Avoiding a Ground Loop

[Benjamin Guerrero]

For future reference, what is the correct order of operation when connecting cables/powering when you are rigging/dissasembling a camera, so that you avoid possible problems such as a ground-loop frying an SDI/HDMI? I have been taught that the correct order is: Battery -> Viewfinder -> Powering -> Media Storage -> Monitor -> Lens/Focus Motor/Other Accesories -> Matte Box. Is that more or less correct? And after a shooting day of an ongoing production, do you just remove the battery straight up or do you first disconnect some cables? And for wrapping the cam, whats the order of dissasembly?

I suppose some of it is left to taste, but I also can't help but think that the principles are there for a reason more often than not. I also have already read the paper by Arri on this topic, but still, it doesnt seem completely clear to me. If powering all from the same source then should I just leave the SDI/BNC/HDMI cable for connecting last? Then I suppose for dissasembly you just disconnect the cable first then everything else. And also when you wrap for the day, does it matter if you just shut down the cam then pull out the Battery, leaving everything as it is, or is there still a risk for a ground loop even then? Sorry for rambling, I'm still wraping my head around this.

The reason I ask is because, and you probably saw this coming, I own a BMPCC 4k and want to avoid damaging it, as ground looping is a very common problem on the Pockets. But still, I want to learn the principles of it all to get better as a Camera Assistant. Thanks in advance!

Edited July 11, 2023 by Benjamin Guerrero

[Benjamin Guerrero]

A little update: A week or so ago I was 2nd AC in an indie music video, with an FX30, and just as I was plugging the HDMI in, the cable lightly hit the border metal plate of the input and it sparked... Thankfully all was well with the plug, and we did end up changing the cable and had no further problems, but to say I was barely scared the moment I saw electric sparks fly would be a huge understatement. Soo, yeah... This is why I'm asking this question in the first place. Hope somebody with more experience than me can chip in on the topic! Thanks in advance.

Edited July 28, 2023 by Benjamin Guerrero

[David Mullen ASC]

I'm not an AC nor an electrician so I can't answer about protocol, but the two times I've been "zapped" on set has been due to power flowing from a BNC cable into the camera. Once it was when a BNC ran from camera to a video village and the script supervisor asked for a reading light so the electrics plugged in a 150w movie lamp on a stand with a hand dimmer on the line, running into the same power strip as the monitor. Turned out that the hand dimmer had been wired backwards and this caused the distro box to send electricity through the BNC back to the camera.

[Benjamin Guerrero]

On 7/28/2023 at 3:24 PM, David Mullen ASC said:

I'm not an AC nor an electrician so I can't answer about protocol, but the two times I've been "zapped" on set has been due to power flowing from a BNC cable into the camera. Once it was when a BNC ran from camera to a video village and the script supervisor asked for a reading light so the electrics plugged in a 150w movie lamp on a stand with a hand dimmer on the line, running into the same power strip as the monitor. Turned out that the hand dimmer had been wired backwards and this caused the distro box to send electricity through the BNC back to the camera.

You're always so kind to answer Mr Mullen. That sounds super scary, thats exactly what I'm worrying about. Did the Video Out kept working after that? I know some high end cameras have surge protection in their connectors. In the kind of productions I work in, I fear messing up the only video output lower/mid end cameras have and having no monitors for anyone the rest of the day, although messing up a video out of a higher end camera doesnt bode well either. How would you and your team handle a problem like that, a camera without video output for the day?

Edited July 30, 2023 by Benjamin Guerrero

[David Mullen ASC]

2 minutes ago, Benjamin Guerrero said:

You're always so kind to answer Mr Mullen. That sounds super scary, thats exactly what I'm worrying about. Did the Video Out kept working after that? I know some high end cameras have surge protection in their connectors. In the kind of productions I work in, I fear messing up the only video output lower/mid end cameras have and having no monitors for anyone the rest of the day, although messing up a video out of a higher end camera doesnt bode well either. How would you and your team handle a problem like that, a camera without video output for the day?

The camera was fine.

I’ve never had the video out not work but if it did it would just be like shooting film before the days of video assist. Mainly the focus-puller would have it the hardest, especially since an EVF is not a great way for an operator to judge focus. We’d be on the phone for a replacement camera with the rental house.

[Benjamin Guerrero]

3 hours ago, David Mullen ASC said:

The camera was fine.

I’ve never had the video out not work but if it did it would just be like shooting film before the days of video assist. Mainly the focus-puller would have it the hardest, especially since an EVF is not a great way for an operator to judge focus. We’d be on the phone for a replacement camera with the rental house.

Huh, it didn't occur to me that it would be like shooting film, but as you said that too comes with its challenges. Makes absolute sense to call the rental to get a replacement, I'll keep it in mind if it ever happens to me. Nowadays most productions do ask for live monitoring as a requirement, and it just makes life so much easier on set. Thanks again Mr. Mullen for sharing your experience. It is always valued.

 

If anybody else can pitch in about protocol for camera cabling, please do. I think it would be of great value for AC's, Video Assistants and DITs!

Edited July 30, 2023 by Benjamin Guerrero

[Dom Jaeger]

Arri released a technical bulletin outlining the correct procedure to avoid shorting the SDI boards. This advice relates to all modern cameras:

https://www.arri.com/resource/blob/194752/d3093e6af632150787ec95d176a39958/download-technical-information-data.pdf

Here's a more informal explanation:

https://www.newsshooter.com/2020/02/15/preventing-damage-to-sdi-outputs/

Essentially, you should always connect the power cables for ALL accessories first, then connect the BNC cables. When tearing down, do the reverse and disconnect all BNC connectors first, then unplug the power cables. 

[Benjamin Guerrero]

5 hours ago, Dom Jaeger said:

Arri released a technical bulletin outlining the correct procedure to avoid shorting the SDI boards. This advice relates to all modern cameras:

https://www.arri.com/resource/blob/194752/d3093e6af632150787ec95d176a39958/download-technical-information-data.pdf

Here's a more informal explanation:

https://www.newsshooter.com/2020/02/15/preventing-damage-to-sdi-outputs/

Essentially, you should always connect the power cables for ALL accessories first, then connect the BNC cables. When tearing down, do the reverse and disconnect all BNC connectors first, then unplug the power cables. 

I was familiar with the paper! For some reason it didn't click before, but now it does. I guess there's still some doubts I have about the issue. What about powering from different DC sources? Are the power cables themselves the only thing that matter, not the power source? i.e. Could you have all cables running from a V-mount plate and just swap batteries from the plate itself without worrying about damage? If so, then you could have everything already plugged in and just chuck in the Vmount battery last. Is that right/safe?

 

Either way, it just seems easier to remember the mantra of power first, BNC last. And of course, thanks for taking the time to reply  Dom!