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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Greg_Cameron on March 11, 2014, 12:12:37 PM
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http://www.proaudiospace.com/forum/topics/ground-issues?commentId=2910192%3AComment%3A184987&xg_source=msg_com_forum
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http://www.proaudiospace.com/forum/topics/ground-issues?commentId=2910192%3AComment%3A184987&xg_source=msg_com_forum
I've got a bad feeling about this. I've seen video trucks do the AC isolation transformer lots of times, but his reasoning about ground loop hums being due to pulling power from multiple phases of 3P power is incorrect. It's all about ground voltage differentials which cause ground loop current which causes 60 (or 50) Hz hum, both audio and video. But this really has nothing to do with the line phase. Also, there's no mention of earth grounding this rig, and since you'll likely have multiple crew members tied to this power, many of whom could be standing in water (video camera operators for a football game, for instance) I think that earth grounding and neutral bonding this isolated AC power would be very important.
I'll read this through again tonight and see if it makes more sense after a brew or two. But at first read I'm not liking it a lot.
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http://www.proaudiospace.com/forum/topics/ground-issues?commentId=2910192%3AComment%3A184987&xg_source=msg_com_forum
From the post:
And no chance of electrocution as there is no connected ground to complete a fatal connection. You're isolated. Which is the whole idea.
Umm...except you, of course. YOU make an EXCELLENT ground. Well, OK...you make a rather poor ground. But when you're the ONLY ground, well, you'll do just fine.
Wasn't there a previous post here regarding RemyRAD and his 'knowledge'?
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I've got a bad feeling about this. I've seen video trucks do the AC isolation transformer lots of times, but his reasoning about ground loop hums being due to pulling power from multiple phases of 3P power is incorrect. It's all about ground voltage differentials which cause ground loop current which causes 60 (or 50) Hz hum, both audio and video. But this really has nothing to do with the line phase. Also, there's no mention of earth grounding this rig, and since you'll likely have multiple crew members tied to this power, many of whom could be standing in water (video camera operators for a football game, for instance) I think that earth grounding and neutral bonding this isolated AC power would be very important.
I'll read this through again tonight and see if it makes more sense after a brew or two. But at first read I'm not liking it a lot.
At minimum, it sounds like some serious code violations going on with that setup.
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From the post:Umm...except you, of course. YOU make an EXCELLENT ground. Well, OK...you make a rather poor ground. But when you're the ONLY ground, well, you'll do just fine.
This is exactly what makes a floated generator dangerous in a distributed power situation. If you're only connected to one thing such as an electric drill at a construction site, then dropping the electric drill in the water won't electrocute you since the generator chassis-ground is isolated from the earth. So the generator chassis is now 120-volts above earth potential, and as long as you don't touch it then there's no problem. But hook a second drill up to the same isolated generator, then the first drill dropped in the water will electrify the generator chassis, which in turn electrifies the second drill chassis in the hands of the other worker.
That's why we should earth-ground generators at outdoor concerts. You can live with one electrically floated piece of gear, but two pieces of gear becomes dangerous because of this reflected hot-chassis condition. And as we all know, in any outdoor event there will be DOZENS of devices hooked to the generator (or isolation transformer) power. And a single failure from something like the old amp the guitar player piked up at a pawn shop can elevate your local ground plane. You REALLY want your local ground plane to be at earth potential. Yup, your sweaty body may not be a perfect ground, but it's good enough to dump 100 mA or so of current through you and your heart. At just 20 mA you can't let go of the metal railing shocking you, and 30 mA for just a few seconds almost guarantees your heart will go into fibrillation and death without CPR and an AED. And 10 mA of shocking current seems to be the low end of where someone with a weak heart or pacemaker is in danger.
Earth grounding of AC power is a good thing, but poorly understood by many sound and video technicians and even electrical engineers I've spoken to.
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http://www.proaudiospace.com/forum/topics/ground-issues?commentId=2910192%3AComment%3A184987&xg_source=msg_com_forum
I have a hard time following anything that RemyRAD character writes about.
The "everything must be on the same phase" mantra is an easy one to debunk however. The studios I work in at the day job have 3 phase power, with loads distributed across all three, and things are perfectly quiet. If we can do it, so can you!
