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Author Topic: Swapped Hot-Neutral  (Read 2261 times)

Mike Sokol

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Swapped Hot-Neutral
« on: February 14, 2016, 09:10:31 am »

I was checking out AC power in a bar last night during load-in, and found a receptacle next to the stage with swapped Hot and Neutral lines. Not dangerous by itself, but a sign that someone didn't understand how to wire an outlet and it was never properly nspected. I predicted significant differential ground voltages between it and the other outlets run via surface EMT, and I was right. But we were able to put in a few DI boxes in the right places to stop the ground loop hum. I really do need to build a ground loop voltage tester for these gigs since there's not enough time to do this properly poking meters in everything. I know how to do it, and I'll post a schematic of the tester once I get it built. 
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John Roberts {JR}

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Re: Swapped Hot-Neutral
« Reply #1 on: February 14, 2016, 09:56:07 am »

I was checking out AC power in a bar last night during load-in, and found a receptacle next to the stage with swapped Hot and Neutral lines. Not dangerous by itself, but a sign that someone didn't understand how to wire an outlet and it was never properly nspected. I predicted significant differential ground voltages between it and the other outlets run via surface EMT, and I was right. But we were able to put in a few DI boxes in the right places to stop the ground loop hum. I really do need to build a ground loop voltage tester for these gigs since there's not enough time to do this properly poking meters in everything. I know how to do it, and I'll post a schematic of the tester once I get it built.

I am not sure what reverse polarity outlets have to do with ground noise?

Inside the products the line connects to one end of the transformer primary and the neutral connects to the other end of the primary.  The transformer primary is completely floating and isolated. Since the mains voltage is AC it makes almost no difference to the transformer. Most products full wave rectify their secondary output so input polarity doesn't matter.

Products with stinger caps could make a difference. Back in the day some guitar amps had polarity switches for the stinger caps to change polarity.

Products using switching power supplies, perform the rectification before the transformer so the high voltage reservoir cap and diodes subject to mains polarity.

AFAIK there could be subtle electrostatic interference differences internally but very modest is any difference to ground currents.

========
This would be easy enough to confirm with bench testing. Plug products into outlets with different polarity and measure safety ground voltage/current. Perhaps with two products, one plugged into each outlet measure between the two chassis?

The differences in electrostatic coupling could make a noticeable difference if the outlet grounds are floating.     

Sorry not trying to be contrary I just don't understand the mechanism for mains polarity to corrupt grounds beyond very small electrostatic coupled currents from lead dress and the like. With the switching PS the whole reservoir cap swinging 120V could couple a little more noise into chassis ground and sensitive circuitry. But again even this should be well managed in competent designs.

Of course if everyone was competent there wouldn't be reversed polarity outlets... (like my test bench outlet that I never bothered to fix).

JR 

 
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Mike Sokol

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Re: Swapped Hot-Neutral
« Reply #2 on: February 14, 2016, 10:40:09 am »

I am not sure what reverse polarity outlets have to do with ground noise?

Well, I guess I misspoke about cause and effect. I didn't mean to imply that the swapped Hot-Neutral lines by itself would CAUSE the ground loop hum. What meant to suggest was that the entire wiring job in the bar was a bit sketchy. And since the main stage power outlets were antiques in surface mount EMT on brick wall, while this receptacle was in a new room addition and powered from a different sub-panel, that there was a high probability that the ground voltages might be different. But since there was only a single 15-amp breaker powering the stage, and this extra receptacle on the side-wall had a 20-amp breaker, the band wanted to use both of them so they wouldn't trip any breakers. Of course, I suggested picking one or the other and turn down the sub-woofers a bit in the tiny bar, but that suggestions fell on (literally) deaf ears...  ;)

Anytime I see one really obvious mis-wiring condition in a bar, my spider-sense starts tingling and I get super paranoid.
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John Roberts {JR}

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Re: Swapped Hot-Neutral
« Reply #3 on: February 14, 2016, 10:49:44 am »

Well, I guess I misspoke about cause and effect. I didn't mean to imply that the swapped Hot-Neutral lines by itself would CAUSE the ground loop hum. What meant to suggest was that the entire wiring job in the bar was a bit sketchy. And since the main stage power outlets were antiques in surface mount EMT on brick wall, while this receptacle was in a new room addition and powered from a different sub-panel, that there was a high probability that the ground voltages might be different. But since there was only a single 15-amp breaker powering the stage, and this extra receptacle on the side-wall had a 20-amp breaker, the band wanted to use both of them so they wouldn't trip any breakers. Of course, I suggested picking one or the other and turn down the sub-woofers a bit in the tiny bar, but that suggestions fell on (literally) deaf ears...  ;)
gotcha
Quote
Anytime I see one really obvious mis-wiring condition in a bar, my spider-sense starts tingling and I get super paranoid.
Like my house?  :o

JR
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Geoff Doane

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Re: Swapped Hot-Neutral
« Reply #4 on: February 14, 2016, 05:16:49 pm »

I am not sure what reverse polarity outlets have to do with ground noise?

It's like Van Halen's brown M&Ms.  Not significant by itself, but indicative of a lack of attention to other [more important] details.

GTD
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Re: Swapped Hot-Neutral
« Reply #4 on: February 14, 2016, 05:16:49 pm »


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