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deadly mic update


Doug Brandon

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The following post stems from the posting 'deadly mic-technique' under GENERAL DISCUSSION

http://jwsound.net/SMF/index.php?topic=483.0

I recieved responses both in public and private on the issues I raised concerning electric safety and the sound team on the set.

I contacted Bill Whitlock who has lectured to the IATSE 695 sound union in these regards. He holds the title: President and Chief Engineer, Jenson Transformers, which manufactures many of the professional generators we use in high budget film production. Coincidentaly, Mr. Whitlock was asked by the National Systems Contractors Association to investigate the tradgedy I referenced where Kyle Lake, a Pastor in Waco, Texas was handed a microphone while standing in a baptismal pool. 800 congregants helplessly witnessed the horror as he was electrocuted to death.

I've included in this posting, the e-mail I sent to Bill Whitlock, followed by his response.

Greetings Mr. Whitlock,

I posted an article on Jeff Wexler's audio forum titled 'deadly mic-technique' wherein I described an incident that took place on a movie set. You may view the post at:

http://jwsound.net/SMF/index.php?topic=483.0

I am a graduate of the University of Michigan program in music technology [*note* run by Mary Simoni for those of you following the post 'where are the women in sound'], and have also installed/re-wired basic house, studio situations. I am a member of IATSE local 695 (y-8) microphone boom operator and am researching the worst possible scenerrio of standing in water while holding a boom pole. Some things seem obvious to me, but not to others, regarding the potential danger of running sound equipment off a location generator that may or may not be grounded to earth. I read your article on prosoundweb.com:

http://www.prosoundweb.com/studyhall/sr/whitlock/grounding.php

and it makes sense there's a resistance between a genny's grounding rod and the dirt of the earth, up to 25 Ohm (?).

Also stated in the article, 10 mA [milliAmp] is enough to cause involuntary muscular contractions, and 50 to 100 mA through the chest can induce ventricular fibrillation which can lead to death.

In my posting I made refrence to a preacher who was killed while standing in a baptismal pool during a batmism and, after reaching for the microphone, was electricuted in front of 800 congregants. I assume this was due to the body of the microphone being grounded, and that somewhere in the system there was a faulty wire.

I am looking for any information regarding the dangers while recording sound on a film set, for a boom operator standing in water, holding a boom pole with a microphone, with or without the sound cart on independent battery supply.

The sound mixer will often operate his/her system on a 'car-sized' 12 volt battery utilizing transformers/inverters to fullfil the mixerboard's and other equipment's 120 volt needs. The microphone operates on a much lower voltage. However, aren't the grounds connected together at some point? Wouldn't a faulty (charged) wire, anywhere in the equipment--coming in contact with the 'ground' anywhere else along the ground chain, be connected to a microphone's grounded body? And if you were standing in a lake, wouldn't one's body be much less resistance than the grounded rod of the power generator (for those mixers that for some reason are running off genny power), or a rod grounding the sound cart (which one respondant stated he does), and therefore, if you were to make an adjustment to the microphone (and suppose you had water on your finger) wouldn't you be at risk for grounding any electron leakage from any equipment being powered by the generator? Does it place the boom operator at risk when operating off battery power, where much lower amperage is involved?

Some of the replies (from around the world) feel there is no risk at all. Others agree the potential for danger is extreme and this situation should be avoided (by using 'AA battey' wireless transmitters from the boom operator rather than  XLR cabling to the cart--while filming in water).

My technical references are crude, I hope they make an order of sense. I would like to create some kind of article pinpointing the dangers for the boom operator in a particular circumstance such as this. I know in the film/television industry, many times a boom operator puts him/herself at risk without realizing it. I would like to make some statements backed up with facts such as, the microphone body does in fact connect with the ground of the mixerboard power supply. Do you know this to be true? Can you advise/help me in any way? Point to already existing material?

Thank you for your time and consideration.

Yours,

Doug Brandon

Hi Doug,

Thanks for writing. I've attached the special column I wrote about the Waco electrocution for Live Sound magazine (I do ongoing columns, call "Clean Signals", for them).

[*note from Doug* I couldn't figure out how to attach the attachments from yahoo--there was no actual address I could access. The articles were similar in manner to the article I pointed out above]

I'm in the midst of preparing for the seminar I'll teach at next week's CEDIA (home theater) convention in Denver, so I don't have time to properly address all the issues raised by your incident. In summary, though, the mixer is guilty of placing other people in harm's way simply because he clearly doesn't understand what safety grounding is all about. He's the one that should be in trouble!