I never did quite figure out where (s)he was headed with the truck power thing though. Our mobile uses a transformer with a multi-tap primary (180 to 260 V in 10V increments), and then a 120-0-120 secondary. The centre point of that secondary is ALWAYS grounded, usually to the same place the 208/240 or whatever is coming from. It does require some operator skill to select the correct tap before powering up the rest of the truck (there is a 250V full scale voltmeter across the secondary of the transformer), but it provides relatively isolated power from either single or 3 phase sources.
The air conditioning and lighting are fed directly from the transformer secondary, and all the technical equipment is fed from a pair of double conversion UPSs, one on each leg of the secondary.
And although it could be run without the ground, it would be extremely unwise to do so. On one remote I worked, where temporary power was supplied, the electrical inspector would not give his blessing until the ground was connected from the chassis of the truck to the electrical service ground rod with one piece of cable. NO cam-locks allowed! I guess he figured it was too easy to cheat if there was a connector in the ground line. At that point the ground is probably there for lightning protection as much as for some sort of fault in the truck. If the truck chassis is floating, almost anything can elevate it to dangerous levels.
GTD
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To be a stickler on code, the only vehicles the NEC applies to are mobile homes and RVs-so a mobile video setup would not necessarily be covered by the NEC, That is not to argue that following the NEC grounding rules would be a bad idea by any means! It does sound like the inspector wanting see an unspliced conductor to a ground rod was following code-if I ever have to tap or splice a conductor going to a grounding electrode, I am required to use an "irreversible" method of connection so his refusal to accept cam locks makes sense "by the book".
What makes this REALLY scary is someone who has all the answers (in their own opinion)-but it is very obvious to others that they really are way off base in some areas. They are unteachable and a hazard to those around them.
I have used isolation transformers to isolate electrical noise before-but it has never required "lifting" a ground to take care of the issue. IF there is a noisy ground issue, code does allow for isolated grounds (individual ground conductors back to the grounding electrode)-but I find hard to believe that properly grounding a truck would cause a problem that could not be solved without taking dangerous shortcuts.
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On one remote I worked, where temporary power was supplied, the electrical inspector would not give his blessing until the ground was connected from the chassis of the truck to the electrical service ground rod with one piece of cable
I can't cite the code, but I believe that between a service entrance panel and the grounding electrode system, the grounding conductor may not have any splices. In a typical installation, there are two grounding rods; a single, continuous wire is run from the panel to the first ground rod, where it is run through the acorn clamp uncut and on to the second ground rod. Alternatively, two conductors are installed from the panel; one to each grounding rod. The inspector was probably interpreting this requirement to apply to your situation.
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Just to clarify - as clarity is really important when dealing with this. The original post referenced an article where the power neutral and safety grounds were isolated? Correct? That is how, in the example of the electric drill dropped in the water example, that no protection is given and the safety ground can become energized? Correct? Which of course is a very very very dangerous situation.
If the neutral and safety grounds are connected at the generator (or power source) and the drill is dropped in the water (or audio gear fails), then the circuit protection would trip.
We aren't necessarily discussing the generator ground rod issue discussed elsewhere - right?
Thanks.
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I'll read this through again tonight and see if it makes more sense after a brew or two. But at first read I'm not liking it a lot.
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I always read better after a couple of brews too. ;)
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If the neutral and safety grounds are connected at the generator (or power source) and the drill is dropped in the water (or audio gear fails), then the circuit protection would trip.
Well, if the isolated transformer or generator (really the same thing) have the neutral and ground bonded together, but no earth ground rod connection, then dropping a drill in the water WILL NOT cause the circuit protection to trip, especially if it was a double-isolated drill without a ground wire. Instead, the transformer or generator neutral-ground bond and anything connected to it will become energized above earth potential and present a shock hazard. Now in a perfect world, a GFCI on the circuit feeding the primary failure should trip, thus protecting the secondary person in the mix. Remember, the GFCI feeding the secondary circuit isn't in the mix since the GROUND wire is what's energized. And we all know that GFCI's are often bypassed for iive shows due to nuisance tripping. That's why hot-grounds are so dangerous. There's nothing to disconnect you from them, no circuit breaker or GFCI or even a power switch. That's because according to code there must be no interruption of the safety ground back to the service panels G-N-E bonding point. And that's exactly what's missing from the isolated transformer scenario in the initial post.
