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Author Topic: What Happens In This Situation?  (Read 1640 times)

Gary Christenot

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What Happens In This Situation?
« on: January 05, 2016, 12:26:19 pm »

This is a follow-up thread to my previous post seeking electrical safety advice

http://forums.prosoundweb.com/index.php/topic,157529.0.html

Would appreciate someone taking a look at the block diagram in the attached photo.

If you read my previous post, the system is a large, rolling road case with a mixer, keyboard, power supply and a couple of other odds and ends designed by my son for a local high school band.  For some reason he designed it to be powered via Powercon connectors that pass through metal plates in the case side with D-shape knockouts.

The thought of AC power passing through/near those plates and having some kid put his hand on that plate and get electrocuted in front of a football half time crowd is giving me cold sweats.

Rather than retype everything out here in text, I've labeled the diagram with areas of question/concern.

Thanks for looking!
« Last Edit: January 05, 2016, 12:36:29 pm by Gary Christenot »
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Geoff Doane

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Re: What Happens In This Situation?
« Reply #1 on: January 05, 2016, 01:24:03 pm »



The thought of AC power passing through/near those plates and having some kid put his hand on that plate and get electrocuted in front of a football half time crowd is giving me cold sweats.



Because you're using this thing outdoors, whatever you plug it into ought be on a GFCI already.  That outlet is somebody else's responsibility, but if the installer followed the electrical code, it will be protected by a GFCI.  ASSuming it was installed properly, that should make whatever you plug into it pretty much fool proof and safe.

However, the rig might also be used inside where there might not be a GFCI, so there are some other precautions you should observe.  The plate that the PowerCons mount in (and the back box around them) should be bonded to the EGC (equipment grounding conductor, or the green wire).  It may not be code, but it would be good practice to also bond the connector plates and the rack rails to ground as well.  I do know that in the Canadian Electrical Code, any metal raceway (an enclosure or tray for wires) MUST be bonded to ground, even if the wires are only audio or data, not power.

There is a chance that the outlet you have plugged into doesn't actually have a good ground.  In that case, your GFCI in the rack will prevent a fault downstream of the GFCI from electrocuting anyone.  BUT if the fault happens before the GFCI (ie, the plate the PowerCons are mounted in), your GFCI won't do anything.  Your GFCI also can't protect against an RPBG (Look it up if that's a new term.  It's all over this forum).  Ultimately, you're still at the mercy of the plug that you're drawing power from.  Of course there would be a problem plugging anything with a metal case into such a miswired plug.

It's a good idea to own and use a non-contact voltage tester (NCVT) when connecting to unknown or otherwise suspect power sources.  These cost about $20, and will tell you if a plate or other metal surface is energized.  There are threads about NCVTs here in this forum as well.  I encourage you to read them if you haven't already.  Used properly, it can alert you to miswired or defective outlets and equipment.

As for the PowerCons, I use them where I used to use Twist-Locs, and as long as you can deal with the limitations, I think they're fine.  You need to connect them before the circuit is energized, or they will eventually fail (stop making contact).  If you really try, you can take them apart with your bare hands and expose live wires, so that is a potential down side.  You also need to use the proper size Sta-Kon for the ground connection on them.  It's not sufficient to just solder a wire on.  This is for safety reasons, since a sufficiently high fault current might melt the solder, causing the ground to come off and then floating all your carefully bonded metal parts.  OTOH, they don't have exposed pins to get bent like Twist-Locs, and they are considerably smaller and less expensive.

GTD
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Gary Christenot

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Re: What Happens In This Situation?
« Reply #2 on: January 05, 2016, 01:47:11 pm »

Because you're using this thing outdoors, whatever you plug it into ought be on a GFCI already.  That outlet is somebody else's responsibility, but if the installer followed the electrical code, it will be protected by a GFCI.  ASSuming it was installed properly, that should make whatever you plug into it pretty much fool proof and safe.

However, the rig might also be used inside where there might not be a GFCI, so there are some other precautions you should observe.  The plate that the PowerCons mount in (and the back box around them) should be bonded to the EGC (equipment grounding conductor, or the green wire).  It may not be code, but it would be good practice to also bond the connector plates and the rack rails to ground as well.  I do know that in the Canadian Electrical Code, any metal raceway (an enclosure or tray for wires) MUST be bonded to ground, even if the wires are only audio or data, not power.

There is a chance that the outlet you have plugged into doesn't actually have a good ground.  In that case, your GFCI in the rack will prevent a fault downstream of the GFCI from electrocuting anyone.  BUT if the fault happens before the GFCI (ie, the plate the PowerCons are mounted in), your GFCI won't do anything.  Your GFCI also can't protect against an RPBG (Look it up if that's a new term.  It's all over this forum).  Ultimately, you're still at the mercy of the plug that you're drawing power from.  Of course there would be a problem plugging anything with a metal case into such a miswired plug.

It's a good idea to own and use a non-contact voltage tester (NCVT) when connecting to unknown or otherwise suspect power sources.  These cost about $20, and will tell you if a plate or other metal surface is energized.  There are threads about NCVTs here in this forum as well.  I encourage you to read them if you haven't already.  Used properly, it can alert you to miswired or defective outlets and equipment.

As for the PowerCons, I use them where I used to use Twist-Locs, and as long as you can deal with the limitations, I think they're fine.  You need to connect them before the circuit is energized, or they will eventually fail (stop making contact).  If you really try, you can take them apart with your bare hands and expose live wires, so that is a potential down side.  You also need to use the proper size Sta-Kon for the ground connection on them.  It's not sufficient to just solder a wire on.  This is for safety reasons, since a sufficiently high fault current might melt the solder, causing the ground to come off and then floating all your carefully bonded metal parts.  OTOH, they don't have exposed pins to get bent like Twist-Locs, and they are considerably smaller and less expensive.

GTD

Thanks Geoff. Yes this will be used in doors, usually in a high school gym.  I was thinking instead of an internal GFCI in the case, how about something like this on the supply cord where it plugs in to venue AC outlet. Then make sure everything on the cart well grounded.  Thoughts?

Gary
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Re: What Happens In This Situation?
« Reply #2 on: January 05, 2016, 01:47:11 pm »


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