ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 ... 7 8 [9] 10   Go Down

Author Topic: Generator question  (Read 38147 times)

TJ (Tom) Cornish

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 4317
  • St. Paul, MN
Re: Generator question
« Reply #80 on: September 23, 2015, 10:25:01 PM »

Given the presence of the GFCIs, the ground rods almost certainly serve no effective purpose whatsoever.  The generator sitting on soil, grass or concrete will have a low enough impedance to soil to provide a path sufficient to trip a GFCI should a fault to soil occur.  Its a bit of a misnomer, but the ground loop impedance required to trip a GFCI is somewhere around 10K ohms.

The NEC is a bit of an ass about installation ground rods anyway, which serve a somewhat different purpose in an installation, but that is a digression.  One can have one ground rod, if the measured impedance is less than 25 ohms.  Or two ground rods, with no requirement to meet any impedance objective.  Thus to achieve code compliance the easiest way is to just drive two ground rods, job done.
Agreed that GFCIs and ground -> neutral bonding are the big deal.  As mentioned above, NEC 250.34 exempts "cord and plug" generators anyway, so unless Jamin's jurisdiction has a special requirement, a ground rod falls into the "nice to have" category (assuming a bonded generator with GFCIs).
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2671
Re: Generator question
« Reply #81 on: September 23, 2015, 11:40:49 PM »

Where I live the ground is like frickin concrete....really. Driving a rod in the ground isn't easy.

What if I'm actually on a concrete or asphalt drive and they don't want a rod driven through? I was on an asphalt drive last Saturday and they wouldn't let the tent folks drive their stakes in.

Grab a teenage guy eager to prove his manhood and let him have at it!

If they don't want a rod driven through, have them sign a release taking responsibility for any bad things that happen as a result-not likely much of a defense-but it might make them think a bit if their name is on the dotted line.

Actually 250.34 exempts generators using "cord and plug connected equipment".  Some inspectors and industry people would argue that distribution wiring (temporary or permanent) doesn't meet the definition of equipment found in Article 100.  A discussion beyond the scope of this thread for sure.

As for bonding Honda generators, let me throw out this thought.  Maybe Mike will want to split this off.

If I am using a typical genny to provide backup for my house with a 4 wire connection, the G-N bond is in my home panel.  If I use the same genny with a distro, G-N bond can be in the distro preferably with an external lug for bonding ground rods. stages, etc.

With a one circuit genny, why not use a similar scheme.  A 120 V 20 cordset to a box with a G-N bond and a GFCI installed which supplies all connected equipment?  Then, if something disconnects the G-N bond, power is disconnected as well.

Same could be done with a 2 circuit genny.  Both 120 V circuits can be run with 3 wires each to one box.  Internal to the box, both neutrals and grounds are connected in one common bond.  Then, no matter the combination-circuit A only, B only or A and B you have one and only one G-N bond-and again if that bond is lost so is power.  This should provide all of the safety of the transformer style bond with a lighter and less expensive setup.  It is failsafe and simple to use and requires no genny modifications.  If made commercially, I am sure an engineer could design a plug that would block the second outlet of a duplex on the genny-this being the main weak link in this scheme.

Thoughts?
« Last Edit: September 23, 2015, 11:55:11 PM by Stephen Swaffer »
Logged
Steve Swaffer

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Generator question
« Reply #82 on: September 23, 2015, 11:44:41 PM »

Given the presence of the GFCIs, the ground rods almost certainly serve no effective purpose whatsoever.  The generator sitting on soil, grass or concrete will have a low enough impedance to soil to provide a path sufficient to trip a GFCI should a fault to soil occur.  Its a bit of a misnomer, but the ground loop impedance required to trip a GFCI is somewhere around 10K ohms.

Remember that another important function of ground rods is to provide an earth-ground path for a nearby lightning strike. That's also why the wire connecting to the ground rod (the Grounding Electrode Conductor or GEC) shouldn't have any sharp bends, and soldering isn't allowed. Lighting has such a high rise-time and peak current, that a sharp bend offers enough impedance that lighting can jump off of the wire, and solder would of course melt and act like a fuse.

