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Author Topic: Continuing thread - Distro box.  (Read 15716 times)

TJ (Tom) Cornish

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Re: Continuing thread - Distro box.
« Reply #60 on: November 07, 2014, 04:10:34 pm »

Agreed but again, provided that the 2 circuits are wired correctly in the first place is the point. If you test the circuits first and they pan out, the PMD does improve things. If you don't test and they aren't wired correctly, you really don't know what you are "combining" with the PMD... Also, If you do find a miswired circuit you may be kind of stuck (and from reading about the bootleg thing, it sounds like there are some situations that may take extensive testing to figure out). You may be able to fix it or it may be upstream somewhere. Maybe you can find another circuit to test; but then how much time can you spend tracking this stuff down?
Id bet that in most places I could find a suitable 240volt source and connect to that before I could properly test multiple 120volt circuits. And I'd run a better chance that there aren't other things on that circuit.
A H/N reversed receptacle is no problem for a PMD (nor does a PMD make that situation better, but at least it's usually not a catastrophic issue).  The only things you're combining in a PMD are grounds.  If you are remiss in your testing and you happen to find a RPBG or plain-old H/G reversal, once again I submit you're better off with a PMD, as the high-energy result(!) will happen inside the PMD, not your gear.  There are several threads here of folks with burnt snakes, DIs, and other equipment that would have been saved with a PMD.

240V power is great, and I'm glad you've gotten to the point where you can demand it.  The problem with a hard line is the temptation to want to source the 240V power no matter what: panel tie-ins, unsafe adapters, etc. 

I use distros when possible, but consider it part of the value of my services to be able to put on a significant event using available power, no matter what that may be.  Turning down 3dB saves you 50% of your audio power - sometimes the answer is not trying to make the show bigger than the venue can support.
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Rob Spence

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Re: Continuing thread - Distro box.
« Reply #61 on: November 07, 2014, 05:10:58 pm »

Debbie - read the articles I linked to.  What you're proposing is fine (and is the essence of the Poor Man's Distro), but going about it with twist-lock receptacles and junction boxes on a board is maybe not the best way forward.  The PMD article contains a link to a 1U rack-mount product that would do what you want, look much more pro, and in the end, probably not much more money.

Here is a link to a commercial PMD
http://www.triktags.com/power.htm
Nicely made and was based on one I built and wrote about. I based mine on some threads in this forum quite some time ago.

I don't use it often but it got out this summer for a wedding reception where I wanted to use two 20a circuits.



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Jonathan Johnson

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Re: Continuing thread - Distro box.
« Reply #62 on: November 07, 2014, 07:15:30 pm »

Connecting hots or neutrals together from multiple circuits is never OK.

If you combined the neutrals of two circuits together, you could overload a neutral.

For example, consider two circuits that are on the same hot leg. Circuit "A" is wired as a 15A circuit with 14AWG wire and a 15A breaker, and circuit "B" is wired as a 20A circuit with 12AWG wire and a 20A breaker.

In your distro, you've made the error of combining the neutrals, and you have loaded both circuits to capacity (15A+20A=35A). The neutral current in this case is additive since both circuits are on the same hot leg, or 35A total. This neutral current will be divided approximately evenly between the two neutrals. As a result, circuit "B" will have about 17.5A on its neutral conductor, and circuit "A" will also have about 17.5A on its neutral conductor, in excess of its 15A rating.

Failure of one neutral conductor will not result in a loss of power in the distro, but will now load the other neutral conductor to the full 35A.

Because the National Electrical Code prohibits interrupting the neutral conductor (without also simultaneously interrupting the hot conductor), there is no overcurrent protection on the neutral wire.

If the circuits happened to be on different legs AND the neutrals were tied together, you probably wouldn't overload either neutral. In fact, if the loads were perfectly balanced, the neutrals will carry no current. But if the circuits happen to be coming from two different services provided by two different utility transformers, the results will be hard to predict and may be undesirable!

This is a fairly simplistic analysis. There are likely other situations where combining the neutrals could create a hazardous situation.

