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Author Topic: Grounding stage sections?  (Read 1870 times)

frank kayser

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Grounding stage sections?
« on: March 02, 2014, 10:24:37 pm »

So I was describing the all-aluminum stage sections our city puts up for various events, and it occurred to me that I don't know if these should be grounded or not. 


The city supplies about ten 4x8 sections of staging on approximately 2' legs with vinyl/rubber caps.


The sections do not bolt or clamp together - there is some motion among the sections.


If I need to ground them, I'd guess the first task would to be to electrically connect all the platforms together.
I would also assume the safety ground on the 20A GFI feeding the plaza would not be the appropriate place to ground the stage, but to drive a ground rod and connect the stage to the ground rod using heavy copper wire.


I can't say I've ever seen something like that grounded (bleachers at the ball park?).  If these things are not to be grounded, I'd love to hear why not.


frank
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Cailen Waddell

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Re: Grounding stage sections?
« Reply #1 on: March 02, 2014, 10:45:30 pm »

I'm not an electrician.  There may be many things I am not considering.  That said - if all the equipment on the stage is properly grounded, the the chance of the stage becoming a better ground path seems unlikely.   

If it gets struck by lightning - everyone is dead anyway. 
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Craig Hauber

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Re: Grounding stage sections?
« Reply #2 on: March 02, 2014, 10:56:42 pm »

So I was describing the all-aluminum stage sections our city puts up for various events, and it occurred to me that I don't know if these should be grounded or not. 


The city supplies about ten 4x8 sections of staging on approximately 2' legs with vinyl/rubber caps.


The sections do not bolt or clamp together - there is some motion among the sections.


If I need to ground them, I'd guess the first task would to be to electrically connect all the platforms together.
I would also assume the safety ground on the 20A GFI feeding the plaza would not be the appropriate place to ground the stage, but to drive a ground rod and connect the stage to the ground rod using heavy copper wire.


I can't say I've ever seen something like that grounded (bleachers at the ball park?).  If these things are not to be grounded, I'd love to hear why not.


frank

Is it bare aluminum on top?  Most of the ones I've seen are still plywood or some kind of textured coating that's generally of insulating quality.  Just clamping a plugged-in but unused lighting fixture to one of the legs would probably suffice.  I also don't see why the grounding of that GFI receptacle wouldn't work either?
If it's fully floating then grounding could help with with static buildup (like the kind you get shuffling across a large carpeted hotel ballroom and touching the console with an audible 'snap' heard through the rig)
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Craig Hauber
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frank kayser

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Re: Grounding stage sections?
« Reply #3 on: March 02, 2014, 11:01:16 pm »

I'm not an electrician.  There may be many things I am not considering.  That said - if all the equipment on the stage is properly grounded, the the chance of the stage becoming a better ground path seems unlikely.   

If it gets struck by lightning - everyone is dead anyway.


A VERY big if... 
And I'm not sure a grounded stage would mitigate a lightening strike. 


However, I would think a well-grounded roof grid, or lighting truss would offer a fair amount of protection from lightening.  Like a lightening rod on a house, or like a mast in a sailboat - it provides a cone of protection to those on deck.


Is it bare aluminum on top?  Most of the ones I've seen are still plywood or some kind of textured coating that's generally of insulating quality.  Just clamping a plugged-in but unused lighting fixture to one of the legs would probably suffice.  I also don't see why the grounding of that GFI receptacle wouldn't work either?
If it's fully floating then grounding could help with with static buildup (like the kind you get shuffling across a large carpeted hotel ballroom and touching the console with an audible 'snap' heard through the rig)



The aluminum decking has SOME anti-slip coating left, but not what I'd call thick enough or complete enough to be considered an insulator.
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Stephen Swaffer

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Re: Grounding stage sections?
« Reply #4 on: March 04, 2014, 09:19:33 am »

The NEC specifies that any metal object that is "likely" to become energized should be bonded to ground.  Bonded is defined as "intentionally connected" and the usual standard is with a "listed" connector.  I am not sure you can get  "listed" clamp for aluminum tubing-you could use a lug under a bolt. The bonding conductor size would be determined by the largest breaker capable of energizing the platform-ie if only 20 amp brreakers feeding a #12 would suffice.  And bondng needs to be to the service ground-NOT a ground rod.