I've thought about mocking this up and gathering some data for a demonstration, which I think would be very interesting. My plan was to do this with a generator once Honda supplies the EU2000i for my experiments. I can also try it with an isolation transformer and show that as well.
Getting interesting, isn't it? ;D
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Ok, got my mind wrapped around that. So, awhile ago there was a discussion about whether generator needed a ground rod? Sounds like it should always have one. My next concern is that the portable stage the "city" provides - it is a trailer that opens up to a stage, needs the metal stage itself to be grounded - hopefully is via the power connection, ground to chassis to the generator. If not, then that could float even with a ground rod at the generator….. Very interesting. We bought the Fluke voltage detector (name escapes me - volt alert?) to check power, guest gear, etc. but the stage continuity to ground can be checked with an ohm meter I assume?
Now, to explain that to the city crew that drops off the generator and barely know how to start it….
Thanks
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Very interesting. We bought the Fluke voltage detector (name escapes me - volt alert?) to check power, guest gear, etc. but the stage continuity to ground can be checked with an ohm meter I assume?
While you can use a Fluke VoltAlert (or similar Non Contact Voltage Tester) to find a "hot" metal stage. It will not detect one that's just floating, but not grounded. The fall-of-potential testers I have uses a short grounding rod about the length of a screwdriver. So sticking a screwdriver in the ground and measuring between it and the stage should give you a pretty low reading, typically between 25 and 100 ohms. Now remember that you can't have any power applied to the stage during this test since that could blow up your ohmmeter. However, measure it for AC voltage first. If there's more than a volt or so difference between the stage and your grounded screwdriver, I would get very suspicious of the grounding. As I've posted on other threads, even an overhead high-tension power line can induce voltage in an ungrounded stage. So I expect there will be SOME residual AC voltage on an ungrounded stage. But I'm not sure exactly what the "normal" voltage would read to a grounded screwdriver. I do know that if your stage is 40 volts AC above ground it will cause a standard sensitivity Fluke VoltAlert to beep/flash, and that a 40 volt shock to wet feet could be deadly.
More to think about, but I believe that ALL portable generators should be earthed to a ground-rod, and have one point of ground-to-neutral bonding. That's how I've always done it for concert power, and that's how I interpret the code. Don't confuse this with the NEC allowance of a floating neutral on the generators under 5 KW. Anything over 5 KW is supposed to be G-N bonded at only ONE point, which can be inside the generator itself, or inside your CB-distro panel. Same goes for an isolation transformer, except in this case you carry the neutral and ground from the main service panel through your "isolated" power transformer. And yes, they need to remain separated there since the incoming service panel performs the single-point G-N-E bond.
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100% agree. Now to convince the city folks with there 55KW diesel Wacker generator that a ground rod is needed. I should find a Wacker owners manual. I assume that the requirement would be in it. They do not understand the unit at all. I have made adjustments on it when the voltage control was set to its lowest setting and we were having issues with power.
Anyways - this forum is awesome!
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100% agree. Now to convince the city folks with there 55KW diesel Wacker generator that a ground rod is needed.
Remember that before you drive any ground rod(s) you need to check for any underground utliities. Of course if this is a city crew they should know already, correct? ;D
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Remember that before you drive any ground rod(s) you need to check for any underground utliities. Of course if this is a city crew they should know already, correct? ;D
They should, BUT I will fully inform them of what they should do.... We place the stage the same place for 5 different Tuesday nights so this should also help them put the stage in the same location every time.
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Having followed this thread, it seems that even when using a small, portable Honda 3000 watt genny, a ground rod would be a good idea.
I don't see any ground connection point on these small gennys.
Thoughts or experiences?
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I downloaded a couple of Wacker operator manuals. Proper grounding is in the manual. I then searched a bit and found this thread. Note the comments about a ground rod providing "no iota of safety".
http://forums.prosoundweb.com/index.php?topic=135427.0 (http://forums.prosoundweb.com/index.php?topic=135427.0)
How can mis-information be purged....