Even if the portable generator(s) are grounded to earth (actual soil), the resistance of such connections isn't nearly low enough to protect against shock or electrocution (and certainly not via a trailer post sitting on top of the ground). Earth connections are basically about lightning. Should there be a signal cable from the audio equipment to another cluster of equipment powered by another source, a fault to earth ground (or a wrong connection) in that cluster could easily force the voltage (relative to earth/soil/water) of the audio safety ground system to lethal levels. 

And you are absolutely correct that safety ground is carried through the shield of the mic cable to the housing of the microphone itself (in every case I know of - it is standard practice for shielding purposes).

When I return from Denver, I would be happy to elaborate if you like.

In the meanwhile, I've also attached a copy of the class handout I use in my seminars - it addresses the difference between safety ground and earth ground, which are NOT interchangeable!

Incidentally, I was recently contacted by Sally Krause about doing another presentation to Local 695.

Cordially,

Bill Whitlock, president & chief engineer

Jensen Transformers, Inc.

(from my home office)

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I have my cart powered by mostly DC, and on one show I was feeling power on the cart.  I was only able to track it down by elimination, and found that it was coming from the video assist person.  Unfortunately, some of these operators are using equipment that is supplied to them by a company, and they may not be that familiar with the operation and/or troubleshooting their systems.  Up until that time I hadn't thought of "receiving" power from the video assist operator's gear.

For the reasons you mentioned, I've been trying to convince my mixer ('like that–– 'my mixer') to go with a wireless video transmitter/receiver from video assist. He's a bit stubborn in some things, especially when it means placing expensive equipment at risk, like leaving video transmitters with a video assist person. Also, as we know, sometime we get interference from these transmitters. Maybe not so much with microwave, but they have inherent problems as well.

Thanks for the support.

Doug

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Hey doug-

A fellow boom op told me about this post after I told him the story of what happen on set yesterday (9-15-06). after reading your post I have to tell you  kudos for standing your ground! Let me tell you my story of what happen on set working on a commercial with Carmen Electra- We were in San Pedro filming in these shin deep tide pools. Luckily it was MOS- *side note* not that it would matter since none of my mixers would be irresponsible enough to send me cabled into the water- Well I was on the shore watching the shoot when I saw the DP, who was hand held and using battery power, scream then dropped to his knees and dump the camera into the ocean! the Dolly grip the jumped after the camera, tried to pick it up, screamed and was shot back into the water. Luckily both guys were ok and continued to work the rest of the day- can't say as much for the camera though! so what or who was the responsible party here? It was caused by bad wiring on the VTR cart that was sending out about 60 volts through the ground. The BNC cable was electrifying the the camera and zapping anyone who touched it! And yes our sound cart was plugged into the VTR cart. VTR was forced to use a remote to send a picture and had to plug into his own put put the rest of the day!

I hope you have more than one mixer you can work for so as not to have to work for an asshole like that again! I'd love to get email you privately to find out who this mixer really is and I can be sure not to have anything to do with him!!!! 08mc@sbcglobal.net if you want! Besides his lack of safety knowledge, why would you want to work with a mixer who isn't gonna have your back! My favorite and most annoying part of your post was the  let's see 1, 2, 3, 4, boats sitting there doing nothing. Come on now that's just a pathetic mixer- hope he has had a chance to see what joke he is becoming!

good luck and stay safe!

Mitch

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VTR carts are well known for strange wiring protocols. I am some what of a technicial guy but try as I might, over the years, I never found a way to isolate a sound kart from the Video guys hum and noise.    So long ago I gave them a comtek feed for audio to avoid any chassis AC leakage. Always be wary of any other external devices!

I also used all battery operation which was "float charged" by what ever AC line I plugged into.  For a potentially unsafe AC situation all that was necessary was to "Pull the AC" plug and run on batteries until safe conditions resumed.  Water Work is one of these dangerous situations but some others are: 

Any generator that has its neutral kicked loose will often send 220+ volts down the 120 volt line you are connected to!

A grounded AC line (metal plug shield at ground potential) can be very dangerous if you plug it into a defective

appliance (such as a work light with a frayed ac line touching an open ground)  because YOUR BODY BECOMES THE CONDUCTOR TO GROUND.  This happened on one of my sets to a Video man-who barely survived.  ALWAYS WEAR GLOVES! 

