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Author Topic: Bonding lighting truss to spider box - Is there a safe method?  (Read 1295 times)

Mal Brown

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Bonding lighting truss to spider box - Is there a safe method?
« on: February 09, 2019, 01:20:55 pm »

I have recently bought into a truss rig for lighting.  Installing rubber quad boxes in the non-Totem segments (so far).  Using DT 3900l lifts .  Iím wondering if there is a way for me to ground one of the lift to my spider.

My shows are mostly run off 50 amps so nothing huge.  Once in a while I am running off separate 20 amp 110v circuits in which case I would have to ground to something else.

Yes, I am not an electrician.  And Yes, I want to do this as safely as possible.
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Cailen Waddell

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #1 on: February 09, 2019, 05:09:03 pm »

I have recently bought into a truss rig for lighting.  Installing rubber quad boxes in the non-Totem segments (so far).  Using DT 3900l lifts .  Iím wondering if there is a way for me to ground one of the lift to my spider.

My shows are mostly run off 50 amps so nothing huge.  Once in a while I am running off separate 20 amp 110v circuits in which case I would have to ground to something else.

Yes, I am not an electrician.  And Yes, I want to do this as safely as possible.

If Iím understanding you correctly... the truss is grounded via all the lighting fixtures clamped to it.  Multiple multiple ground points.  You should be ok. 


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Mal Brown

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #2 on: February 09, 2019, 05:24:58 pm »

There have been cases where musicians have been electrocuted by having a mic or guitar in hand and contacting a truss that was charged up somehow.

I sat through an electrical safety class at a Namm this year.  The instructor/presenter talked about grounding the truss in a 3 phase distribution setting.  I believe he used a separate green lug in his disto rack.   

He also checks the truss with a meter and an extension cord ground.

I was thinking maybe I could get there with my spider which is a Leviton ĎThe Boxí GFCI.
« Last Edit: February 10, 2019, 02:04:25 am by Mal Brown »
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Fb Gorge Sound and Light
WillyandNelson.com

Rob Spence

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #3 on: February 09, 2019, 06:43:41 pm »

If you are concerned, run a wire from the truss to your distro. If there is no ground lug available but perhaps an open twist lock connector, put a male end on the wire and plug it in. On the truss you could use a spare fixture clamp and use the fixture screw with a crimped lug on the wire.

I always ground metal stages when I provide power.



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Tim McCulloch

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #4 on: February 10, 2019, 01:35:27 pm »

The most common means of musician electrification is the instrument in their hands and the amplifier it's connected to being energized, and the muso completes the circuit when she/he comes into contact with a grounded object.

While energized truss or other objects happens, too, it's not as frequent as "vintage" guitar amps or bass rigs at full service voltage...

I'd be surprised if a musician can reach your truss.  If you're worried about the crank stands becoming energized because of a light fixture failure (LEDs/movers don't seem to frequently have this problem, unlike poorly maintained PAR cans or aged theater lights) I suggest you insulate the truss from the crank stand's truss carriers.

I'm not suggesting you're on a fool's errand but you're creating another source of potential failure.  Any other conductor that shorts to ground elsewhere in your system will then energize your crank stands and truss until a breaker trips or the dog peeing on it starts yelping.
« Last Edit: February 10, 2019, 01:38:58 pm by Tim McCulloch »
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Mal Brown

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #5 on: February 10, 2019, 06:31:08 pm »

Yep. fixture or cable failure is what I was thinking about.  Sounds like I may be over thinking this...
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Stephen Swaffer

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #6 on: February 10, 2019, 09:43:01 pm »

The NEC requires anything "likely" to become energized in building to be bonded to ground.  I like the term "bonded" better than "grounded"-it means "intentionally connected together with a metallic conductor".  Making sure anything that might become energized has a metallic path to the grounding electrode of the building is key to making sure breakers trip when a fault occurs.

As Cailen mentioned, the fixtures should be grounded.  How often do those bonds and continuity get checked?

