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[Nitin Sidhu]

Hello!

GLD112 + Stage Racks.

Gig in an auditorium. We setup sound for live performances outside of the glass window control room. Control room has Projector and video, so video crew sets up there. The video crew give us Laptop audio out (via laptop audio out). Laptop VGA out is connected to a video switcher, video switcher is connected to high power (I think 10/13Kl) projectors.

We are sourcing power from a single phase house receptacle available back stage feeding our amps/foh. We are well measured and all is in order. We are seeing a higher voltage of 245v, but assured will be down as soon as the AC kicks in.
Control room has an Online 3ph UPS supplying power to all gear in Control Room. This is not measured.

As soon as we plug the laptop EP cable to our local inputs on the console, our RCBO trips. Console is also on a UPS, so remains powered. Console then no longer outputs any audio, anywhere, local or to stage rack. Nothing on headphones but a buzz either. A&H local tech tells me we have fried a 15v rail (if that is correct terminology) . We have a Qu as backup and up and running without much fuss. No tie in to control room now.

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. Additionally, the laptops sound has also stopped working. Almost as though the soundcard chip on the mb is fried.

I am not educated in the field. And only try my best to follow good practices. So I am seeking advice on how to prevent such scenarios. This is not the first time I have seen consoles go down as soon as they are tied into any feed coming from the Video vendors.

Thank you all for your time.
Sidhu

[Tim McCulloch]

Hello!

GLD112 + Stage Racks.

Gig in an auditorium. We setup sound for live performances outside of the glass window control room. Control room has Projector and video, so video crew sets up there. The video crew give us Laptop audio out (via laptop audio out). Laptop VGA out is connected to a video switcher, video switcher is connected to high power (I think 10/13Kl) projectors.

We are sourcing power from a single phase house receptacle available back stage feeding our amps/foh. We are well measured and all is in order. We are seeing a higher voltage of 245v, but assured will be down as soon as the AC kicks in.
Control room has an Online 3ph UPS supplying power to all gear in Control Room. This is not measured.

As soon as we plug the laptop EP cable to our local inputs on the console, our RCBO trips. Console is also on a UPS, so remains powered. Console then no longer outputs any audio, anywhere, local or to stage rack. Nothing on headphones but a buzz either. A&H local tech tells me we have fried a 15v rail (if that is correct terminology) . We have a Qu as backup and up and running without much fuss. No tie in to control room now.

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. Additionally, the laptops sound has also stopped working. Almost as though the soundcard chip on the mb is fried.

I am not educated in the field. And only try my best to follow good practices. So I am seeking advice on how to prevent such scenarios. This is not the first time I have seen consoles go down as soon as they are tied into any feed coming from the Video vendors.

Thank you all for your time.
Sidhu

Galvanic isolation is your only protection from this level of silliness.

Transformer isolation from and to *anything and everything* that you do not have 100% control over.  We learned this from Clair - as a matter of corporate policy and field practice they supply iso transformers for any input or output connection that is not part of their touring package.

As a point of clarification, the *console* didn't go down because of this connection, the mains AC was interrupted by the RCBO/RCPD/GFCI device.  Presuming your gear and electrical stuff is 100% correct, these interruption devices did the job they were supposed to do - prevent a hot chassis.  You should beat the offending video suppliers and burn their cables.  This is the kind of shit that kills people and you should note that I didn't use a "smiley" emoticon.  I'm very serious.

I realize India is very much a 'whatever works, works' place and that if you take a moral high road you may forfeit some work.  Use the transformers on all 'outside' audio sources and destinations and don't look back.  Demand that your crew fully understand WHY you're requiring this - it's not just to prevent nuisance AC interruption, it's to prevent injury to death to your crew, client or client's presenters/bands/employees.

[Nitin Sidhu]

Thank you Tim!

I take electrical safety very very seriously. Hence wee have RCBO's across all our distros, do not work if we do not get proper ground. Refuse to plug in gear which comes with Suko type plugs (which is not the indian D-type, so no ground) etc. Fortunately, crew is also trained to understand and have standing orders not to compromise.

While I was upset with them for plugging in the laptop without using the available DBT, I can understand why they thought it was a non issue, they were not aware of the video in the loop, and even if they were, they probably would've overlooked it. One lives and learns.

You say the console did not go down because of the connection. Why would it be damaged then ? I understand the RCBO did its job, which is satisfying. But my impression is that the audio cable coming from the laptop was Hot. Hence.

Thank you again.
Sidhu

[Tim McCulloch]

Thank you Tim!

I take electrical safety very very seriously. Hence wee have RCBO's across all our distros, do not work if we do not get proper ground. Refuse to plug in gear which comes with Suko type plugs (which is not the indian D-type, so no ground) etc. Fortunately, crew is also trained to understand and have standing orders not to compromise.

While I was upset with them for plugging in the laptop without using the available DBT, I can understand why they thought it was a non issue, they were not aware of the video in the loop, and even if they were, they probably would've overlooked it. One lives and learns.

