ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: Split 240 plug for stage power?  (Read 2066 times)

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Split 240 plug for stage power?
« on: June 10, 2015, 08:50:26 am »

I just taught a church sound seminar in Columbia, SC and had a strange AC power situation. This was in a gymnasium that had a permanent/portable stage for the praise band. The lead technician said there was only one "special" plug on the floor which provided 220-volt power for the stage which was split to 110. Note that modern code and electricians call this 240 and 120 volts, so that's the first hint. Since my load-in crew was down to a single guy instead of the four they promised me, I was in a big rush to get everything hooked up for a 9AM start. But I still took time to do a quick inspection of this "special" plug since I've been burned before. It was a 3-pole twist lock receptacle with what appeared to be a 50 ft 12-2 or 12-3 plus ground extension cord feeding a double-wide metal body outlet strip up on stage with two rows of six outlet. I did my normal quick test using a NCVT to make sure none of the metal was hot, then plugged in my Amprobe INSP-3 to check for proper voltage and ground continuity before plugging in any of my seminar gear. The INSP-3 calculated voltage drop under a 20 amp load was a bit high at 15%, and the ground impedance showed up around 1/2 ohms which was perfect (should be under 1 ohms, but not zero as that suggests a bootleg ground). I plugged in and powered up my FOH gear and everything seemed fine with no hums, buzzes or weird things. However, when I tied my video switcher into the VGA line going back to their video projector powered on the opposite wall by a different AC circuit, there was a huge 15 Hz shake in the video image. Since my frame-store was showing it was sending 75 Hz video, I suspect this was from a huge ground loop current in my video line between my special stage power and the video projector power. Again, no time for troubleshooting so I hooked up my own video projector into my "special" stage power and everything worked fine for the day.

I suspect (but didn't have a chance to measure) that this special "220" receptacle was indeed two hots and a neutral without a separate EGC ground, and their electrician had created a bootleg ground in each of the sides of the double-wide power strip. They had complained about a lot of hum from their own sound system since the mixing console was plugged into the same power outlet as their video projector. But I wasn't there to work on their system since they were just hosting my seminar.

I'll try to get more info and follow up later, but it's 8 hours away from me so I'll need to get someone in the area to do some more measurements. But I'm sure this is a code violation that's sure to create all sorts of ground loop hum for anyone connecting gear to multiple outlets.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2382
Re: Split 240 plug for stage power?
« Reply #1 on: June 10, 2015, 09:32:09 am »

With 3 wires and 220 (240?) you either have no EGC or no neutral depending on its use-if you are using 120 volt loads, it must be a neutral so no EGC.

Hopefully, with code requiring 4 wire plugs/recepts for dryers and ranges, this kind of stuff will become less common.  For years, the only "220" plugs most non-electricians ever saw were 3 wire, so its not too surprising how often this "solution" has been implemented.
Logged
Steve Swaffer

Alex Donkle

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 69
Re: Split 240 plug for stage power?
« Reply #2 on: June 10, 2015, 05:12:03 pm »

With 3 wires and 220 (240?) you either have no EGC or no neutral depending on its use-if you are using 120 volt loads, it must be a neutral so no EGC.

Hopefully, with code requiring 4 wire plugs/recepts for dryers and ranges, this kind of stuff will become less common.  For years, the only "220" plugs most non-electricians ever saw were 3 wire, so its not too surprising how often this "solution" has been implemented.

I believe dryers were only required to use 4-wire after the 1996 code update, so I'm sure there's a lot of existing buildings that have 3-wire 240V power.
Logged
Consultant

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Split 240 plug for stage power?
« Reply #3 on: June 10, 2015, 08:51:33 pm »

With 3 wires and 220 (240?) you either have no EGC or no neutral depending on its use-if you are using 120 volt loads, it must be a neutral so no EGC.

Hopefully, with code requiring 4 wire plugs/recepts for dryers and ranges, this kind of stuff will become less common.  For years, the only "220" plugs most non-electricians ever saw were 3 wire, so its not too surprising how often this "solution" has been implemented.

Apparently this "split" fix is pretty recent and done in the last few years. There was probably a 3-wire/240-volt twist lock plug in the gym floor for the buffer, and this electrician came up with a custom fix. But this bootleg ground creates all sorts of ground loop problems when interconnecting the stage gear with other circuits.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2382
Re: Split 240 plug for stage power?
« Reply #4 on: June 10, 2015, 11:13:49 pm »

If this is existing in the floor, there may not be a good way to add a 4th wire.  Do they really need multiple circuits?  From a technical standpoint it would be better to convert it to a single 120 volt circuit.  If the plug was a 30 amp, you could conceivably configure it as a 30 amp 120 V circuit, then add OCP and convert it 2 15 amp circuits.

From an electricians POV for power, extra work-but many electricians forget about the code requirement banning "objectionable" current in an EGC.  Outside of A/V work, it rarely creates a noticeable issue.

Logged
Steve Swaffer

Alex Donkle

  • Jr. Member
  • **
  • Offline Offline
  • Posts: 69
Re: Split 240 plug for stage power?
« Reply #5 on: June 11, 2015, 06:48:17 am »

From an electricians POV for power, extra work-but many electricians forget about the code requirement banning "objectionable" current in an EGC.  Outside of A/V work, it rarely creates a noticeable issue.

Except for the absurd grounds the telecom guys typically ask for. BICSI standard is to design the ground connections between IT closet busbars for 0.005 ohms resistance, and field measure no more than 0.1 ohms.

If the split power is coming from the same panel (or at least same transformer) as the projector power, is there any hope of them pulling a new ground conductor in the existing conduit? Otherwise, might be a good time for a fiber upgrade.
Logged
Consultant

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Split 240 plug for stage power?
« Reply #6 on: June 11, 2015, 07:29:53 am »

Except for the absurd grounds the telecom guys typically ask for. BICSI standard is to design the ground connections between IT closet busbars for 0.005 ohms resistance, and field measure no more than 0.1 ohms.

If the split power is coming from the same panel (or at least same transformer) as the projector power, is there any hope of them pulling a new ground conductor in the existing conduit? Otherwise, might be a good time for a fiber upgrade.

This was a pretty low-tech church with 20 year old carpet covered speakers for FOH and a really old Mackie SR24-4 mixer with a bunch of dead channels. Nobody I talked to seemed to have any concern about the grounding issues and how it influenced the hum. Since they always run their AV computer from the same outlet as the video projector they likely don't have any video hum issues. But it sure bugged me.

So this is one more thing to be on the lookout for at worship venues. Whoda Thunk?
« Last Edit: June 11, 2015, 07:32:11 am by Mike Sokol »
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

ProSoundWeb Community

Re: Split 240 plug for stage power?
« Reply #6 on: June 11, 2015, 07:29:53 am »


Pages: [1]   Go Up
 



Page created in 0.059 seconds with 22 queries.