"Likely" is a pretty vague term.  Bottom line is someone is going to be held responsible if something bad happens.  If you are in a lawyers potential target field, might be wise to consider.  IF all equipment is grounded, you should be ok-but do you have a broken ground pin in the chain?  Are you SURE you don't?
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Steve Swaffer

TJ (Tom) Cornish

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Re: Grounding stage sections?
« Reply #5 on: March 04, 2014, 10:11:58 am »

There are a few different things to consider here: as others have said, grounding is not the same thing as bonding.  Grounding implies a connection back to earth, and if the legs are aluminum, you already have some amount of "grounding" just by them being on the ground.  If the ground is wet and the aluminum is unpainted and in direct contact with the earth, you already have an excellent ground, whether you want it or not.  I disagree with Cailen about being sure that the stage would be a less-preferable path than the actual ground wire - there's potentially a lot of metal with a lot of ground contact, not to mention that there's no way to ensure that every piece of equipment is always properly grounded; - an SO cord could become pinched between stage sections causing a fault during the actual performance, even if the cord was in perfect condition a moment ago.

For life safety, you need to think about the potential for fault currents.  If a metallic object has the potential to be energized - something very possible with lots of power distribution and instruments of unknown condition right on your conductive deck, then it should be bonded to the electrical service, such that GFCIs and OCPDs will function.  This likely also means the metallic object is grounded, but not necessarily (see below).

There are some nuances here depending on the nature of your power source.  The small Honda EU-series generators are unbonded - no ground to neutral connection at the generator.  The EU6500 can be externally bonded, but the 120v-only models cannot be bonded, as they make "balanced power" - 60V hot to ground, 60v neutral to ground.  For these small generators, the ground screw goes basically nowhere, - especially the EU6500. 

If you are using a 120v only Honda generator, your protection comes from the fact that the supply is isolated - there's only one power source - the hot wire of the single circuit, and only one power sink - the neutral wire.  There's actually no way to have a ground fault, as there can be no completed circuit to ground.  A dangerous situation with this type of supply would require two faults - one fault to hot, and the other to neutral.  This could potentially happen with two cord failures - a hot to one deck, and a neutral to another deck.  Touching these two decks would complete the circuit.  Bonding the decks together would eliminate this risk.

If you are using an EU6500 with an external distro, your distro should bond neutral->ground, as this is not present internally on the EU6500.  Per NEC2014, the receptacles must be GFCI, and I would want to bond the deck sections to each other, and to the distro, which creates a fault current path back to the generator that would enable GFCIs and OCPDs to function.

Code requires generators greater than 5KW to have an earth ground rod, I believe.

If you are using shore power, receptacles should be GFCI, and the decks should be bonded together and to ground.
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Tim McCulloch

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Re: Grounding stage sections?
« Reply #6 on: March 04, 2014, 10:18:09 am »

So I was describing the all-aluminum stage sections our city puts up for various events, and it occurred to me that I don't know if these should be grounded or not. 


The city supplies about ten 4x8 sections of staging on approximately 2' legs with vinyl/rubber caps.


The sections do not bolt or clamp together - there is some motion among the sections.


If I need to ground them, I'd guess the first task would to be to electrically connect all the platforms together.
I would also assume the safety ground on the 20A GFI feeding the plaza would not be the appropriate place to ground the stage, but to drive a ground rod and connect the stage to the ground rod using heavy copper wire.


