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[Rob Spence]

I always isolate outputs to others (video feeds or band recorder for example) and inputs from consumer devices. Most often for laptops and phones I use a Radial SB-5 which is convenient for having a built in 1/8" trs cable. I also use the Radial PCDI.


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[Tim McCulloch]

thank you Tim!
I await Mike's revert and i do hope he stumbles in here, I have learnt so much from his generous advise! But do take the time to share your impressions.

Also, what sort of isolation would you be using in the shop ? We use the DBT+ http://www.laaudio.co.uk/product_dbt_1339.aspx by LA Audio.

Thank you
Sidhu

Lift the grounds with the provided switches.  DBT looks like a nice, basic device that does exactly what you need.

The Radial AV-1 and AV-2 and Whirlwind PC-DI are in my personal inventory.

Sketch out the diagram Mike asked about.  That's kind of where I started (in my head, not on paper) trying to figure out how unexpected voltage got into the console.  The AC service polarity flip and combination of grounded and ungrounded AC circuits between production elements is, I think, the key to this.

[brian maddox]

This topic is one that should always be revisited from time to time.

Recently did a job where we had to feed a lobby system that was about 1000 feet away and of course on a different power supply.  The signal was run analog on a custom multicore that also carried 2CAT5 signals and 20 AMP power to the stage RIO8 box about 400 cable feet away.  The rest of the way it was run on a regular XLR cable.

Anyway, when our A2 went to plug something in to the input on that RIO box with everything connected, he got bit by a pretty good shock.  This of course prompted a lot of metering and such to make sure we weren't doing anything super dangerous.

In the end it was determined that it was a combination of induced voltage on that custom RIO cable along with disparate voltages on the local and distant grounds that caused the episode, and while there was little amperage and therefore little real danger, it was still a bit alarming.

I say all that to say this.  If I had simply had the good common sense to put a transformer on that distant line, none of it would have happened.  I know that 10 years ago i would have never run such a long cable to a distant system on a different ground without one.  But days of snakes being run over fiber and UTP have made me more complacent about ground loops and other potential dangers.  And complacency has no place in this business.

Lesson Learned.

[Mike Sokol]

I say all that to say this.  If I had simply had the good common sense to put a transformer on that distant line, none of it would have happened.  I know that 10 years ago i would have never run such a long cable to a distant system on a different ground without one.  But days of snakes being run over fiber and UTP have made me more complacent about ground loops and other potential dangers.  And complacency has no place in this business.

Especially true if you're running an audio feed 500 ft to the video truck. Those are often powered from a completely different service panel, POCO transformer, or even a generator with no ground at all. You really don't want your audio feed to become the ground fault path if something goes wrong on the far end of the line. If it does, there's usually too much series impedance in your line so the over-current circuit breaker won't trip immediately. But there could be a dozen or more amperes of fault current in the shield of your XLR feed, and that's enough to cause them to melt down right in front of your eyes. If there was a really low resistance path, then the over-current circuit breaker on the far end would trip quickly, limiting damage. That's why the EGC is supposed to have an impedance back to the G-N bonding point of less than 1 ohm. You want that circuit breaker to trip quickly in the event of a line-to-chassis short, not just sit there and cook your wires. 

[Scott Holtzman]

Mike nailed it on the head.  Any reversed cables on the distant end and signal cables become the fault path with sometimes spectacular consequences.  The potential is only limited by the series loss of the cabling.

I recall I had a UPS in my home office at it turned out it was wired backwards for years (neutral and hot reversed).  My give away should have been I got a shock one day hooking up the cable modem.  I passed it off as an issue with the cable distribution amp as there was no current, the voltage was very quickly shunted.

We were without power for about a week after the East Coast winter storm about 6 years back.  We had been running for days off a construction level generator.  I had one TV in the living room run off an extension cord up to the UPS to smooth out the power from the generator.  The cable company forgot to gas up their little generator on the pole mounted amp that was running our block.  I pulled out a laptop and patched it into the TV and we were watching something.  The battery in the laptop wore down so my wife grabbed the charger and plugged it into wall outlet that was being fed by the generator.  That was all she wrote.  That HDMI cable was able to carry enough current to blow the laptop up in spectacular fashion. 

[Mike Sokol]

Mike nailed it on the head.  Any reversed cables on the distant end and signal cables become the fault path with sometimes spectacular consequences.  The potential is only limited by the series loss of the cabling.

I think these three sentences from Sidhu tell the story: Inspection then reveals that all power stringers used by the video crew (Suppose only for laptop and switcher) were 2 core power cables, so no ground. Live and Neutral on the same were reversed. And my crew connected to an alien audio source without galvanic isolation.

