# How Can Changing AC Sockets Cure Hum?



## Stevens R. Miller (May 11, 2016)

I'm working with a local community theater group. We've rented a middle school auditorium for our show. Last night was our first tech rehearsal in the space. We brought our own audio mixer. Setting up, we found two pairs of AC sockets. One had a sticker on it that said, "For Harmony Middle School Audio Only." Dutifully, we plugged into the other one.

Well, we had a nasty hum, even when all of our inputs were muted and all of our faders were down. After trying everything else we could think of, we unplugged from the socket we had used and plugged into the one "For Harmony Middle School Audio Only." The hum went away (not entirely, but almost).

What could be different about two AC sockets that would cause one to cure the hum the other allowed?


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## sk8rsdad (May 11, 2016)

They're probably running on different phases or through different distribution panels. Google _ground loop_ for hours of learning fun.


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## RonHebbard (May 11, 2016)

sk8rsdad said:


> They're probably running on different phases or through different distribution panels. Google _ground loop_ for hours of learning fun.


All of the above and additionally one of the receptacles could be a proper IG device c/w a legitimate insulated isolated ground conductor clear back to the common (and grounded) point of the transformer supplying the breaker panel. Initially IG receptacles were always orange but it wasn't long before many other colors came on the market with only a tiny, green, IG symbol to differentiate them. Unlikely perhaps but still a possibility.
Edited to add: I suppose I should mention it's not unheard of for painters to paint right over receptacles removing all traces of IG identification. 
Toodleoo!
Ron Hebbard.


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## BillConnerFASTC (May 11, 2016)

Or they lifted the ground on the receptacle - really isolated it. They may have gotten tired of breaking off the ground pin from the plug and getting fired for it by inspectors.


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## RonHebbard (May 11, 2016)

BillConnerASTC said:


> Or they lifted the ground on the receptacle - really isolated it. They may have gotten tired of breaking off the ground pin from the plug and getting fired for it by inspectors.


I fully agree with you Bill, especially when they purchase a proper IG receptacle to avoid grounding via the mounting strap and then do as you suggest to ensure it's totally ungrounded. Some people never learn while some never ground.
Toodleoo!
Ron Hebbard.


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## cdavisnyc (May 11, 2016)

In our space, audio only AC outlets are on a separate conditioned power supply (to prevent spikes & drops). We only use it for audio & video equipment. Thus, no hum. 

I suspect that the label meant "audio only" instead of "Harmony Middle School only"


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## AlexDonkle (May 12, 2016)

cdavisnyc said:


> In our space, audio only AC outlets are on a separate conditioned power supply (to prevent spikes & drops). We only use it for audio & video equipment. Thus, no hum.
> 
> I suspect that the label meant "audio only" instead of "Harmony Middle School only"



Unfortunately, power conditioners don't really prevent group loop hum like is being described here. If you're powering all your audio gear from a single outlet, that's the easiest method of having no hum (since there's only 1 ground), with or without a power conditioner.


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## BillConnerFASTC (May 12, 2016)

I'm use to isolation transformers with isolated ground feeding all av power receptacles, usually identified by color but also within a panel that is all av.


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## cdavisnyc (May 12, 2016)

AlexDonkle said:


> Unfortunately, power conditioners don't really prevent group loop hum like is being described here. If you're powering all your audio gear from a single outlet, that's the easiest method of having no hum (since there's only 1 ground), with or without a power conditioner.



No, you're right about that. The power conditioner doesn't prevent the ground loop hum. I didn't mean to imply that it did. 

Keeping audio power separate just means we don't have to spend time hunting down the source of the ground loop.


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## Fountain Of Euph (May 13, 2016)

I have to run 80ft of power directly from the audio rack on the third floor boot to FOH on the first floor to specifically prevent ground loops...

Sent from Taptalk for Android, this was.


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## Wheezy (May 13, 2016)

Stevens R. Miller said:


> What could be different about two AC sockets that would cause one to cure the hum the other allowed?



The short answer is no two ground points are exactly the same potential.

There could be other causes as well, such as a lighting circuit near the other ground, causing power influence. But, maybe by accident or intention, the school found the ground on this outlet is more direct.

If, for instance, your mixer is better grounded than your amp in another outlet, the amp bleeds some of its return through your XLR cable.


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## BillConnerFASTC (May 13, 2016)

Steven - next time you are there - plug in a simple outlet tester and see if there is a ground. I did interpret your comment about the label to suggest it was a homemade label - like a building maintenance person might do.


