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Author Topic: Generator and House power?  (Read 2453 times)

Jamin Lynch

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Generator and House power?
« on: October 15, 2013, 11:11:45 am »

Mike, will you explain the problems that may occur (if any) if somebody were to use a generator for the main power source and also using an outlet from the house power at the same time? Is that a bad idea?

Thanks
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TJ (Tom) Cornish

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Re: Generator and House power?
« Reply #1 on: October 15, 2013, 12:43:26 pm »

Mike, will you explain the problems that may occur (if any) if somebody were to use a generator for the main power source and also using an outlet from the house power at the same time? Is that a bad idea?

Thanks
If both systems are designed and installed correctly by competent unlicensed technicians ::), and all equipment is in good working order - i.e. no ground faults anywhere - there should not be any life safety issues, as all the power conduction between systems will be separate.  Since this cannot be guaranteed, the systems should be grounded together.  Without a common grounding system, a ground fault could electrify part of the system.  This grounding should be done in a substantial manner - i.e. not through the shields of your XLR cabling, otherwise your PA effectively becomes the grounding connection between systems. 

A generator operating in this fashion is a "separately derived system", and therefore must be ground-neutral bonded:
http://www.mikeholt.com/technical.php?id=grounding/unformatted/Separatesystems&type=u&title=Separately%20Derived%20Systems%20%5BTransformers,%20Generators,%20etc.%5D%20(4-10-2K)

Some of this depends on the scale of what you're talking about.  The portable generator world is in flux right now, as the NEC2014 is changing requirements for small generators, many of which are unbonded, or cannot be bonded (smaller Honda EU series).  The new changes require GFCI protection, and potentially make existing units non-compliant.

http://www.fohonline.com/current-issue/74-tech-feature/8858-generators-and-portable-primer-part-1.html
http://www.fohonline.com/current-issue/74-tech-feature/9030-generators-and-portable-primer-part-2.html
http://soundforums.net/lighting-electrical/8248-nfpa-decision-new-section-nec-445-20-small-15kw-portable-generators.html

Generally it would be desirable to keep all of the audio on one system or the other, as this significantly reduces the likelihood of your PA becoming current-carrying, and would likely be advantageous for noise as well.
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Mike Sokol

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Re: Generator and House power?
« Reply #2 on: October 15, 2013, 05:15:21 pm »

Mike, will you explain the problems that may occur (if any) if somebody were to use a generator for the main power source and also using an outlet from the house power at the same time? Is that a bad idea?

Thanks
I'll add more to this later, but for all the reasons that TJ has noted, there's lots of potential audio noise and life-safety issues if both house and generator power grounds aren't tied together properly. And that ground/bonding wire needs to be as heavy as any of the potential mains currents. So a piece of #8 or even #6 wire is the traditional gauge required. I've done outdoor gigs with generators and wondered why my XLR cables were getting hot. Now I realize there was probably a LOT of grounding current flowing between various pieces of ungrounded/unbonded gear.

Of course, any ground loop currents flowing through the shields of an XLR cable can cause hum due to the pin-1 problem which still exists on a lot of modern gear.

So the short answer is this. Unless you know exactly how both generator and house power is grounded, then it's safest to pick one or the other. I can also visualize a really crazy ground-loop condition that would have very low frequency currents from the non-phase locked systems. For instance, if the generator was running at 61 Hz and the house power was at 60 Hz, then there could be a 1 Hz loop current created that could do some crazy stuff like resetting computers, making light dimmers slowly shift intensity, and heaven only knows what other kinds of problems.
 
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Mike Sokol
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