You say the console did not go down because of the connection. Why would it be damaged then ? I understand the RCBO did its job, which is satisfying. But my impression is that the audio cable coming from the laptop was Hot. Hence.

Thank you again.
Sidhu

Sorry, Sidhu, I somehow overlooked the blown 15v rail part of your report.  I have a pretty good guess as to the electrical failure mode but I think Mike Sokol will have a much better explanation of it...

[Ed Hall]

Galvanic isolation is your only protection from this level of silliness.

Transformer isolation from and to *anything and everything* that you do not have 100% control over.  We learned this from Clair - as a matter of corporate policy and field practice they supply iso transformers for any input or output connection that is not part of their touring package.

Tim,

Would a Radial ProAV2 provide the isolation you're talking about here?  I always use this or similar passive whenever someone connects to my system. I just want to make sure I'm getting the protection I think I am.

Ed

[Keith Broughton]

Tim,

Would a Radial ProAV2 provide the isolation you're talking about here?  I always use this or similar passive whenever someone connects to my system. I just want to make sure I'm getting the protection I think I am.

Ed

Yes.
They have transformers on the outputs and you get complete isolation with the ground switch in the lift or off position.

[Ed Hall]

Yes.
They have transformers on the outputs and you get complete isolation with the ground switch in the lift or off position.

Thanks Keith.  BTW It was nice to meet and chat with you the other day.

[Mike Sokol]

Sorry, Sidhu, I somehow overlooked the blown 15v rail part of your report.  I have a pretty good guess as to the electrical failure mode but I think Mike Sokol will have a much better explanation of it...

I find that it often helps to sketch out all the connection paths including audio, video, line, neutral and grounds. Then you can more easily trace the potential current paths and predict possible failure modes. Sidhu, can you draw up a simple diagram of how this was all hooked up? Doesn't have to be fancy, it could just be a picture of a pencil drawing on paper. That would allow a more educational discussion by everyone on the forum. This is a really important topic that we should pursue. In fact I was doing a banquet production last night when this thread popped up, and I had a discussion about audio transformer isolation with one of my A-V techs. We were able to trace out all the copper paths of the gear around us, and note potential failure modes if this A-V system spanned to a secondary room or even another building. Many of our church gigs request audio sends to overflow rooms that can be in a different building on a separate service panel. That's when it gets really dangerous. 

[Nitin Sidhu]

I have a pretty good guess as to the electrical failure mode but I think Mike Sokol will have a much better explanation of it...

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

[Nitin Sidhu]

I find that it often helps to sketch out all the connection paths including audio, video, line, neutral and grounds. Then you can more easily trace the potential current paths and predict possible failure modes. Sidhu, can you draw up a simple diagram of how this was all hooked up? Doesn't have to be fancy, it could just be a picture of a pencil drawing on paper. That would allow a more educational discussion by everyone on the forum. This is a really important topic that we should pursue.

Mike, Thank you for dropping by! I wish I had the width to do a thorough inspection myself, but I was a bit tied up firefighting the situation, and asked my crew to measure all coming in from the Control room. Im on the road now, but I will ASAP speak to as many ppl possible and get you your drawing.

Thank you again!
Sidhu

[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.


Sent from my iPad using Tapatalk HD

[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.

[Speed Daemon]

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.

Thanks for the lesson, Levi.  I've used residential power in Europe where there are two hots (balanced line) and a ground, with no voltage divider like for US home service.  Never had to deal with more industrial venues though.  IIRC they split 480 single phase over there.

I can't say that I've ever had to deal with a mixed power situation where the control room has 3-phase inverter power (left over from cinema projectors?), but FOH has only the mains.  Worked in plenty of data centers and other critical facilities where every outlet was fed by regenerated 3-phase UPS power, so there was uniformity in power quality and grounding methods.  I can only imagine that a split system can't just tie the 3-phase inverter neutral to the mains neutral, and that the potential for spurious voltages and ground loops would be high.

Best of luck to Sidhu, who has a not insubstantial power engineering challenge!

[Nitin Sidhu]

Thank you all so much for the time and effort.
I am attaching a diagram to illustrate the connections to the best of my knowledge. I hope it provides clarity.

@Daniel, CEE7/3 is correct. A lot of equipment here is sold with this AC Power plug, and I am aghast how it is legally possible. The L and N
pins will fit Indian sockets, but offer no ground protection. Additionally, there is a gauranteed 50% chance that the user will plug it in reversed.

I long ago saw that if we have a 3 core power cable connected to a 2 core outlet (hence no ground connected), the ground cable will have some voltage induced in it. As an experiment, I took a short strand of 1mm wire, and placed it next to a live wire, sure enough, a simple Voltage tester lit up when connected to the end of this free strand. This voltage is then perceptible on the chassis of, for example, a mixer.

If something else is critical to the diagnosis, I am willing to visit the center again and measure everything.  This will take time and permits etc. But well worth it I would think. I am also going to do a thorough inspection of all our power supplies.

Thank you all once again!

Regards,
Sidhu

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