The bottom line is that things that are bonded together and bonded to the building grounding system do not electrocute people.
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Craig Hauber

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #7 on: February 13, 2019, 12:09:21 pm »

The NEC requires anything "likely" to become energized in building to be bonded to ground.  I like the term "bonded" better than "grounded"-it means "intentionally connected together with a metallic conductor".  Making sure anything that might become energized has a metallic path to the grounding electrode of the building is key to making sure breakers trip when a fault occurs.

As Cailen mentioned, the fixtures should be grounded.  How often do those bonds and continuity get checked?

The bottom line is that things that are bonded together and bonded to the building grounding system do not electrocute people.
Was helping on a quick setup the other day and noticed that all the fixtures were cheap LED -mostly plastic and really lightweight secured to the truss by ADJ plastic o-clamps (mini plastic cheseboro-looking)  There was nothing on the truss even incidentally grounded as it was sitting on linoleum floor tile. However there was a rats-nest of wiring throughout it so the potential for the whole assembly being made hot did still exist via faulty cordage power strip or cube tap.

So should we be iron c-clamping at least one old conventional up there that's properly grounded through its line back to the dimmer?
-or should one of those water-pipe grounding clamps for residential wiring be used? 

Many of LED lights are 2-prong and with plastic-housings so even using a metal clamp won't ground them.

Unfortunately I also can't envision a single strand of wire clamped to a truss leg and draped through the entire production back to the distro as "code-friendly" 

So what would be the best way to implement this?

Also keep in mind the truss is coated with some kind of clear glossiness so you would have to mar it considerably with a conventional c-clamp or water-pipe grounder to dig through into conductable material.
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Stephen Swaffer

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #8 on: February 13, 2019, 12:24:23 pm »

If it were me,  I would use this:

https://www.cesco.com/Thomas-Betts-T-B--L70-Blackburn-Elastimold-L70-Type-L-Slot-Screw-Mechanical-Lug-Connector-14-AWG-Solid-4-AWG-Stranded-1-Hole-Mount-Copper/p2121042

or this:

https://www.cesco.com/Nsi-Electrical-Products-L614-NSI-L614-Compression-Lug-1-Hole-1-4-Inch-Stud-6-AWG-Blue/p2108303

properly sized to the ground wire, which should be sized to the largest breaker to the truss.  Use a screw in a tapped hole-not a sheet metal or self driller to meet code and bypass any coating.  Ground clamps are also acceptable (they are not resi only) but this would look nicer.  Noting wring with a single strand of ground wire as far as code-but SO might be more durable and draw less unwanted attention.
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Steve Swaffer

TJ (Tom) Cornish

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Re: Bonding lighting truss to spider box - Is there a safe method?
« Reply #9 on: February 13, 2019, 12:36:05 pm »

If it were me,  I would use this:

https://www.cesco.com/Thomas-Betts-T-B--L70-Blackburn-Elastimold-L70-Type-L-Slot-Screw-Mechanical-Lug-Connector-14-AWG-Solid-4-AWG-Stranded-1-Hole-Mount-Copper/p2121042

or this:

https://www.cesco.com/Nsi-Electrical-Products-L614-NSI-L614-Compression-Lug-1-Hole-1-4-Inch-Stud-6-AWG-Blue/p2108303

properly sized to the ground wire, which should be sized to the largest breaker to the truss.  Use a screw in a tapped hole-not a sheet metal or self driller to meet code and bypass any coating.  Ground clamps are also acceptable (they are not resi only) but this would look nicer.  Noting wring with a single strand of ground wire as far as code-but SO might be more durable and draw less unwanted attention.
While not disputing the desirability of grounding metal things that can become conductive, drilling into a rated truss structure isn't something I would be comfortable with due to the structural liability of altering/damaging the truss (and I've never seen this done on any show size, largely because the truss is in the air and therefore not likely to be able to be touched).  Grounding the truss stands would seem to be more effective as they are probably steel instead of aluminum (which oxidizes quickly), and would be more likely to be a shock source due to being on the ground.  There may be a place to put a ring terminal going to an Anderson connector or something, though again if I would have to balance the liability of drilling the stand to get an NEC-approved bonding method vs. using an existing hole for a ring terminal, I think I would choose not modifying the lift in a way that could put me in a position of liability.
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