bridged, and fortunately I shouldn't have to get into them very hard at this gig.  Likewise, back line and lighting are light eaters: The client is completely on IEM with amp simulators, and my stage lighting is comprised of non-moving LED instruments.

I'm considering using the 20A branch circuit to power the backline, lighting, full-range amps and FOH.  I'd like to split the sub amps, two to each generator, with audio ground lifts where necessary.  The battery pack can charge my phone  ::)

Now for a couple of quick questions:

1> Any thoughts about splitting the available power in a different manner?
2> Do I need tie the branch circuit and the generators into a common electrical ground? Can this be done without freaking out the GFIs?


Thank you for taking the time to help me out  :)

Tim ==

.

I would not run any audio off of a no spec-ed generator. The generator may be ok for lights. For audio I would run it all of of the available 20 amp circuit, which means I would not be running 4 sub amps. For a 300 person, even outdoors, on a shoestring budget, I would put up 2 sets of 1 top over 1 sub and call it good. Those people interested in the music will move closer.
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TJ (Tom) Cornish

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Re: Power from multiple sources
« Reply #3 on: May 18, 2012, 02:33:21 pm »

I would not run any audio off of a no spec-ed generator. The generator may be ok for lights. For audio I would run it all of of the available 20 amp circuit, which means I would not be running 4 sub amps. For a 300 person, even outdoors, on a shoestring budget, I would put up 2 sets of 1 top over 1 sub and call it good. Those people interested in the music will move closer.
+1 - that's where I was headed.  Sometimes we do ourselves a disservice by overproviding what the gig deserves or can handle.  A charity gig that ends in broken equipment due to cruddy generators isn't a good situation.
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g'bye, Dick Rees

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Re: Power from multiple sources
« Reply #4 on: May 18, 2012, 02:58:38 pm »

I've just learned that electrical power for this Saturday's gig will be provided from several sources.  It's a favor to a friend, and the budget can't sustain the rental of a right-sized entertainment generator.

Here's what I'll have to work with:

1> One 110VAC 20A branch, on a GFI, with 5 downstream duplexes.
2> Two jobsite generators (it's proving difficult to get the kW rating from the promoter)
3> A battery pack/inverter

The gig is one performance in a series of smallish outdoor summer concerts given by a town in our metro area.  Judging the venue, a metal gazebo facing a ~3/4-acre grassy hillside, I'd say no more than 300 people will attend.

My sub amps are 4x PLX2402 at 4-ohms bridged, and fortunately I shouldn't have to get into them very hard at this gig.  Likewise, back line and lighting are light eaters: The client is completely on IEM with amp simulators, and my stage lighting is comprised of non-moving LED instruments.

I'm considering using the 20A branch circuit to power the backline, lighting, full-range amps and FOH.  I'd like to split the sub amps, two to each generator, with audio ground lifts where necessary.  The battery pack can charge my phone  ::)

Now for a couple of quick questions:

1> Any thoughts about splitting the available power in a different manner?
2> Do I need tie the branch circuit and the generators into a common electrical ground? Can this be done without freaking out the GFIs?


Thank you for taking the time to help me out  :)

Tim ==

.

Pass.  If they don't value your contribution enough to fork over for a proper generator just walk away.
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Mike Sokol

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Re: Power from multiple sources
« Reply #5 on: May 18, 2012, 03:02:42 pm »

+1 - that's where I was headed.  Sometimes we do ourselves a disservice by overproviding what the gig deserves or can handle.  A charity gig that ends in broken equipment due to cruddy generators isn't a good situation.

+2 on running all audio power off of one generator. And make sure you bond the generator to earth, even if it's a piece of rebar and pair of vice-grips. I did a gig for Chumbawamba once and arrived to find an ungrounded industrial generator for my power. Something in the distro must have been leaking current to earth because I measured 90 volts AC from the mics to the railing around the stage, which in this case was 4 stories up on the roof of the garage. To make it even more complicated, this was a British band who needed 240 volts for their stage gear, and I was running a British sound system with 240 volt power amps and 120-volt mixer and processing. Lots of things that could go wrong. The last thing I wanted was a musician to take a shock between a guitar and the railing, and topple 50 ft to the sidewalk below.

To make things safe, I found a long piece of rebar on the site behind the garage, pounded it 8 ft into the ground, and bonded it to the generator frame with a pair of vice grips and a length of 10 gauge wire. Yes, I taped the handle in lock position with e-tape and then measured the system ground voltage as close to zero (within a volt or two).

Not exactly code, but it was safe. And yes, a non-contact tester pointed at the generator frame should not beep.
« Last Edit: May 20, 2012, 07:01:46 am by Mike Sokol »
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Mike Sokol
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Mike Sokol

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Re: Power from multiple sources
« Reply #6 on: May 18, 2012, 06:18:04 pm »


I'm considering using the 20A branch circuit to power the backline, lighting, full-range amps and FOH.  I'd like to split the sub amps, two to each generator, with audio ground lifts where necessary


I missed this on the first read, but worry in general about using AC ground lifts in any situation. (I hope you were referring to pin-1 XLR lifts) If proper audio isolation transformers are used between the drive rack and amp racks, there should NEVER be any hum from ground loops and no need to lift grounds. Both Whirlwind and Ebtech make appropriate audio isolation transformers such as the ISO-2 and Hum Eliminator.

