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Author Topic: Useing separate buildings  (Read 4615 times)

Stephen Kirby

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Re: Useing separate buildings
« Reply #10 on: July 31, 2015, 07:10:29 pm »

Similar to the multi house fireworks thread, I would think that commercial buildings would have some higher end service that the organizers can negotiate access to.

The only street festivals I've done have had municipal CA outlets ready to go.  But I'm curious for people who do this more often with generators, do you drive the ground rod right though the asphalt?  No guaranties that there will be open ground reasonably close to where then generator goes in today's concrete jungles.  I don't think a planter box counts.
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Mike Sokol

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Re: Useing separate buildings
« Reply #11 on: July 31, 2015, 07:29:08 pm »

Mike...

I'd change just one thing in your response:

Rent your own generator and add it into the event cost/your charge.  Expecting any outfit proposing such risky expediencies to get a proper generator is unlikely.  Hence my previous mention of the shop welder...
You're probably right. They think a generator is a generator is a generator. >:(
« Last Edit: August 01, 2015, 06:27:07 am by Mike Sokol »
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Mike Sokol
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Ray Aberle

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Re: Useing separate buildings
« Reply #12 on: July 31, 2015, 08:37:31 pm »

Gus, if this is next weekend (7th, 8th, 9th) I'd be happy to send my dad up there with our 45kW for a great price. Pair of spider boxes, couple of 100' 50A cables, and you will be good to go.

-Ray
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Jonathan Johnson

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Re: Useing separate buildings
« Reply #13 on: August 01, 2015, 02:12:26 am »

Just don't.  The reason why isn't even clearly explained on the above-linked-to page, though there is a hint as to why there can be mega danger.

If you have to use two supplies then you need wireless or unshielded fibre between the two sets of powered things, and keep each consumer of power near its source and very far away from each other.

The reason is that power from two different sources can have very different voltage potential on the grounds. Since the audio ground is bonded to the chassis ground  of the equipment is bonded to the equipment grounding conductor of the circuit is bonded to to neutral conductor of the supplying transformer, this difference of potential can result in significant current on your audio grounds.

Besides introducing noise ("ground loop hum"), these currents can melt audio wires and, if a person completes the circuit between the two systems (as when touching a microphone and guitar strings at the same time), there is a high possibility of electric shock, and a very real danger of electrocution. And that's with everything wired properly and according to code and in good repair.

Another consideration is electrically bonding the metal structure of stages to the grounding system of the power source. There is at least one thread here that discusses that.
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Stop confusing the issue with facts and logic!

Mike Sokol

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Re: Useing separate buildings
« Reply #14 on: August 01, 2015, 06:58:38 am »

The reason is that power from two different sources can have very different voltage potential on the grounds. Since the audio ground is bonded to the chassis ground  of the equipment is bonded to the equipment grounding conductor of the circuit is bonded to to neutral conductor of the supplying transformer, this difference of potential can result in significant current on your audio grounds.

Ah yes.... and here's a PPT slide from one of my NSZ classes that shows how the current flows between two pieces of gear connected by XLR cables. Note that since the current loop includes the XLR cable shield as well as its twisted pair, you don't need to split out the conductors like you need to for an extension cord with both outgoing and return currents that null out the meter readings. Just clamp your ammeter around the XLR cable and you'll be amazed to see how much ground loop current flows at times. Even 100 mA of current can make a lot of powered speakers hum. 

I've done enough experiments with my Ground Loop Maker rig to find that current flow between interconnected audio gear is typically around 1 ampere per volt of Ground Loop Differential. So even at a moderate 2 or 3 volt difference between two different "grounds", you'll measure 2 or 3 amps current with a clamp meter around the interconnecting XLR cable. Now if you look inside a lot of audio gear you'll find that Pin 1 doesn't immediately terminate at the chassis as one would expect. Lots of times it's routed though a connector and traces on the circuit board. That trace on the board isn't rated for that level of fault current, and can melt like a fuse. (Ask me how I know this).

I've personally measured 5 volts difference in the ground potential between two sides of a single large warehouse building with an all steel structure. Imagine how many volts potential can exist between two different buildings. And there will be hundreds or even thousands of amperes of current behind this voltage difference, so ALL the voltage drop will occur between your audio gear. See the picture of an IMP2 DI box that got in the middle of a big voltage difference. Thanks, Al Keltz from Whirlwind for sending me the pic.

So don't put your audio gear in the middle of HUGE potential currents and voltages. Pulling all power from a single source is always the best way to eliminate ground loop hum, equipment damage, and most importantly, human injury or death from electric shock.
« Last Edit: August 01, 2015, 07:12:57 am by Mike Sokol »
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Re: Useing separate buildings
« Reply #14 on: August 01, 2015, 06:58:38 am »


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