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3209
  • Southwest Washington (state, not DC)
Re: Generator question
« Reply #83 on: September 24, 2015, 01:41:24 AM »

I use a pair of vice grips...

Vise Grip is a brand of locking plier.

Vice grip probably has something to do with a prostitute's technique.
Logged
Stop confusing the issue with facts and logic!

David Buckley

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 565
Re: Generator question
« Reply #84 on: September 24, 2015, 06:46:53 AM »

Remember that another important function of ground rods is to provide an earth-ground path for a nearby lightning strike. ..........
Not going to disagree a word of your post.

But, in context, its not relevant, and doesn't provide a reason to have an earth rod.

A tiddly Honda generator is just a couple of foot tall, and as such would be really unlucky to be the subject of a direct strike.  The stage setup which the generator is powering is a far more attractive target, and may well get struck.  However, the generator is not necessarily bonded to the metalwork of the stage (unless you get the vice grips and a green wire!) or have an unintentional connection, DI box on steeldeck, or parcan bolted to scaff pole.  If the stage does get struck, then the stage is likely in direct contact with the soil or concrete and that will be the path that most current will take.  The cable between the generator and stage will, as you note, act as a choke, and thus there will be little current getting to the genset, and thus any ground rod would be moot anyway.  A direct strike will inflict considerable damage on everything on the stage and connected to it and nearby it.  This protection is really about induced currents from nearby strikes which is far more common, and again, because of the fast risetimes, the same considerations apply.

So, again, in this type of operation, with ground/neutral bonded, and GFCIs, a ground rod doesn't add any safety to the setup.  In entertainment electrics, where there are electrical risks, GFCIs are simply the best thing since sliced bread, and correctly used, will save lives.  If your show gets whacked by lightning, then all bets are off.  Its best not to have anyone anywhere near the stage or anything connected to it if the lightning breezes through.
Logged

Steve M Smith

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3381
  • Isle of Wight - England
Re: Generator question
« Reply #85 on: September 24, 2015, 07:25:40 AM »

Vise Grip is a brand of locking plier.

Vice grip probably has something to do with a prostitute's technique.
It's more to do with correct English spelling vs. your Webster corrupted version!


Steve.
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Generator question
« Reply #86 on: September 24, 2015, 07:49:06 AM »

Not going to disagree a word of your post.

But, in context, its not relevant, and doesn't provide a reason to have an earth rod....


While I'm talking about ground rods in general for permanent structures, I do believe they provide additional equipment protection from lightning for outside shows. Now I'm not talking about a direct lightning hit as you imply. There's virtually NO WAY to protect gear from that amount of voltage and current. I'm referring to nearby ground strikes which branch out over the surface of the earth for many hundreds of yards. And of course this isn't just a single generator poking up above the earth. It's many hundreds of feet of XLR and AC cable snaked around the ground to the FOH mixer, speakers which are often flown 20 feet or more above the earth, and the stage itself. Without solid earth grounding this becomes a great antenna for capturing and distributing lightning into something called a "side flash" where the lightning exits multiple pieces of gear and enters the earth, destroying all sorts of things.

Now, we could start an entire thread on lightning safety for humans in the area and probably should. But in this particular case I'm talking about saving the gear AFTER you've evacuated the stage crew, musicians and crowd. 

Also, while GFCIs are great for protecting humans using backline and frontline gear, I'm not sure they're the best idea for protecting your amp racks. In that case, there's typically some sort of twist-lock plug feeding a rack of gear with internal distribution to multiple amps. As I've noted before, everything leaks a little current to chassis ground, so a rack of ampllfiers fed by a single GFCI with a 5mA trip point will probably nuisance trip too often to be useful. And separate GFCI circuits for every amp in a rack won't fly. However, I do think that industrial level of GFCI trip points (30 mA) for an amp rack would likely be useful, but probably not to code. Yikes!!!

So the real question is how important it is to drive a ground rod for a portable generator on a small stage? And since everything's not going to be GFCI protected, and there's power distribution to multiple locations (Stage, Monitor World, FOH Console, etc...) then I believe a ground rod should be standard procedure for portable generators used for festival power. In recap, I believe neutral bonding and earth grounding your portable generator, then using GFCI outlets for stage power, and making sure the metal stage is bonded to the generator ground should be standard procedure.

