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50amp outlets

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Jamin Lynch:
Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks

RYAN LOUDMUSIC JENKINS:

--- Quote from: Jamin Lynch on September 19, 2013, 10:21:46 AM ---Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks

--- End quote ---

Are you sure it as no ground or is it no neutral?  In almost 25 years running audio, I have never come across a 50 amp without a ground.  The 50 amp 125/250 volt receptacle does exist in the Nema 10-50R but I have never come across one.  Are you working in some really old venues?   We do have an old hotel here that had a couple 10-30R recepticals that they supossedly were upgrading and may already be done with but no 50s.

Jamin Lynch:

--- Quote from: RYAN LOUDMUSIC JENKINS on September 19, 2013, 10:49:54 AM ---Are you sure it as no ground or is it no neutral?  In almost 25 years running audio, I have never come across a 50 amp without a ground.  The 50 amp 125/250 volt receptacle does exist in the Nema 10-50R but I have never come across one.  Are you working in some really old venues?   We do have an old hotel here that had a couple 10-30R recepticals that they supossedly were upgrading and may already be done with but no 50s.

--- End quote ---

Yep. 2 hots and a neutral.



Mike Sokol:

--- Quote from: Jamin Lynch on September 19, 2013, 10:21:46 AM ---Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks

--- End quote ---

Hmmmm..... I see a lot of 3-wire 30-amp/240-volt dryer outlets with a single ground/neutral contact in older buildings, but I can't remember seeing any 3-wire 50-amp/240-volt outlets in recent memory. But they're entirely possible. Since you're working from a new ground-neutral bonding point, any current on the line conductor will create a voltage drop in that side of the line, which will cause an equal and opposite voltage drop in the neutral conductor. Since your ground wire is also the neutral wire in that case the safety ground voltage will move around a lot, perhaps as much as 10 volts or so.  That suggests you'll want to avoid connecting some of your sound or stage gear to any other receptacles in the building unless you use audio isolation transformers such as a Whirlwind ISO-2 or similar, and you'll certainly want good DI's with a ground lift switch in the "lift" position. If you do cross connect something with shied isolation, lets say the amp rack connected to the 30 or 50-amp/240-volt 3-wire outlet and the mixing console to a handy Edison outlet on the back wall, you'll create a classic ground loop problem and lots of hum. In fact, I think this would be an excellent GLID (Ground Loop Inter-modulation Distortion) situation where power being pulled from the power amp rack will induce 60-Hz ground loop hum that you'll hear only during bass transients (Don't Doubt the GLID). ;D 

So, the basic rule is it. Either EVERYTHING is powered from your own distro connected to this 3-wire, 50-amp/240-volt outlet, or you need to provide audio isolation transformers between gear on the two power feeds. And you might be able to get away with pin-1 lifts on the XLR inputs depending on the CMRR of your balanced inputs on the amp rack. But I personally would avoid cross connecting gear powered between standard Edison wall outlets and your 3-wire distro in general. Too many things to go wrong.   

Mike Sokol:

--- Quote from: Jamin Lynch on September 19, 2013, 11:03:46 AM ---Yep. 2 hots and a neutral.

--- End quote ---
Note that the neutral is also the ground as well. So you have a double-bonded G-N in your power system.

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