The 2-channel units are good for isolating the grounds between a stereo feed from your drive rack to the FOH amp rack with crossovers. I have the 8 channel version in my main FOH rack that allows me to do a stereo send on 7 & 8, plus six monitor sends on 1 thru 6. This works especially well with powered monitors that will be powered from a variety of outlets around the stage.

The key idea is to "ground locally and isolate remotely" (you can quote me on that). Each amp rack, drive rack, back-line amp and powered wedge should be independently grounded by its local power plug, and any interconnecting XLR cable shields isolated on one end if there's any hum at all. If you have properly balanced inputs on your amps and powered speakers with decent CMRR, then you might be able to get away with a pin-1 lift on the XLR cables, but I feel that a proper audio isolation transformer with a mu-metal shield will offer superior audio quality with no hum or buzz under nearly any power condition.

UPDATE: This is especially important for back-line stage amps which really need a transformer isolated DI box to fulfill this mission. NEVER use a phone plug to XLR adapter cable to feed your mixing board from a pre-amp output. And be aware that many XLR direct outputs on bass guitar amps DON'T have transformer isolated outputs. Nope... some are not even balanced outputs, just a drive signal on pin 2 with pin 3 tied to the chassis. Yikes!!! For these outputs I generally use a Female XLR to 1/4" adapter cable to feed a proper external DI box. Use any flavor DI you like, but a good old WW IMP-2 has never failed to properly isolate ground loop currents from bass amps for me.   

Also, note that a standard clamp-on ammeter will find ground loop currents in XLR cables. No need to split out the wires like you do when measuring AC power draw. Just clamp the meter right around the XLR cable. My lab experiments show you'll get approx 1 amp of ground-loop current per volt of ground differential. As little a 1/10th volt ground differential (100 ma ground loop current) will make Mackie powered wedges hum noticeably.
« Last Edit: May 20, 2012, 07:15:47 am by Mike Sokol »
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Timothy J. Trace

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Re: Power from multiple sources
« Reply #7 on: May 18, 2012, 07:07:20 pm »

Thanks, guys, a lot of good information here.

Don't own a hum eliminator, and can't acquire one on short notice.  I have a Radial JDI Duplex which I can use for an isolator after I convert my sub drive line to a TS plug at the input side of the DI.  Should I feed the DI the high and the low, or the high and the shield?  And are these machinations going to be worth a damn?

As for the rest, I think mostly we agree that the grounds need to be tied together.

I can test all power sources to make sure that the outlets are wired correctly -- hot, neutral and ground where they belong.

I can drive a 6' ground rod easily with a proper post hammer.  I can put it right next to the generators and connect each chassis to ground with at minimum #8 THHN and clamps.

How, then, should I add the utility power to the ground scheme?  Can I use an Edison plug to wire a duplex to the ground rod (using the U-ground pin only)?

If I don't find ground continutity between the gazebo frame and the branch duplexes, should I tie the frame into the ground scheme for safety?

Dick, please don't think that I don't appreciate your point...I'm just not going to go there on this particular gig.
« Last Edit: May 18, 2012, 07:10:56 pm by Timothy J. Trace »
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Mike Sokol

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Re: Power from multiple sources
« Reply #8 on: May 18, 2012, 07:19:56 pm »

Thanks, guys, a lot of good information here.

Don't own a hum eliminator, and can't acquire one on short notice.  I have a Radial JDI Duplex which I can use for an isolator after I convert my sub drive line to an unbalanced TS connector at the input side of the DI.  Should I feed the DI the high and the low, or the high and the shield?  And are these machinations going to be worth a damn?

As for the rest, I think mostly we agree that the grounds need to be tied together.

I can test all power sources to make sure that the outlets are wired correctly -- hot, neutral and ground where they belong.

I can drive a 6' ground rod easily with a proper post hammer.  I can put it right next to the generators and connect each chassis to ground with at minimum #8 THHN and clamps.

How, then, should I add the utility power to the ground scheme?  Can I use an Edison plug to wire a duplex to the ground rod (using the U-ground pin only)?

If I don't find ground continuity between the gazebo frame and the branch duplexes, should I tie the frame into the ground scheme for safety?

Dick, please don't think that I don't appreciate your point...I'm just not going to go there on this particular gig.

My head is reeling thinking about the legal aspects of all this, but if you're pulling power from both generators and the utility at the same time, you certainly want to tie your generator grounds to the utility ground somehow, and all metal frames on the stage should be properly bonded to this "earth" ground as well. Remember that generators typically float both their neutral and hot lines above the frame ground, so just like in all incoming power panels the generators' grounds and neutrals need to be bonded with a ground rod in order to create a ground plane. The utility ground rod is probably sufficient for this case and could substitute for the ground rod you suggest. A secondary ground rod at the generator "could" be driven, but should not be necessary from a NEC standpoint. However, connecting all these various grounds together and bonding them to earth properly is a recipe for getting somebody killed on stage or gear blown up if not done exactly right. I would see if there's an electrician who can do you a free favor and tie the grounds of all three systems together. Just remember that the neutrals from the generators also have to be bonded to the earth reference ground (rod) as well.
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Mike Sokol
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Re: Power from multiple sources
« Reply #9 on: May 19, 2012, 08:09:00 am »

Thanks, guys, a lot of good information here.

Tim, please report back after the gig and take pictures of the power setup if you can. Hope everything works out for you....
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Mike Sokol
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Re: Power from multiple sources
« Reply #9 on: May 19, 2012, 08:09:00 am »


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