Yeah, ground rods are a real PITA to remove, so let's get working on an alternative to the standard 8 ft rod driven in with a min-sledge hammer. I'll show you my plans next week for discussion by the swarm.

Jamin Lynch

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1999
  • Corpus Christi, TX.
Re: Generator question
« Reply #87 on: September 24, 2015, 09:34:08 AM »

Would the same things apply to let's say a 25K Whisper Watt?

All the ones I've seen were just dropped off by the rental company, started up and plug and play. I've asked about ground rods and such but only get a dumb look just before they drive off.
Logged

Stephen Swaffer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2671
Re: Generator question
« Reply #88 on: September 24, 2015, 10:08:19 AM »


 However, the generator is not necessarily bonded to the metalwork of the stage (unless you get the vice grips and a green wire!) or have an unintentional connection, DI box on steeldeck, or parcan bolted to scaff pole.



These types of unintentional "bonds" are very dangerous-they can create a hazard yet not be a good enough connection to clear a fault-far better to have a solid bond and a ground connection.  Really can't leave safety to happenstance.


Another thing to keep in mind is that there is nothing magical about "ground rods".  A metal light pole is fastened to a concrete pier that comes close to meeting the requirements of a concrete encased electrode.  If this is a venue that is frequently used, consider having them install a permanent grounding point..  A local venue I use has a FOH location set up in front of the bandstand with a buried snake and power in a fiberglass underground box-pop off the cover and you have access, put the cover on and it provides no hazard for other uses at the park. Do something similar but stick a ground rod in.

If there are stands that are attached to concrete piers-again you have a decent Ufer ground.  If there is a tent, use one of their tent stakes if you can.

IMO, safety is important enough that we should not give reasons (excuses??) that it can't be done or that it is inconvenient.  The reason venue's don't make provision is because we let them get away with it.  I understand that if you are the jerk that won't just do it then it can cost business-OTOH if you present a reasonable suggestion-maybe an idea no one has thought of you might get them to appreciate it. A fiberglass hand box and ground rod wouldn't run over $200-consider a couple trips to the chiropracter and that wouldn't be a bad investment if it is a repeat venue for you. 
Logged
Steve Swaffer

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3361
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
Re: Generator question
« Reply #89 on: September 24, 2015, 12:52:16 PM »

If there are stands that are attached to concrete piers-again you have a decent Ufer ground.  If there is a tent, use one of their tent stakes if you can.
I did a gig last month where we had to park the 25KW genny in the parking lot on the asphalt, so I wasn't going to mess up their parking lot with a ground stake. But there was a tent already pitched nearby on the lawn with big 3 ft stakes and I had a clamp big enough to attach the genny ground wire to a stake already in place. Best part was I didn't have to drive anything or take it out later since that was the tent company's job. Still, if I had the ground rod and a mini-sledge I would have installed it, even though that wasn't "supposed" to be my job. Oh well...

Now, no inspectors showed up, even though we had a head's up that one would likely stop by. And I did confirm the neutral-ground bond on the genny was about 1/4 ohm, and we were on a concrete pad without a metal stage so no bonding to the stage  was needed. And while I wasn't grounded with an 8-ft ground rod, everything measured correctly with a Ground Loop Impedance Tester (of course) except for one of the stage distro boxes supplied by the rental company. The contractor who brought the stage power was a little embarrassed to find a broken ground lug on the distro connector. But that made me feel like I was doing my job.

As part of my bed-o-nails testing, I'm going to compare the ground impedance of a 3 ft tent stake to an 8 ft ground rod and my bed-o-nails Sokol Ground. And I do know a few guys on the NEC code committee who are discussing RV grounding requirements, so I can actually discuss this with an NEC committee that can give us an official answer that could be written into the next code cycle. I don't want to propose anything unsafe or illegal, just come up with a methodology that we all can use.     

ProSoundWeb Community

Re: Generator question
« Reply #89 on: September 24, 2015, 12:52:16 PM »


Pages: 1 ... 7 8 [9] 10   Go Up
 



Site Hosted By Ashdown Technologies, Inc.

Page created in 0.032 seconds with 21 queries.