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[Cailen Waddell]

So I do some work on a yearly outdoor exhibit at a state fair.

The exhibit takes place in a tent in a parking lot. There are 3 power sources:
1. Whisper watt genny at 277/480 feeding the portable a/c unit
2. Whisper watt genny at 120/208 feeding exhibit lighting and dimmer racks (400a 3 phase unit)
3. 50a 120/240v shore power connection feeding a small power distro

Each genny is grounded on a proper ground rod.  The air handler is grounded to its genny. It does not touch the exhibit except through plastic ductwork.  The lighting genny has a ground neutral bond, and the lighting distro is grounded through the cam lock cables.  All the truss in the tent is ground supported and sits on a temporary wooden floor laid in the tent.  The truss itself is not directly grounded but all the fixtures are, and are clamped to the truss.

Finally the 50a disconnect - this is where i think we could get in trouble.  The tent has a lot of technology in it.  LCD screens, computers, networking, and a little sound gear for a couple presentation areas.   This is run off the 50a distro.   TVs are hung on scenic walls which are wood covered in carpet.  None of this gear touches the truss, although some of the power cables for it runs on the truss.   There are 2 data cables from the lighting rig that connect to two pieces of gear powered from the 50a distro.  There are opto isolators on these dmx lines

Thoughts?  Do I need to be concerned about the lack of ground between any of the systems?  Part of me is paranoid that if something got cross plugged there could be a grounding issue between Systems. 


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[Jonathan Johnson]

If it's at all possible for a person to touch exposed metal parts that are "grounded" to separate systems, then absolutely the separate systems must be bonded. If there is any possibility of a cross-connected ground (such as through the shield of a audio or video signal line), then the separate systems must be grounded, otherwise you could melt the signal line in the event of a fault.

Since you cannot expect that no one will create a cross connection, or that no one will move things around so separately grounded metal is within reach, it would be a really, really good idea to bond everything.

The danger isn't solely in the cross connection, or in the touching-both-systems by itself, but if one of those scenarios plays out in the presence of another fault in the system, bad things can happen. Bonding the systems helps prevent the bad things.

[Mike Sokol]

If it's at all possible for a person to touch exposed metal parts that are "grounded" to separate systems, then absolutely the separate systems must be bonded.

I would agree. See this thread about an unbonded fence that killed a 13-year old at a football game. http://forums.prosoundweb.com/index.php/topic,151746.0.html I've contacted the reporter and local authorities to find out exactly what happened, but it's always difficult to get real answers after the fact. Apparently, a light pole had never been ground-bonded properly, yet had passed inspection. That's also what happened in Seattle last year which electrocuted a dog that was peeing on the light pole that became energized.

It only takes something like an extension cord pinched between metal frames or steps, and then you've got a lot of energized metal. I also think it's also a good idea to do a quick stage test with a NCVT (Non Contact Voltage Tester) after setting up and powering the system. A standard sensitivity NCVT (rated for 90 to 600 or 1,000 volts) will light up from a foot or more away from a large energized surface. That will get your attention.

[Cailen Waddell]

I would agree. See this thread about an unbonded fence that killed a 13-year old at a football game. http://forums.prosoundweb.com/index.php/topic,151746.0.html I've contacted the reporter and local authorities to find out exactly what happened, but it's always difficult to get real answers after the fact. Apparently, a light pole had never been ground-bonded properly, yet had passed inspection. That's also what happened in Seattle last year which electrocuted a dog that was peeing on the light pole that became energized.

It only takes something like an extension cord pinched between metal frames or steps, and then you've got a lot of energized metal. I also think it's also a good idea to do a quick stage test with a NCVT (Non Contact Voltage Tester) after setting up and powering the system. A standard sensitivity NCVT (rated for 90 to 600 or 1,000 volts) will light up from a foot or more away from a large energized surface. That will get your attention.

Thanks all - this is exactly what I needed.   How would you recommend I bond the grounds?  The hvac system is completely isolated.  I'm not worried about that.  I should be able to ask the fair electricians to do anything I need.

Trying to decide where to derive the ground from for the shore power....  From the sub panel in the tent or the pole panel outside?   Do we bond back to the ground rod on the lighting generator? Or am I better running a ground wire directly between the generator lugs and something?   Could it be as easy as two male plugs with just a ground wire connecting them?   The power Distro is in a secure area....

Edit - I should add mike that my crew in my day job, have ncvt's and a multimeter.   We metr and test power always when a temporary setup is going in....

[Stephen Swaffer]

I would prefer to go from generator lugs to outside panel.  The wire should be sized to the largest breaker on a feeder leaving the generator-#3 if it is 400 amps.  Personally, I pefer to make ground connections as solid and secure as possible-I really want someone to have to go find a tool to remove it, if I can.

[Scott Holtzman]

Exothermic material like cadweld is the gold standard.  I have accumulated so many molds over the years I can do some real fancy stuff.

I am also play around with amateur radio.  Mostly shortwave listening so I have about 1/2 dozen feeds coming into the house.  I used a copper bus bar designed for a telco central office and attached Polyphaser silicon avalanche protectors to keep lightning out of the house.  That is bonded to the halo a trench dug around the foundation that has sectional groundrods with a 3/0 buss cadwelded.  This also feeds the service entrance.  I meg it out every couple a years and it has held up for 10 years now.

Bottom line is you can't do too much.  I don't think a cadwelded for a temp generator is overkill.

[Cailen Waddell]

UPDATE:  site electrician has grounded everything correctly for us.

The generator on the other hand almost really screwed us.  I was focusing some fixtures when I noticed they dimmed. Lighting crew wasn't near console.  Went out to generator, had dropped to 90 volts L to N.  I dropped the tent power...  Waiting on generator company to determine issue.  They think it is a bad voltage regulator pot.  I think it's deeper.  Either way, changed the other genny over to 120/208 operation and disconnect our HVAC, got lighting rig back up...  We open at 3p today.... 

I suppose proving why a good ups is important on critical gear....


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