This was probably a classic RPBG (Reverse Polarity Bootleg Ground) ground fault scenario. As you can see from the diagram, anytime there's 2-conductor wiring without an EGC (safety ground), there's the possibility that someone wired a "grounded" outlet up on the far end. And if you also notice swapped hot and neutral conductors on the wiring, then an RPBG is at least at 50/50 chance. As I've noted here before many times, you may never know it's an RPBG outlet if you only plug in non-grounded gear. And a standard 3-light outlet tester won't warn you at all, and tell you all is well with two amber lights and no red light. Even if you do plug in a piece of gear with a grounded plug to an RPBG outlet, everything will operate normally, even though its chassis is now energized to 120 or 230 volts. But interconnect that "hot-chassis" gear to your own properly grounded equipment, and your HDMI, SDI, XLR or USB cable becomes the fault current path with full circuit breaker current available. And that's when the fireworks start. 

[Nitin Sidhu]

hello Mike!

i got to speak with my crew today, and have my impressions as stated by them. I take everything with a grain of salt unless measured by myself. Im flying to the middle east tomorrow, and hopefully will have time to elaborate with a diagram here.

Thank you for sharing your diagrams. I see in both scenarios stated, an RBPG is in play. I know that the auditorium was commissioned by professionals, so I will be very surprised if something as a bootleg ground is in play.

I do know that there was a swap of Hot-Neutral on the video vendor stringers. I also have been told that the 3-phase power supply (control room + Backstage) was from same source. We used 1 of those phases.

Thank you.

[Mike Sokol]

I do know that there was a swap of Hot-Neutral on the video vendor stringers. I also have been told that the 3-phase power supply (control room + Backstage) was from same source. We used 1 of those phases.

A hot-neutral swap by itself shouldn't cause a fault current. But it's possible that some piece of gear on the far end had a hot-to-chassis short. I saw one of those over the summer when a bass player had a power cord on his amp taped up as it passed through the bushing hole where the power fed through. Yup, no rubber/plastic bushing. Just bare wires rubbing on the sharp metal of the chassis.

[Speed Daemon]

As an aside, I just found an outlet in my house that had no neutral, and a bare copper ground wire connected to the neutral.  Someone had used a length of Romex to wire up a light switch, then added a duplex outlet using the same hot wire and the ground wire.  This really irked me because I had double-checked that same outlet myself during the electrical inspection, and recall the inspector dismissing my inquiry about visually inspecting an outlet box with "this thing tells me everything" (holding up a $5 3-light tester that had just failed to detect the ground fault).  Just a reminder that it's just as possible here in the US of A.  So far I've found 3 wrongly wired outlets that the inspector missed.

Ever since I started using computers for music playout, I've stuck with Toslink to electrically isolate the IT from the AV parts.  When I can, I keep the power on different legs, and use separate AC filtering to keep any switch-mode PSU back currents out of my old-fashioned pi filter-supplied power amps.  (I know, the DAC is both fish and fowl, but at least it's the only suspect.)

At one of my work gigs, I occasionally have to take laptop audio from a podium (usually in hotel ballrooms) to the mixer.  The AV contractor (usually the hotel) supplies a 3.5mm TRS to dual 1/4" TS cable and a standard direct box.  It works, but doesn't provide a high-Z load to both 1/4" inputs, effectively shorting the two stereo channels together.  (My own stereo-mono transformer DI box has a 470 Ohm resistor on each tip, and insulated TS jacks to make sure that the floating ground of the laptop is not tied to the earth ground of the mixer by default.)  So far no problems, but I wouldn't do it that way.  As far as I'm concerned, a laptop with a "brick" for AC power is the same potential danger as the dodgy guitar amp that shocks its owner every time his lips hit the mic windscreen.  If you can't isolate it with glass fiber, use a Whirlwind Isopod or similar product that has a transformer and no ground tie.

I hope I'm not getting too off-topic about this, but I'm not sure what the EP cable is, and am not sure that there is a neutral with 240VAC in India.  The same principle applies to video--there are isolation transformers for HD-SDI that used to be used in analog times to remove "hum bars".  It's also noteworthy that some UPSes have galvanic isolation, but most don't.  In this case, having a UPS with an isolation transformer built in saves one component that you might leave behind by accident!

http://www.apc.com/us/en/faqs/FA157465/

[Daniel Levi]

In India 240V will be L-N-E supply i.e. live and neutral with a protective earth, IIRC the standard socket is the olders BS546 standard similar to the modern BS1363 but with round pins, no fuse and there is separate 6A and 16A plugs (the original British standard being 5/15A). The larger of these plugs being used as the standard plug for stage lighting in Britain.

The Suko type plugs originally mentioned were probably referring to Schuko (CEE7/3 & 7/4) which whilst they may fit in an Indian socket there will be no earth connection.

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