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Having followed this thread, it seems that even when using a small, portable Honda 3000 watt genny, a ground rod would be a good idea.
I don't see any ground connection point on these small gennys.
Thoughts or experiences?
Here's my thoughts, but I'm going to run this by my Honda technical support contact first. If you look at my article about neutral bonding Honda generators at http://www.noshockzone.org/generator-ground-neutral-bonding/ you'll see that I've made a simple Edison kludge plug with a jumper between the Ground and Neutral screws. This has been tested lots of times by hundreds of RV owners running Honda 2 and 3 KW generators, and Honda tech support has admitted (off the record) that it's a workable fix. Now imagine adding one more wire coming from this G-N bonding point to a piece of #6 stranded wire that's connected to a properly earthed ground-rod. That's exactly how your home or business system is "earthed" by the PoCo. You should also run a heavy (#6) wire from from the metal stage (big clamp) to the same ground rod bonding point as well. This is all electrically valid and to code (as far as I can tell), but I don't like the fact that kicking out the G-N kludge plug accidentally opens up both your G-N bond as well as the Earth connection.
If you all agree that this is a workable solution, I'm going to run it by my code monkeys at Mike Holt's forum to get the NEC spin on this. But the NEC does NOT require grounding of small portable generators ONLY because they're considering you'll only have one thing at a time plugged into it, such as the drill at the construction site example I used at the top of this thread.
This is exactly the type of experiment I want to do with the EU2000i that Honda has promised to send. Once I get that done and vetted by my NEC contacts, I'll publish it as an official fix. In the meantime, this is all experimental guessing on my part. An educated guess, to be sure, but until I set up the test and gather data I don't want anybody risking their lives. So please, let's critique this proposed fix rigorously.
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Mike,
I agree that your solution is NEC compliant. I am trying to remember and will check code later today (book is the office),but do non-bonded gennies also have to have gfci protection?
I am not sure of the purpose for not bonding the neutral to ground. A 3-5K genny is plenty to run multiple pieces of equipment, and if you are using a drill with a grounded case that shorts hot to the case, you now have energized the genny with no path to trip OCPD. Touching another piece of equipment that uses a polarized plug could potentially put you in the circuit with 120 V between neutral and "ground". Seems dangerous to me without a GFCI to detect an imbalance between hot and neutral on a given circuit?
IMO the ground-neutral bond is the most critical, but a ground rod still plays an important part in safety-especially in wet conditions. Though lightning protection is the primary purpose, keeping the frame/ground of the genny bonded to earth should prevent any significant voltage difference between the two.
Slightly OT but relevant. Yesterday I dealt with a farmers bin that had had some bad things happen-arced the bus on a QO panel,burnt a contactor coil, etc. I finally realized the panel was fed 3 wires-2 hots and a neural, the EGC had not been carried to the bin. There was no neutral bond to the panel and all wiring relied on conduit to panel for EGC. bottom line, no metallic fault path from ground to neutral. I found where a rodent had chewed through insulation, but with no metallic path things just arced and burned instead of tripping the OPCD. Out of curiosity, I dropped one probe of my voltmeter in a puddle against the concrete pad of the bin (the bin/pad should be equivalent to a concrete encased electrode and arguably equivalent to a metal stage sitting on damp ground)) and measured 1.56 volts-but that was with things running and no faults. I do not think I would want to touch the bin with a wire shorted to the conduit! Rewiring the bin was already in the works, and though I do not have to have ag jobs inspected, you can bet there will be grounding changes.
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I am not sure of the purpose for not bonding the neutral to ground. A 3-5K genny is plenty to run multiple pieces of equipment, and if you are using a drill with a grounded case that shorts hot to the case, you now have energized the genny with no path to trip OCPD.