I can't say I've ever seen something like that grounded (bleachers at the ball park?).  If these things are not to be grounded, I'd love to hear why not.


frank

Ask your city manager and inspection department if they've heard of Salina, Kansas and the 12 year old girl who was electrocuted because of apparent non-bonding.  The first lawsuit has been filed, and my guess is the City of Salina or their insurance company will be paying out.  See topic elsewhere in this forum.
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frank kayser

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Re: Grounding stage sections?
« Reply #7 on: March 04, 2014, 12:23:59 pm »

Ask your city manager and inspection department if they've heard of Salina, Kansas and the 12 year old girl who was electrocuted because of apparent non-bonding.  The first lawsuit has been filed, and my guess is the City of Salina or their insurance company will be paying out.  See topic elsewhere in this forum.
Tim,
It was precisely that article that got me thinking about the safety of our staging.  As the outdoor season is rapidly approaching, I would like to be able to put the City Manager on notice of a potential safety issue and be able to state the case in some "intelligent" fashion, and use the Salina, Kansas as an incentive for them to pay attention and act.


Of course, I'd like to know enough to be able to see if something is actually done, and done effectively.
thanks
frank

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frank kayser

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Re: Grounding stage sections?
« Reply #8 on: March 04, 2014, 01:31:21 pm »

There are a few different things to consider here: as others have said, grounding is not the same thing as bonding.  Grounding implies a connection back to earth, and if the legs are aluminum, you already have some amount of "grounding" just by them being on the ground.  If the ground is wet and the aluminum is unpainted and in direct contact with the earth, you already have an excellent ground, whether you want it or not.


I disagree with Cailen about being sure that the stage would be a less-preferable path than the actual ground wire - there's potentially a lot of metal with a lot of ground contact, not to mention that there's no way to ensure that every piece of equipment is always properly grounded; - an SO cord could become pinched between stage sections causing a fault during the actual performance, even if the cord was in perfect condition a moment ago.

<snip - generator discussion>

For life safety, you need to think about the potential for fault currents.  If a metallic object has the potential to be energized - something very possible with lots of power distribution and instruments of unknown condition right on your conductive deck, then it should be bonded to the electrical service, such that GFCIs and OCPDs will function.  This likely also means the metallic object is grounded, but not necessarily (see below).

If you are using shore power, receptacles should be GFCI, and the decks should be bonded together and to ground.


Thanks, TJ, for some very salient points.  Some of it is a bit confusing, and some may, according to my reading here, may or may not be in conflict.  I'd like to explore the answer a bit, if you please. 


One thing that is confusing my direct situation is the generator discussion.  If you please, I am very interested in that, too, but I'd like to explore that a bit in a different post.


I'd like to explore the shore power aspect of your answer in this post.
First, I have to assume the staging in question (aluminum deck and legs) is poorly grounded as it sits.  I have never worked with the staging on anything but a slab of concrete.  (wet and dry). The legs have a vinyl cap to protect a surface from scratching, acting as an insulator.  The  square legs fit loosely into sockets.  Aluminum oxidizes quickly left open to the air, and creates a high resistance surface on the aluminum further degrading the natural grounding.


Second point is grounding vs. bonding.  As I understand it, and discussing only shore power, the safety ground in the GFCI eventually is connected to a ground rod somewhere (close to the power panel, I would assume).  Neutral would be bonded to ground in only the first panel (and only bonded once - not every panel/sub-panel) after the power leaves the pole.  (lay explanation)  So, in a shore power situation, bonding and grounding are the same, correct? (Understanding gennys are a different kettle of fish)


Third point: "... deck, then it should be bonded to the electrical service, such that GFCIs and OCPDs will function"  I'm assuming this is a comment for generators, as your last statement "If you are using shore power, receptacles should be GFCI, and the decks bonded together and to ground"


Fourth point: I fully agree - any cord on this type of stage can (and has) been caught between sections, though we've been lucky to have caught them before the cord insulation could be compromised.


Fifth point: I think I've read here, that just because metal is in contact with the ground (wet or dry), that constitutes a good ground.  I'm out on a limb here, but I'm thinking that just being in contact is inadequate given the length and composition of the grounding rod.


And thanks for helping me gain a better understanding.
frank
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TJ (Tom) Cornish

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Re: Grounding stage sections?
« Reply #9 on: March 04, 2014, 02:52:24 pm »


Thanks, TJ, for some very salient points.  Some of it is a bit confusing, and some may, according to my reading here, may or may not be in conflict.  I'd like to explore the answer a bit, if you please. 


One thing that is confusing my direct situation is the generator discussion.  If you please, I am very interested in that, too, but I'd like to explore that a bit in a different post.


