Portable stage grounding

[zbang]

Got into a discussion recently with a coworker about this- do portable stages like the one in the picture need ground rods? How about bonding? The supply is 3ph 5 wire, and that generally supplies listed dimmers and PD boxes. There usually isn't a direct bond from the EGC to the structure, although with all the lighting instruments, I'd call it "effectively grounded" if not compliantly grounded. I have my own opinion about this, but want to hear some others.

The stage in the picture is about 40' square and unfolds off a 48' trailer. Setup takes only a couple of hours. (Then there's sound, bands, etc, that part takes a little longer).

 

[LawnGuyLandSparky]

I would guess no. Since to power supply is most likely a portable SDS, a live wire on some exposed metal isn't seeking ground anyway.

 

[don_resqcapt19]

The portable stage is a structure and this structure is fed by a feeder. 250.32(A) applies. That being said, I don't see where the installation of grounding electrodes will provide any safety benefit.

 

[iwire]

The portable stage is a structure and this structure is fed by a feeder. 250.32(A) applies. That being said, I don't see where the installation of grounding electrodes will provide any safety benefit.

Even with everything being cord and plug connected?


Another issue could be the multiple feeders usually brought onto these stages.

 

[zbang]

To combine a couple of answers-
Stages of this size are usually fed by a single feeder (400a) unless they have insane lighting. Larger stages will have multiples, although they might come from the same source.

Definitely a feeder (w/ EGC). I'm not sure why using a SDS matters- when using 'shore power', the service has the SBJ, and when using a large genset, that has the SBJ. (How do I know? I check. Fat lot of good the EGC does without it .)

Multiple feeders is an interesting case, although if it's shore power, there's only one GES and if it's gensets, their grounds there are tied together (I check here, too).

Everything at the stage is cord/plug connected.

I don't think I've ever seen one of these stages to be bonded. I also don't think it would enhance electrical safety. I could see installing rods for lightning protection, although even that's dubious as the entire thing is either sitting on the earth or only separated from it by a 2x8 plank.

 

[cripple]

Portable stage grounding

My opinion Article 520 would cover is installation, and there are not requirement to treat the stages as a building or structure therefore no grounding electrodes required. The grounding requirements can be found in section 520.81 which requires metal frames and enclosures of all equipment to be connected to the equipment grounding conductor. If the power supply is a generator it would need to have a SBJ, and if supplied by a utility the service would need to meet Article 230, and a grounding electrode at the would be required.

 

[mxslick]

I found out the hard way what happens when you use a genny without the SBJ..luckily the equipment damage was minimal.

Thankfully it was my own gear that suffered, not the clients.

 

[dbuckley]

The problem with these types of setup is ground to ground shocks.

The five wire supply may be from some significant distance away, then thus there may be a measurable potential difference between the stage electrcial ground and the soil around the stage. So a bare footed concert-goer standing in the mud that touches something that has a proper ground to the five wire supply may get a tingle, or worse.

Event power is such a problematic area that the British Standards Institute has crafted BS 7909:2008 - Code of practice for temporary electrical systems for entertainment and related purposes because, frankly, qualified electricians were consistantly getting event power wrong.

 

[zbang]

Event power is such a problematic area that the British Standards Institute has crafted BS 7909:2008 - Code of practice for temporary electrical systems for entertainment and related purposes because, frankly, qualified electricians were consistantly getting event power wrong.

I've been trying to get a hold of a copy of BS7909 for a while, but I don't fancy paying $100+ for it myself. I think that ESTA is also working on this, and had some pointers to other docs & policies, but I can't find them at the moment. You're very correct that many people get it wrong.

 

[dbuckley]

It is true - the price of standards in general is outrageous.

Though if I were in the business of doing eventing electrics regularly I'd spring for it.

 

[quogueelectric]

Having worked extensively with stage lighting systems They have common stage pin connectors for the lighting and also many theatres have a 400a switch with a drop of cam lock welding cables . This is for venues which just want a feed and then deal with power distribution from thier own needs. A gut feeling would say for a mobile sytem you should provide some sort of stable ground. Ie 2 ground rods.

 

[LawnGuyLandSparky]

Having worked extensively with stage lighting systems They have common stage pin connectors for the lighting and also many theatres have a 400a switch with a drop of cam lock welding cables . This is for venues which just want a feed and then deal with power distribution from thier own needs. A gut feeling would say for a mobile sytem you should provide some sort of stable ground. Ie 2 ground rods.

I say, in the grand scheme of things it's safer NOT to ground an SDS.

 

[iwire]

I say, in the grand scheme of things it's safer NOT to ground an SDS.

Perhaps if we are talking a very small system but capacitance coupling can lead to fatal shocks to ground anyway.

 

[zbang]

Let's take any SDS out of the picture for the moment. The stage pictured was fed by two feeders (400a and 200a breakers) from the same service and the pad-mount transformer and service disconnect are within 20' of the feeder disconnects. Stage feeders were maybe 150' and went into listed distro boxes. In this case, the supply is already bonded & grounded per code. I suppose there could be a step potential at that distance, however I think the EGCs for the lights will provide some measure of protection to a hot lead touching the stage frame anywhere. I'm sure this protection would be enhanced by bonding the frame to the feeder EGC, but.... that's the original question.

Now, suppose the stage is feed from a generator (with SBJ) that's 300-400' away. Will ground rods at the genny compromise overall safety? I don't think so. Is there a step potential problem at the stage? Well, maybe. Would bonding the stage frame help that? I think so. Is it required?

Article 520.81 could apply, but it seems written to the equipment, not to the structure. Art. 525.30(3) could apply although the staging doesn't necessarily "contain or support electrical equipment". Art. 552 doesn't apply (stage is too big), and I don't see anything in 590, either. About the best I can do otherwise is 250.104(C) and 250.32(A) as Don pointed out.

Me? I think it's a really good idea to bond the frame to the feeder's EGC, but I seldom see anyone do that. I've never seen rods in use.

 

[quogueelectric]

I have seen mobile job trailers pose the same problem you are asking about. People were getting shocked when leaving the GCs trailer into the mud. Upon further reviewI went to yhe panel supplying temp power and noted that all the breakers were bumped up to 30 a on 12 wire. After further investigation the rx had a burn mark down the center conductor blistering the ground wire the entire length of the run. The Qualified electrician knew he had a problem and decided to fix it by putting a larger breaker in. This was the guy my boss at the time saw as one step below Jesus himself. You cant fix stupid!!

 

[quogueelectric]

It is amazing more people dont get electrocuted.

 

[dbuckley]

Now, suppose the stage is feed from a generator (with SBJ) that's 300-400' away. Will ground rods at the genny compromise overall safety? I don't think so. Is there a step potential problem at the stage? Well, maybe. Would bonding the stage frame help that? I think so. Is it required?

A generator fed system is, from the perspective of ground-to-ground shocks, much safer that an PoCo fed system.

Because largely of MGN distribution, voltage drop along the neutral gets refelected into the soil by multiple ground rods, thus there are step potentials all over the place. A genset, on the other hand, with a five wire between the genset and the distro, even over many many feet, doesn't have the same problem, as there is only one neutral / ground bond, and thus the neutral voltage drop does not get reflected to the ground conductor.

Even if a metallic stage structure is not intentionally bonded, it is generally accidentally bonded due to the lighting equipment being affixed to the structure. In my opinion, its still better to intentionally bond the stage metalwork to the electrical ground to stop (again) ground-to-ground shocks as the first few bits of kit go in.

 

[fireryan]

Whats a SBJ

 

[dbuckley]

Whats a SBJ

System Bonding Junction or jumper - where the ground and neutral meet.