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Author Topic: Is it just me or is there some really dangerous stuff going on here...  (Read 6347 times)

Mike Sokol

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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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Jonathan Kok

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http://www.proaudiospace.com/forum/topics/ground-issues?commentId=2910192%3AComment%3A184987&xg_source=msg_com_forum
From the post:
Quote
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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Greg_Cameron

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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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Mike Sokol

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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.   
« Last Edit: March 11, 2014, 03:01:16 pm by Mike Sokol »
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Geoff Doane

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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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Stephen Swaffer

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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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Steve Swaffer

Jonathan Johnson

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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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BobWitte

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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.



Jamin Lynch

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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.   
[/quote]

I always read better after a couple of brews too.  ;)
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