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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: George Dougherty on May 26, 2016, 12:17:56 PM
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The best answer is always test so you avoid, however I'm often dealing with short setups and volunteer assistance during setup which could get ahead of me.
I'm looking to properly build my amp rack powercon inputs into a chassis with more flexibility and a common ground. My brain isn't thinking through the potential ramifications at the moment so I figured I'd ask the experts. Does the common ground setup in a poor man's powercon distro box make the Reverse Bootleg Ground situation any more immediately hazardous if jumpers are used to power more input sections?
As a slightly tangential question related to the box. Are there any improvements that can make the setup safer? Do I need any sort of breaker protection in-line?
I'm planning on 4 input with 2 parallel outputs up front and 3 parallel outputs on back tied to each input.
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I am assuming that the only things connected amongst the inputs are the grounds? Neutrals and hots are kept separate. On that case, as long as SO cabling is sized correctly for the receptacles supplying the load, no supplementary protection should be needed.
As long as you test first, it should not increase the danger-it could even reduce it.
If all four outlets are RPBG, then you have the same danger.
If only one is RPBG, and at least one of the others has a good earth ground, you will get sparks and tripped breakers.
If you have an RPBG, and no connected grounds on the other receptacles, then you can certainly multiply the danger.
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If you have an RPBG, and no connected grounds on the other receptacles, then you can certainly multiply the danger.
When you say no connected grounds, would that include bootleg grounds being a problem on the other outlets I'm puling power from? All my connectors are grounded and I don't use ground lift adapters. Trying to make sure I'm clear when that scenario would be an issue.
Otherwise, very cool. I'd much rather pop a breaker than fry someone or my gear once its plugged in.
Yes, only the grounds are tied between inputs, all my rig cabling is SO grade and my plan was to have 4 circuits instead of my current 3 so I could also use the amp rack as the central point for power, feeding the backline and powered monitors from the two outputs on front as well.
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I am assuming that the only things connected amongst the inputs are the grounds? Neutrals and hots are kept separate. On that case, as long as SO cabling is sized correctly for the receptacles supplying the load, no supplementary protection should be needed.
As long as you test first, it should not increase the danger-it could even reduce it.
If all four outlets are RPBG, then you have the same danger.
If only one is RPBG, and at least one of the others has a good earth ground, you will get sparks and tripped breakers.
If you have an RPBG, and no connected grounds on the other receptacles, then you can certainly multiply the danger.
Yep. A poor man's distro doesn't replace the need for testing receptacles, but it will "pre-blow" any RPBG receptacles for you, which is a lot better than doing that by burning up your audio snake.
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Yep. A poor man's distro doesn't replace the need for testing receptacles, but it will "pre-blow" any RPBG receptacles for you, which is a lot better than doing that by burning up your audio snake.
+1
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Wow, finding 4 circuits to pull power from? At that level I would be looking for a larger feed to a distro. The most I have ever found that were reasonable distances were 3 circuits.
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But the real advantage of the poor mans distro is that all the outlets on it have, from the stage perspective, a common ground. OK, the installation wiring might be a bit screwed and one of more of the sources have their N/G swapped, but from the stage ground perspective, that's invisible, and this is especially important if a source ground or source bootleg ground lets go; the stage remains safe between items of equipment on it, and GFCIs will all still behave correctly.
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Wow, finding 4 circuits to pull power from? At that level I would be looking for a larger feed to a distro. The most I have ever found that were reasonable distances were 3 circuits.
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Yeah, I rarely find that many. When I do use my 50A distro I do tend to run the backline straight from there anyway. I'm also down to really needing just two circuits to the rack since I moved to active wedges. It costs the same to punch my enclosure for 4 as it does 12-16 from Redco and powercon jacks are just a few bucks a pop, so I might as well. Eventually I'll have two sub amps on separate inputs and a pair of amps each for main L/R which could be on their own inputs. It gives me flexibility and room to grow.
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When you say no connected grounds, would that include bootleg grounds being a problem on the other outlets I'm puling power from? All my connectors are grounded and I don't use ground lift adapters. Trying to make sure I'm clear when that scenario would be an issue.
If you tie into an RPGB with one circuit and the rest of the circuits you connect either have an RPBG or no ground connection, then you will energize your entire rig. At a bare minimum use a NCVT to verify that nothing is energized once it is connected. That test takes 2 minutes, but should probably be considered THE most important pre-show test.
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If you tie into an RPGB with one circuit and the rest of the circuits you connect either have an RPBG or no ground connection, then you will energize your entire rig. At a bare minimum use a NCVT to verify that nothing is energized once it is connected. That test takes 2 minutes, but should probably be considered THE most important pre-show test.
Thanks, That's an easy check since I keep a NCVT in the power cable trunk.
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Wow, finding 4 circuits to pull power from? At that level I would be looking for a larger feed to a distro. The most I have ever found that were reasonable distances were 3 circuits.
Yeah, in most of the clubs I played in over the years there would be a line of receptacles across the back of the stage, but they were all powered by a single circuit breaker. And other receptacles in the room might be shared by the kitchen or lighting. This is still common in many of the churches I've taught in, and I've had my sound system taken down by someone in the kitchen turning on a commercial coffee urn which draws around 15 amperes by itself. Ugh...
That's also why ground loop hum seems to come on go randomly at times. Other things in the building can be turned on and off, creating different voltage drops on various circuits. If the ground and neutral is intermingled on a branch circuit (standard bootleg ground) or even a swapped ground and neutral (happens more than you think, even in new construction) then you'll get all sorts of changing ground loop currents, which can make hum come and go randomly. So by bonding all your grounds together from the various pwer circuits, you force these currents to occur inside the distro itself, and not via the shields of your snake or XLR cables. One thing you can do as an interesting experiment is to test for current using a standard clamp-ammeter around the entire feed wire, not splitting out the hot like a traditional test). Any current you read with this test will be the ground loop current you've diverted from your XLR interconnects.
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commercial coffee urn...
A large old church I do a lot of work in has coffee "urns" that are about 20 gallons, and conveniently the receptacle that powers them is a 14-50. :)