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Poll

Should you pre-twist the wire for wire-nuts?

Yes
- 4 (23.5%)
No
- 6 (35.3%)
Doesn't Matter
- 4 (23.5%)
Wire Nuts Are Evil
- 3 (17.6%)

Total Members Voted: 17


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Author Topic: No Box, Safe?  (Read 39966 times)

Mike Sokol

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Re: No Box, Safe?
« Reply #90 on: September 17, 2014, 10:45:26 AM »

One way around it most places is to use isolated ground receptacles.  The strap that connects the receptacles to the box is NOT connected to the safety ground terminals in the receptacle.  They are connected with a separate ground wire back to the panel,  not to the box.



Frank, but once the EGC (ground) wire is bonded to the steel at the receptacle, the damage to the audio ground is already done. At that point it doesn't matter if it's an isolated ground receptacle or not. You have to have an EGC/Audio ground all the way back to the service panel's EGC/GEC bonding point that DOESN'T touch building steel anywhere on its path. That's the tricky part, since by definition any armored cable and metal boxes will be attached to building steel. And it only takes one connection like that (accidental or on purpose) to contaminate a technical ground. I did a consult with a church in Texas last year that used metallic conduit for the ONLY ground. Of course, it hummed like crazy since there was several volts difference various grounds that the audio was plugged into. Rather than rewire everything (which, by the way was recently passed by a local inspector) we just added in a bunch of WW ISO-2 audio isolation boxes between the mixer outputs and the amp rack that was 100 feet away. They already had a bunch of DI boxes with appropriate ground lift switches on stage, but their main hum was from a ground loop current between the console position and the amp rack. I didn't measure the actual current, but I'm guessing it was at least a couple amps since the voltage differential varied around 1.5 to 2 volts AC between the various outlets. That was the hint that something was terribly wrong with the audio ground.

Frank DeWitt

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Re: No Box, Safe?
« Reply #91 on: September 17, 2014, 11:04:43 AM »

Frank, but once the EGC (ground) wire is bonded to the steel at the receptacle, the damage to the audio ground is already done.

Yes,  I think we are in agreement.  Mount and ground the box any way you wish / required by code, Run a ground wire to the box, boxes, or whatever. Attach the box to the building steel ETC.  Then run a separate wire from the ground screw on the receptacle (different color wire)  back to the panel.  Yes, it is a pain. It is easy for someone to mess it up and hard to find, and it is only practical in new construction, but it can be done.  I worked in one company that had a isolated ground system in a large experimental lab (think football field size)  They actual monitored the current on the isolated ground system. If it changed dramatically a alarm went off.

Me in my old church.  I use a lot of audio isolation transformers, just like you did.   
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Mike Sokol

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Re: No Box, Safe?
« Reply #92 on: September 17, 2014, 11:52:40 AM »

Me in my old church.  I use a lot of audio isolation transformers, just like you did.

+1 - Many times that's the cheap way out. What I do for this type of testing is keep a couple of cheap/orange extension cords in my truck. They're really not good enough for audio power, but plenty fine for testing grounds. Then plug the male ends into outlets at the console and amp rack positions. Hold the two "hot" ends next to each other and meter for AC voltage between the two grounds. It should read VERY close to 0 volts. My tests show that even 1/10th of a volt difference between two outlet grounds can make a lot of amplifiers and speakers hum. (Yes, the pin-1 problem). You can confirm this by simply clamping an ammeter around the outside of the XLR return from the board to the amp rack. Since the return current path is external to the XLR shield (the building steel) there's no need to split out the twisted pair from the shield. In fact, you can clamp a $50 ammeter around the entire analog snake to check for currents. Even 100 mA is a significant ground loop current that can cause a lot of grief. The audio iso transformers isolate the two shields from each other. And so no shield current = no ground loop hum. It really is easy to troubleshoot like that since a clamp ammeter is completely passive and you can use it in the middle of the production with the sound system running to find a hum, then swap in an audio iso transformer at break. And fixing the hum in a pa system during a show or worship service elevates you to super audio dog status (or something like that).

Steve M Smith

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Re: No Box, Safe?
« Reply #93 on: September 17, 2014, 12:40:34 PM »

Do you actually need the isolation of a transformer in these cases or would the normal 10 ohm resistor and 0.1uF capacitor as used in active DIs to 'lift' the ground do the trick?


Steve.
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Jonathan Johnson

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Re: No Box, Safe?
« Reply #94 on: September 17, 2014, 02:17:38 PM »

Going strictly by the NEC, non-metallic boxes are acceptable anywhere you are allowed to use nonmetallic sheathed cable, or knob and tube wiring or open wiring on insulators(NEC 2011 314.3).  NM (romex) is allowed -by the NEC-in certain types of construction (wood frame essentially) no matter the use.  I realize that most local AHJ will not allow NM in commercial-but that is actually going above and beyond the NEC.

Which are better?  PVC may get hard and crack or strip threads.  Metal often rusts-and threads get stripped.  Pick your poison-they all can be a problem.