If nothing else, tying the neutrals of two circuits together WILL result in tripping of upstream GFI devices.
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Steve M Smith

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Re: Continuing thread - Distro box.
« Reply #63 on: November 08, 2014, 04:42:33 am »

For example, consider two circuits that are on the same hot leg. Circuit "A" is wired as a 15A circuit with 14AWG wire and a 15A breaker, and circuit "B" is wired as a 20A circuit with 12AWG wire and a 20A breaker.

In your distro, you've made the error of combining the neutrals, and you have loaded both circuits to capacity (15A+20A=35A). The neutral current in this case is additive since both circuits are on the same hot leg, or 35A total. This neutral current will be divided approximately evenly between the two neutrals. As a result, circuit "B" will have about 17.5A on its neutral conductor, and circuit "A" will also have about 17.5A on its neutral conductor, in excess of its 15A rating.

Whilst this is absolutely correct, I would be very surprised if a cable rated at 15A couldn't handle 17.5A.

I'm not suggesting it's good practice though!!

Steve.
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Mike Sokol

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Re: Continuing thread - Distro box.
« Reply #64 on: November 08, 2014, 11:08:50 am »

Whilst this is absolutely correct, I would be very surprised if a cable rated at 15A couldn't handle 17.5A.

Perhaps, but I know what will happen with 30 amperes of current on a 15 amp rated extension cord:

http://www.youtube.com/watch?v=WZznobYGF_c

Gotta love my "Glo-Melt" transformer.
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Mike Sokol
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Steve M Smith

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Re: Continuing thread - Distro box.
« Reply #65 on: November 08, 2014, 12:45:38 pm »

"Glo-Melt" transformer.

Is that a registered trademark?!!


Steve.
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Mike Sokol

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Re: Continuing thread - Distro box.
« Reply #66 on: November 08, 2014, 02:51:38 pm »

Is that a registered trademark?!!

Indeed it is. This is an American Beauty / Wassco GLO-MELT resistance soldering transformer that makes a maximum of 3 volts AC at 40 Amps current with an isolated secondary output. A radio engineering buddy of mine gave it to me a few years ago after he bought it for $5 at a Ham-Fest and discovered it didn't make 120-volts output. Of course for me it's a perfect ground-loop current injector since most ground loop differential voltages are perhaps 1 or 2 volts, but can reach dozens of amperes of current. The later Wassco GLO-MELT transformers used a Triac controller on the front end to reduce the output voltage/current. But this is one of the old-school ones with a Vaiac transformer front end that has a 3-volt/40-amp secondary winding, so it doesn't inject any dimmer harmonics into my ground loop experiments.  8)

So now you all see why I'm getting good at troubleshooting ground loop hums. I can create them at will under all sorts of conditions with any sound gear I like, change the amount of ground loop current and paths at will, then figure out how to measure them and make the hum go away with a variety of pin-1 lifts, audio isolation transformers, and anything else I want to try.  ;)

I also have a B&K 150-volt AC power supply with an isolated secondary that I use to bias the chassis grounds of sound gear and RVs to see how that can be detected and corrected as well. Sadly, it doesn't have a cool name like the GLO-MELT transformer.  :'(

Here's a package of the resistance soldering sticks. See, I don't make this stuff up...  :o
« Last Edit: November 08, 2014, 07:03:28 pm by Mike Sokol »
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Mike Sokol
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Mike Sokol

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Re: Continuing thread - Distro box.
« Reply #67 on: November 10, 2014, 03:10:47 pm »

Pics of my Wassco GLO-MELT resistance soldering transformer and how I use it in my No~Shock~Zone seminars. Yes, that's an electrically hot mic. And yes this is all very dangerous, etc... But I have a GFCI on the incoming power so if I accidentally get between a hot line and an earth ground I'm safe. However, there's no good way to protect me from an accidental hot-to-neutral contact, so I'm VERY careful with this demonstration rig. So kids, DON'T try this at home.
« Last Edit: November 10, 2014, 05:17:58 pm by Mike Sokol »
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Mike Sokol
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ProSoundWeb Community

Re: Continuing thread - Distro box.
« Reply #67 on: November 10, 2014, 03:10:47 pm »


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