There's an exception in the code which allows for portable/contractor generators under 5KW to not require GFCI protection if they have a floated neutral. I think this was originally added to avoid GFCI nuisance tripping at work sites, but has since been adopted for all small portable generator situations. The second reason for the floated neutral (at least from my discussion with Honda tech support) was that if you used your little 2KW generator to power your entire house during a power outage via a transfer switch, then the house circuit panel would provide the G-N bond, and a second G-N bond inside the generator was a code violation. Of course, that secondary G-N bond would also trip any GFCI in the path during safe use. So while that's true, very few small generators are tied into a house circuit panel via a generator transfer switch. The flip side of this is that many contractor generators OVER 5KW already have an internal bonded neutral, which makes it a code violation to tie it into your house panel via a transfer switch. Yikes! >:(
This little bit of intel took me a long time to gather simply because of the confusing and incorrect use of words such as grounding, safety-ground, bonding, earth-grounding, etc... If you note, I use the term G-N-E bonding when discussing circuit panels because that's where the neutral bus, safety-ground bus, and ground-rod all come together physically and electrically (or at least they're SUPPOSED to).
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I have installed quite a few generator transfer switches set up for 4kw to 7kw generators-have to love that 5 kw break they put in there! Pretty common here (rural Iowa)-a few years back we lost power for 4 days-no heat and no well pump makes for conditions that really aren't intolerable-but "we" have been accustomed to such comforts!
I understand the second bond is a code violation-but to me it would seem to be a less serious issue than a missing bond but what do I know? It also seems really odd that all outdoor recpts have to be GFCI, as well as all outlets used during construction have to be GFCI-but a generator, which you can not safely run indoors does not need to be GFCI. Must be nice to not have to be reasonable!
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I have installed quite a few generator transfer switches set up for 4kw to 7kw generators-have to love that 5 kw break they put in there! Pretty common here (rural Iowa)-a few years back we lost power for 4 days-no heat and no well pump makes for conditions that really aren't intolerable-but "we" have been accustomed to such comforts!
I understand the second bond is a code violation-but to me it would seem to be a less serious issue than a missing bond but what do I know? It also seems really odd that all outdoor recpts have to be GFCI, as well as all outlets used during construction have to be GFCI-but a generator, which you can not safely run indoors does not need to be GFCI. Must be nice to not have to be reasonable!
NEC2014 requires all "user accessible" receptacles to be GFCI if there is the possibility of multiple circuits - i.e. 240v operation. In the case of a single circuit floating generator, there's no circuit for the fault current to go to that would trip the GFCI.
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Getting back to the RemyRAD character. It would appear that he is using a Signal Transformer Model DU-7.5, 7.5 KVA Power Isolation Transformer. When he speaks of 3 phase power, he is only using one phase and stepping 208V down to 120V.
http://www.signaltransformer.com/home/step-updown-power-isolation/
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Getting back to the RemyRAD character. It would appear that he is using a Signal Transformer Model DU-7.5, 7.5 KVA Power Isolation Transformer. When he speaks of 3 phase power, he is only using one phase and stepping 208V down to 120V.
http://www.signaltransformer.com/home/step-updown-power-isolation/
It also seems Remy is saying the there is no neutral ground bond on the secondary side of that transformer either in the transformer or load center. If that's the case, that could be very dangerous. Waiting for clarification on that one.
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It also seems Remy is saying the there is no neutral ground bond on the secondary side of that transformer either in the transformer or load center. If that's the case, that could be very dangerous. Waiting for clarification on that one.
That's what the link suggested, a totally floating secondary on the iso transformer with no connection to the PoCo neutral or ground connections. And I agree that it's a very dangerous situation.
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That's what the link suggested, a totally floating secondary on the iso transformer with no connection to the PoCo neutral or ground connections. And I agree that it's a very dangerous situation.
NEC 250.21 Lists AC systems that need not be grounded. Audio/Video equipment did not make the list. 250 .20.B requires a grounded system anytime you are working with 120/208/240 systems with a common conductor if you are supplying any type of distro.
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NEC 250.21 Lists AC systems that need not be grounded. Audio/Video equipment did not make the list. 250 .20.B requires a grounded system anytime you are working with 120/208/240 systems with a common conductor if you are supplying any type of distro.
The only isolated power I worked with was for very specific test gear (we were building and measuring R2R ladders for missile guidance systems) and there were all kinds of safeguards in place. And even these isolated power systems had their own ground rods that were kept completely separate from the PoCo ground and any building steel. We were required to do regular measurements on the isolation transformer as well as our isolated ground rod as part of our measurement SOP.
None of this isolated power is safe or relevant for any type of AV system, and a violation of code.