stef Posted December 12, 2024 Report Posted December 12, 2024 Basically, adding power to a cart is like plugging in a big power strip on your cart. Yes, that should probably be a 3-prong grounded cable. But should I bond that ground conductor to the metal cart and rack, or should I actually try to insulate against grounding anything (In the US, if that matters, and it must be within NFPA/NEC codes. I have a codebook but have no idea what section it's in because filmmaking uses a lot of standalone generators.) I can see issues either way. One issue is if it's on genny power and someone touches both the grounded cart and something on house power, there could be a high enough voltage bias between grounds that could shock. But if you have failing equipment on the AC side, it might be safer to have the cart grounded properly to the genny's ground, with a proper ground wire on any AC equipment. A second, related question is if the cart has a 120v inverter on it, like a car inverter, should chassis/neutral be bonded to the cart, too? These generally do not have a connected ground, but I believe they can be chassis grounded and that will act as 0V to bias the waveform around it. (Yes, I realize it is somewhat redundant to have an inverter plus an active genny powering it, but an inverter can run off your battery to power any AC devices without worrying about whether you lose power.) Quote
Philip Perkins Posted December 13, 2024 Report Posted December 13, 2024 Most of my carts used equipment that ran on DC power supplies, ie were made to run off batteries (I know, quaint). The solutions I used for powering them were at first always big gel cel batteries (even on a stage) with bespoke DC-DC convertors, then later the excellent old PSC "Cart Power' box (AC to DC, with power distro). The Cart Power had a 3-pin AC plug, and I don't recall any issues with using its U-ground really ever. My power issues were never with properly designed professional audio gear, they were always with flaky cheap monitors, low budget powered speakers, work lights or other lesser gear that did not have good power design and ran off wall-warts. Quote
The Documentary Sound Guy Posted December 13, 2024 Report Posted December 13, 2024 I'm curious too ... seems like a good question for testing the knowledge of your gaffer... Quote
RadoStefanov Posted December 13, 2024 Report Posted December 13, 2024 Why would you need an inverter? Quote
Paul F Posted December 13, 2024 Report Posted December 13, 2024 Regarding your first scenario, it's not clear to me what you mean by "touches....something on the house power". I assume you mean they are in contact with house ground and the genny ground and you are concerned about the differential voltage being harmful. The following suggests that that is not a possibility if the practices called out by Local 728* are followed. It states "When mobile generators supply power to location production systems IN ADDITION TO the building's electrical system, the generator's grounding connection SHALL BE BONDED TO THE MAIN BUILDING GOUNDING ELECTRODE SYSTEM AT THE SERVICE." (Their caps, not mine) *Local 728 paper entitled: Basic Electrical Safety Precautions for Motion Picture and Television Off Studio Lot Location Productions . Published August 01, 2000. Issued by the City of Los Angeles Dept. of Building and Safety - https://cinematography.net/edited-pages/Generators-----Grounding.htm Quote
Philip Perkins Posted December 13, 2024 Report Posted December 13, 2024 If there was both genny and local power available, and you really did need to power from something other than your own DC system, I'd want a pretty good reason to not use the local (wall) power. I've been really hosed by relying on genny power, even when the electrics were friendly. You don't see cameras running on AC, right? Quote
Derek H Posted December 14, 2024 Report Posted December 14, 2024 Look for the “cable connection” series of articles by Jim Tanenbaum in 695 quarterly editions of old. Lots of good info on this subject. Quote
PMC Posted December 16, 2024 Report Posted December 16, 2024 On 12/13/2024 at 5:19 PM, Philip Perkins said: I've been really hosed by relying on genny power… Add me to that list. Long ago It only took one time of the genny op pulling power on me without notification for me to go out and buy a UPS (uninterrupteable power supply) for my cart. Quote
stef Posted December 18, 2024 Author Report Posted December 18, 2024 On 12/13/2024 at 3:26 PM, Paul F said: Regarding your first scenario, it's not clear to me what you mean by "touches....something on the house power". I assume you mean they are in contact with house ground and the genny ground and you are concerned about the differential voltage being harmful. Yes. And I'm not saying we're actually using the house power, but maybe someone leans on both the cart and grabs the washing machine next to it. Or an exposed pipe. That can cause a differential between two grounds, which shouldn't ever be much of an issue, but all depends on the genny being grounded properly, which is a point of failure. But thanks for that answer, it answered another question I didn't know I had. On 12/13/2024 at 1:20 AM, RadoStefanov said: Why would you need an inverter? Why would I need AC power when I can just run on batteries? I can give 10 scenarios where I might want an inverter, but that's not relevant to whether to bond an inverter to a cart. Plenty of other departments probably have to consider this, too. On 12/12/2024 at 9:35 PM, Philip Perkins said: The solutions I used for powering them were at first always big gel cel batteries (even on a stage) with bespoke DC-DC convertors, then later the excellent old PSC "Cart Power' box (AC to DC, with power distro). Hah! That's what I'm using (cart power II). I'm leaning towards "Yes" and "Yes" for both questions, but I don't have a definitive, verifiable answer. Quote
stef Posted December 18, 2024 Author Report Posted December 18, 2024 On 12/13/2024 at 3:26 PM, Paul F said: Local 728 paper entitled: Basic Electrical Safety Precautions for Motion Picture and Television Off Studio Lot Location Productions This safety bulletin is really interesting. They say that everything has to be bonded to the frame of the generator, but if it's not possible to completely insulate the genny from earth, then they have to make a very solid ground connection. That makes a lot of sense. Basically, this means there's no way touching just the hot wire from the genny could shock you, because it cannot complete a circuit if the genny is isolated from earth ground (while you're standing on earth). However, that also tells me the genny might frequently be isolated from earth. In that case, bonding a cart to the ground pin could be a very bad thing if I can't guarantee the cart is also insulated from ground. (If an accident or equipment failure, or even a lightning strike happened, it could cause the cart to act as earth ground, and all sorts of things could go sideways, and I'd bet most of us don't want our cart acting as the sole earth ground in such an event.) I realize the lunchboxes placed around sets to tap power are all probably bonded and sitting on earth, but still.... This is enough to convince me the cart should NOT be bonded to the ground pin of incoming power, because we cannot guarantee it is insulated from earth, which the genny might be. It also tells me any AC equipment (required to use 3 prongs) should be insulated from the cart. Yikes. The answer to both questions is probably No and No. And now I have to check if my Cart Power 2 has a chassis ground, and if so, might need to insulate it from anything metal. If someone has valid information, especially contradictory, please let us all know. Quote
Philip Perkins Posted December 18, 2024 Report Posted December 18, 2024 Dudes...stay off the genny power.... Quote
Derek H Posted December 18, 2024 Report Posted December 18, 2024 If you use rack gear and run AC into a power supply/distro like a PSC or Meon or even into a Furman you may find that the cart ends up grounded through the rack rails. Pretty sure on my Furman at least there’s continuity between the safety ground pin and the rack ears. You could insulate it I suppose. But I think you’ll find ground continuity all over your cart once cabled up. For what it’s worth we always plug into whatever power our electric dept makes available to us and have never had an issue. Maybe we’ve just been lucky. Quote
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