If manufacturers' sales literature is to be believed, plastic is superior to metal for fire resistance.

http://www.carlonsales.com/techinfo/codesstandards/CS-Fire_PVC_Building_Codes.pdf
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Frank DeWitt

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Re: No Box, Safe?
« Reply #95 on: September 17, 2014, 02:28:10 PM »

+1 - Many times that's the cheap way out. What I do for this type of testing is keep a couple of cheap/orange extension cords in my truck. They're really not good enough for audio power, but plenty fine for testing grounds. Then plug the male ends into outlets at the console and amp rack positions. Hold the two "hot" ends next to each other and meter for AC voltage between the two grounds. It should read VERY close to 0 volts. My tests show that even 1/10th of a volt difference between two outlet grounds can make a lot of amplifiers and speakers hum. (Yes, the pin-1 problem). You can confirm this by simply clamping an ammeter around the outside of the XLR return from the board to the amp rack. Since the return current path is external to the XLR shield (the building steel) there's no need to split out the twisted pair from the shield. In fact, you can clamp a $50 ammeter around the entire analog snake to check for currents. Even 100 mA is a significant ground loop current that can cause a lot of grief. The audio iso transformers isolate the two shields from each other. And so no shield current = no ground loop hum. It really is easy to troubleshoot like that since a clamp ammeter is completely passive and you can use it in the middle of the production with the sound system running to find a hum, then swap in an audio iso transformer at break. And fixing the hum in a pa system during a show or worship service elevates you to super audio dog status (or something like that).

Heads up.  This needs to go on the wall or in the note book or some place handy.  In one paragraph you explained.
1   what a ground loop is
2   How to tell if you have one
3.  How to tell with a clamp on amp meter
4   What to do about it.
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Jeff Bankston

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Re: No Box, Safe?
« Reply #96 on: September 17, 2014, 02:33:57 PM »

One way around it most places is to use isolated ground receptacles.  The strap that connects the receptacles to the box is NOT connected to the safety ground terminals in the receptacle.  They are connected with a separate ground wire back to the panel,  not to the box.


we can only install what the customer wants or what the blueprints specify. you must understand WE the electricians HAVE TO follow the plans. also if we decided to install ig recepticals when we are told to install regular ones WHO pays for that ? we could get sued and our business license pulled for selling a customer something they did not ask for. the problem is some of you guys dont have a clue when it comes to the business of electrical work. if i install ig recepts when a customer didnt ask for them i would have to pay the extra and that aint gonna happen. what needs to be done is the code changed for places that have sound systems.
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Frank DeWitt

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Re: No Box, Safe?
« Reply #97 on: September 17, 2014, 02:43:40 PM »

Do you actually need the isolation of a transformer in these cases or would the normal 10 ohm resistor and 0.1uF capacitor as used in active DIs to 'lift' the ground do the trick?
Steve.

The resistor trick is used in cheep active DIs.  Not in ones like Radial

If we take Mikes numbers that 100 ma causes a lot of hum then if you start with 100 ma and lets assume your resistance from the whole ground loop (audio cable shield and building grounds) is 5 ohms then adding 10 ohms you have  .5 volts across 15 ohms you cut the current to 33 ma so you cut your hum to 1/3 of what it was.

If you had 2 volts like Mike has reported seeing then your 10 ohm resistor gets the current down to 133 ma.  HUM.

Going from a mixer to a amp rack you don't need phantom power so I would try a jumper cable with a open pin 1 first.  Then when I still had hum I would put the transformer in.  Grin

Transformers are a great solid fix.  The good ones (that don't change the sound) cost money but the problem goes away.
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Frank DeWitt

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Re: No Box, Safe?
« Reply #98 on: September 17, 2014, 03:02:48 PM »

we can only install what the customer wants or what the blueprints specify. you must understand WE the electricians HAVE TO follow the plans. also if we decided to install ig recepticals when we are told to install regular ones WHO pays for that ? we could get sued and our business license pulled for selling a customer something they did not ask for. the problem is some of you guys dont have a clue when it comes to the business of electrical work. if i install ig recepts when a customer didnt ask for them i would have to pay the extra and that aint gonna happen. what needs to be done is the code changed for places that have sound systems.

Absolutely but hopefully some people reading this list or other good lists will specify a AC power system that is audio friendly.  As one example,  I belong to a growing church and I have a folder marked "New Building"  There is info on isolated grounds, separate Huge raceways for signal between the booth and back stage, amp rack placement and orientation, alternatives to floor pockets ETC.   In other words, I may be the customer one of these years.

Wait till the electrician hears I want the hot and neutral twisted.
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Mike Sokol

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Re: No Box, Safe?
« Reply #99 on: September 17, 2014, 03:09:43 PM »

Heads up.  This needs to go on the wall or in the note book or some place handy.  In one paragraph you explained.
1   what a ground loop is
2   How to tell if you have one
3.  How to tell with a clamp on amp meter
4   What to do about it.

Should I make this a new topic and write a short essay on it? That could be useful, I think....

ProSoundWeb Community

Re: No Box, Safe?
« Reply #99 on: September 17, 2014, 03:09:43 PM »


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