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Guest Jeffrey Colon

VTR carts are well known for strange wiring protocols. I am some what of a technicial guy but try as I might, over the years, I never found a way to isolate a sound kart from the Video guys hum and noise.    So long ago I gave them a comtek feed for audio to avoid any chassis AC leakage. Always be wary of any other external devices!

I also used all battery operation which was "float charged" by what ever AC line I plugged into.  For a potentially unsafe AC situation all that was necessary was to "Pull the AC" plug and run on batteries until safe conditions resumed.  Water Work is one of these dangerous situations but some others are: 

Any generator that has its neutral kicked loose will often send 220+ volts down the 120 volt line you are connected to!

A grounded AC line (metal plug shield at ground potential) can be very dangerous if you plug it into a defective

appliance (such as a work light with a frayed ac line touching an open ground)  because YOUR BODY BECOMES THE CONDUCTOR TO GROUND.  This happened on one of my sets to a Video man-who barely survived.  ALWAYS WEAR GLOVES!   

One can always check for potential from a mic or equipment chassis to an earth ground by using a meter.  I have always checked for voltage between amplified musical instruments and stage rails and microphones... usually it's a vintage two wire tube amp that will present some problems... from a lip buzz on the mic to a deadly voltage potential.

A poorly wired VTR or light housing (i.e. not grounded with a short to chassis) and it's voltage potential to someone who is grounded is absoulutly deadly.  In my ENG EFP world I am rarely dealing with shore power, but I have seen more than a couple of pignose ground lifters used on lights in older buildings.  I have tossed many of these ground lifters (widowmakers) in the trash.  I am appalled at the either ignorance or faith shown by many techs when it comes to electricity.

  I try to keep my gear transformer and wireless insulated... Today 50% chance of thunderstorms... I promise I won't be standing under a tree with a carbon fiber pole... but that is a whole nother issue.

Jeff C.       www.nolavox.com

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With all this talk of zapping the boom op, I have to wonder how many people use wireless boom around here? I personally love it, and use it whenever I'm able to be close to set. I have 2 comtek BSTs, so I can send PL to the Boom op(s). I have had zero problems with interference (Yet) and if so, I have my snake one click away...

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With all this talk of zapping the boom op, I have to wonder how many people use wireless boom around here?

I know I am totally in the minority here, but we almost NEVER use wireless for the boom microphone. Only when the shot cannot be cabled will Don opt for going wireless for the fishpole. This may change in the near future, even for us, as production shooting styles change with more shots requiring a wireless approach. Also, with the Zaxcom Digital Wireless having superior audio to most all other systems we have used, and with Zaxcom coming out with the TRX-990 all-in-one boom box, we may find ourselves doing more wireless boom work.

Regards,  Jeff Wexler

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Hey Jeff, I guess I'm in the minority as well as to cable Vs wireless.  Might be us older timers remember all to well the radio mics we came up with, not a pretty picture or sound track as I recall. The new products are changing the dynamics of this choice these days and I realize the value of wireless boom oping. We use this technique for steady cam shots alot, or for shooting in the alleys of L.A. or the other nasty locations we encounter in our set life. Maybe the new Zaxcom units will change my mind about cable, but to my ears we give up something when we record wireless boom. This is probably an old arguement, but it's my 2 cents

Max Respect, Old School

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Lastly, a note regarding the topic "deadly mic update," we should all use an isolation transformer when feeding audio to video assist operators.  This will help stop the transmission of AC back down the cable to our carts.  Unfortunately, receiving video from them is still an issue.

Does not the Jensen ISO Max transformer that Wolf recommends offer protection from the picture feed from video assist?

My audio feed to video village has been 100 % wireless since 1999.

My stolen cart was 12 volt with a swithing power supply floating to keep the battery topped off.  If the shore power (or genny) went to 240V, only the switching power supply would suffer.  The new cart will have an upgraded supply similar to the old one plus isolated DC feeds using DC to DC converters.

The thing that scares me about wireless boom is the same thing that scares me about all the wirelesses I am buying since the break in.  When will the 700MHz stuff be blown off the air?  It's crazy for a small guy like myself to buy a $6,000 wireless that may be useless in 18 months.

David Terry

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