I'd like to explore the shore power aspect of your answer in this post.
First, I have to assume the staging in question (aluminum deck and legs) is poorly grounded as it sits.  I have never worked with the staging on anything but a slab of concrete.  (wet and dry). The legs have a vinyl cap to protect a surface from scratching, acting as an insulator.  The  square legs fit loosely into sockets.  Aluminum oxidizes quickly left open to the air, and creates a high resistance surface on the aluminum further degrading the natural grounding.
"Ground quality" is a continuum.  Leg caps or boots can frequently be compromised, and stage decking gets banged around all the time.  Aluminum Oxide cannot be depended on in any fashion as an insulator. Bottom line - you have to assume that in the absence of intentional insulators, any metal is conductive and potentially able to be energized.  The larger the metal and/or the larger the contact area with the ground, the larger the current potential.  There's nothing magic about a 6' or 8' ground rod in the ground - a few legs of an aluminum stage with compromised feet on wet grass may have a lower-impedance ground connection than an intentional ground rod in drier soil.



Second point is grounding vs. bonding.  As I understand it, and discussing only shore power, the safety ground in the GFCI eventually is connected to a ground rod somewhere (close to the power panel, I would assume).  Neutral would be bonded to ground in only the first panel (and only bonded once - not every panel/sub-panel) after the power leaves the pole.  (lay explanation)  So, in a shore power situation, bonding and grounding are the same, correct? (Understanding gennys are a different kettle of fish)
My language may not have been precise enough to fully explain this - an unfortunate reality of this not being billable time.  Two points here that could be expanded on later:

Point 1:
 GFCIs absolutely need a ground connection to function.  This is implemented by the G->N bond in the service entrance, which has the function of completing the circuit of the earth back to the neutral wire.  This is required, otherwise if you touch the hot terminal of your GFCI protected outlet and ground, there is no completed circuit path for the leakage current to go - your body and the ground you're standing on (or the generator, depending on your perspective) would just float at the potential of the hot wire.  This is exactly the isolating scenario of the 120v only Honda Generators I mentioned - a GFCI would never trip, as there is nowhere else for the current to go, since the only possible current sink is the neutral wire. 

Note 1: A GFCI receptacle itself does not necessarily need to be grounded - they are often used to retrofit homes with no ground wire at the receptacle, however the GFCI depends on the environment - the house, stage, etc., being bonded to neutral (done at the service entrance) for there to be a leakage current sink back to the panel.

Note 2: The intent of a GFCI receptacle is to provide a measure of life safety for currents that leak from hot to ground.  A GFCI receptacle provides no safety for a hot -> neutral fault.

Point 2 (Re: in a shore power situation are bonding and grounding the same thing):
From a cord and plug device standpoint, yes.  The green wire in the cord provides the path to earth ground as well as the bonded path back to neutral.  I used the language I did because in the example of staging given, some may think it would be acceptable to drive a ground rod in at the stage and then it would be "grounded".  This is inadequate, as there needs to be a low-impedance path from the conductive stage back to the G/N bond point at the service entrance to sink fault currents sufficiently well to trip the OCPD.  This, functionally, is bonding -  accomplished with a ground wire.



Third point: "... deck, then it should be bonded to the electrical service, such that GFCIs and OCPDs will function"  I'm assuming this is a comment for generators, as your last statement "If you are using shore power, receptacles should be GFCI, and the decks bonded together and to ground"


Fourth point: I fully agree - any cord on this type of stage can (and has) been caught between sections, though we've been lucky to have caught them before the cord insulation could be compromised.


Fifth point: I think I've read here, that just because metal is in contact with the ground (wet or dry), that constitutes a good ground.  I'm out on a limb here, but I'm thinking that just being in contact is inadequate given the length and composition of the grounding rod.


And thanks for helping me gain a better understanding.
frank
I don't understand your third point question, but absolutely the stage deck should be grounded and bonded in a shore power situation, or any time you have more than a single circuit generator.
« Last Edit: March 04, 2014, 03:05:17 pm by TJ (Tom) Cornish »
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