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Question on grounding in a stage stringer


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I'm making up 2 stage stringers. This is for small venues so they are 30' with 3 boxes(one recepticle/box), one @ 10', 20' and 30' the end. 

I've got the both greens from the in and out ( of the box) tied together on the recepticle (Leviton) ground lug which is obviously one plate that screws into the metal box. The box has plastic stress relief connector holding the 14/3 600V SOOW cable. The boxes are painted on the outside. The recepticle ground is attached to a plate on the box by both mounting screws.

I've seen the Windsor related video on youtube about making up a stringer. No real surprises but he uses rubber boxes. Mine is metal but like I said it has two grounding points to the box from the recepticle. 

So here's the question. Would "Code" say I still need to run a round wire from the recepticle to the metal box even though it's being grounded from two points on the recepticle already?

Thanks

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Perhaps my terminology isn't exactly correct.

The fitting into the box is plastic and has a built in strain relief.

The obvious question needs to be asked if a screw holding down a clip attached to a wire screwed into the recepticle plate with the other end attached to a clip, also screwed to the box is good then why isn't two screws holding one plate to another good? If the recepticle ground plate is good for a wire ground why not for a direct ground?

Please enlighten me as to why it wouldn't be OK for portable applications? The only dif between my hookup and the one in the video is that I'm using a metal box and theirs is rubber. ??

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Gorquin wrote: If the recepticle ground plate is good for a wire ground why not for a direct ground?

 

 

 

The people who write the electrical code probably want to see the box grounded by an indenpendent conductor and screw as opposed to double dipping with a structural screw and receptacle brace. There is more chance of structural screws loosening with use

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I added a solid ground wire to the metal box so all of the ground wires are common to each other and the box.

Before I plugged anything in a checked all of the connections to each other for shorts....none.

I checked the three outlets for continuity with the plug. All good.

I plugged a lamp into all fo the sockets and it worked fine.

Here's a twist I didn't expect so you engineers can tell me if this is normal or there's a possibility that I did something wrong.

We had band practice tonight and I used one of the stringers. (I haven't finished the other yet.)

I'm using an old BF Princeton that still has a 2 prong non-polarized AC cord on it.

At one point while holding my guitar I could feel current at my hand when I touched the mic. The PA and amps are plugged into a separate outlet than the amp.

I'm aware that if one has two separate units with non-polarized plugs that there can be a line voltage potential between the two grounds depending how how they are plugged in.

I didnt think this could happen with two separate units when at least one had a three prong cord?

OK, so, am I wrong? Could there be a problem with my house wiring? Or, could there be something dumb I missed when connecting the wires in my stringer? Black(Hot) to Gold, White(Neutral to Silver), Green to ground. ALL of the ground lugs are both crimped and soldered and go to the metal case described above.

What say you?

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Thanks guys. 

I was in audio service for a number of years but the exposure to AC is very limited. Once you get past an open or shorted transformer, bridge, or cap one typically deals with DC even though some of those voltages can be high as in the case of a 500V plate.

I know some guys like Torres Eng. in Ca are proponents of having caps on the polarity switch even if you have a three prong cord. A few of my guitars that I've had to rewire don't have a ground directly to the bridge. They have either a resistor or a cap in series with the ground wire to add at least some measure of isolation. The guitar I was using yesterday was a stock guitar. I'll have to isolate that guitar as well as replacing the AC cord on the old BF.

Thanks for the insights.

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Gorquin wrote:

 

Thanks guys. 

 

I was in audio service for a number of years but the exposure to AC is very limited. Once you get past an open or shorted transformer, bridge, or cap one typically deals with DC even though some of those voltages can be high as in the case of a 500V plate.

 

I know some guys like Torres Eng. in Ca are proponents of having caps on the polarity switch even if you have a three prong cord. A few of my guitars that I've had to rewire don't have a ground directly to the bridge. They have either a resistor or a cap in series with the ground wire to add at least some measure of isolation. The guitar I was using yesterday was a stock guitar. I'll have to isolate that guitar as well as replacing the AC cord on the old BF.

 

Thanks for the insights.

 

I'd be curious to see how they expect a "polarity switch" to work on a 3-wire grounded cordset.

The purpose of the polarity switch was to flip-flop the "hot" and "ground" relative to the receptacle for the pair orientation that was quieter, when a 2-wire unpolarized cordset was used.

The purpose of a polarized 3-wire cordset is to ensure that the three connections remain in the same configuration at all times. NEC identifies the three connections as "ungrounded" (or Hot), "grounded" (or Neutral), and "grounding" (or Ground...but note the difference between grounded and grounding).

There should never be a situation that causes the ungrounded and grounded connections to be reversed, but that's exactly what a "polarity switch" will do. It basically takes a safe three-wire wiring arrangement and makes it dangerous again. The capacitor or pair of capacitors will not correct this wiring mistake or make it safe...applying ungrounded potential to the amplifier chassis is never safe. This is what causes the guitar strings to become electrically "hot"...the hot potential of the ungrounded conductor is connected to the amp chassis, the amp chassis has the guitar input jack shield connected to the chassis, the guitar cable shield is now hot, and this energizes the guitar strings. Your body touches the strings, then the mic, then the mic cable and back through the PA to the PA's grounded outlet. Zap!

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They're not "switching" the polarity of the Black or White leads. They are grounding the Green wire to AC ground but Switching a cap from there to either Off (NC) or the Black or White lead even with a three wire setup. On some older models that have 3 prongs I've read where they run a 600V cap(if memory serves I think it was .1) from the Black and White leads to ground. But, this sounds like it could eventually cause a problem as well if either of those caps shorted.

They also run a 250V MOV across the Black and White leads as a transient spike protector. 

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"So here's the question. Would "Code" say I still need to run a round wire from the recepticle to the metal box even though it's being grounded from two points on the recepticle already?"

Code works this way: All ground wires from every circuit in the box are tied to together. The box shall be grounded with a wire based on the largest circuit. 

Next question...your installing a plug: It shall be grounded. You can use a wire based on the size of the conductors feeding the recept, or use a self grounding recpt if you follow the rules. The self grounding recpts still have a ground screw. Hmmm wonder why?

Clever wiring can make it easy :)

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Gorquin wrote:

 

They're not "switching" the polarity of the Black or White leads. They are grounding the Green wire to AC ground but Switching a cap from there to either Off (NC) or the Black or White lead even with a three wire setup. On some older models that have 3 prongs I've read where they run a 600V cap(if memory serves I think it was .1) from the Black and White leads to ground. But, this sounds like it could eventually cause a problem as well if either of those caps shorted.

 

They also run a 250V MOV across the Black and White leads as a transient spike protector. 

 

This was my guess as to how they were achieving "switching", and it still makes no sense whatsoever in a 3-wire supply circuit. K.I.S.S. prevails.

The MOV is another potential problem, as they only "work" once, and have the potential to fail spectacularly. If the device doesn't emit the magic smoke when it fails or is "used", then the user has no clue whether "protection" is still in effect or not.

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