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## Aaron Becker (May 13, 2016)

I agree with what everyone else said. 

Are the outlets labeled by panel/circuit number by chance? You would significantly reduce your chances of having hum (ground loops, as others have mentioned) by using power off the same circuit, and further reduce it by pulling from the same outlet. 

I've been in a couple schools that were on the older side where dedicated "sound system power" was pulled through the crawl space or ceiling (by a qualified electrician, after getting the appropriate permits) for this exact purpose so the FOH equipment could be tied to the drive racks and on-stage sound. It virtually eliminated the hum issues that venue was having. 

I agree with Bill, the label seems to be a homemade style, curious to see if the ground is present on that outlet or not... 

As a general practice, I now test all outlets (including those on power strips) before plugging ANY of my equipment in. I've found too many places with hot/neutrals reversed (in 120 applications) or missing neutrals that could potentially cause issues to my equipment racks.


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## Stevens R. Miller (May 16, 2016)

BillConnerASTC said:


> Steven - next time you are there - plug in a simple outlet tester and see if there is a ground. I did interpret your comment about the label to suggest it was a homemade label - like a building maintenance person might do.



Yes, the label is just a sticky paper thing with handwriting on it. The outlet must have been an afterthought, as it is just inches away from another one. Well, maybe they were installed at the same time, but it just doesn't look that way to me (and there is no such outlet in the other middle school where I've done tech, and the two schools are nearly identical, down to the nuts and bolts).

Am I reading this correctly: some of you are suggesting we make sure the mixer is _not_ using the ground wire in the socket? Is that safe?

Regarding running a power line from the rack in the wings: I thought of doing that, on the theory that, if I could be sure the rack mixer and my technician's mixer (the one whose output is going into a channel on the rack mixer) had a truly common ground (or as close to one as the laws of physics allow), there couldn't be a ground loop. Any downsides to that? Safety issues? Fire code? What's the longest run of a power cord I should consider?

The connection from the tech's mixer to the rack mixer is via a single XLR-3 cable. We did swap that with another, to no effect. Regardless, if there's enough 60-cycle energy in the house, is it possible the cable is playing a role? FWIW, when I dim the house incandescents to 50%, the hum gets worse.


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## BillConnerFASTC (May 16, 2016)

But did you test it?

Toodbox staple. (I unfortunately have a too addiction disorder - TAD - and have many tools of his sort - and have my eye on a megger, without a real need for one.)


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## Stevens R. Miller (May 16, 2016)

BillConnerASTC said:


> But did you test it?View attachment 13330
> Toodbox staple. (I unfortunately have a too addiction disorder - TAD - and have many tools of his sort - and have my eye on a megger, without a real need for one.)



Not yet. I have one of those things, somewhere...

I've long had the fantastical dream that my workshop will someday look like this:



But, for now, it looks more like this:


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## RickR (May 16, 2016)

The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article. 

A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground. Noise can't travel back up the "iso-ground" so any unintended ground loops or leakage in the power system will be avoided. Even a separate ground to the panel is better than sharing one with whatever is on the other circuit, especially if it's a dimmer or motor.


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## BillConnerFASTC (May 16, 2016)

RickR said:


> The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article.
> 
> A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground. Noise can't travel back up the "iso-ground" so any unintended ground loops or leakage in the power system will be avoided. Even a separate ground to the panel is better than sharing one with whatever is on the other circuit, especially if it's a dimmer or motor.



Not only performers but a few pastors in the the midst of a baptism as well.


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## Stevens R. Miller (May 16, 2016)

RickR said:


> The question is whether you have an isolated ground or no ground at all. Not grounding things is NOT good! It's unfortunately become a solution by sound techs that don't understand the dangers involved. I was recently in an AV tech seminar and some of the best sound operators in town all admitted they sometimes break the ground. Performers have been killed by broken grounds! See this Richard Cadena article.



That's a great article. I never knew how GFCIs worked before. It's also full of pretty sobering stuff.


> A true isolated ground bypasses the grounding buss at the breaker panel and connects to the bonding point (transformer, source ground, etc) where the neutral and ground meet earth ground.



Okay, help me keep up with you here, if I can: By "neutral," I believe you mean the side of the two-wire pair that provides the 120 VAC to whatever is plugged into them that is _not_ one of the two phases delivered to my house (and that, when used together, provide me with 240 VAC). The ground provides a way for a chassis or enclosure to be connected to earth ground. What I've never understood is why there is a need for the ground wire if the neutral wire is connected to ground. For example, we have a counter-top mixer (Kitchen Aid, five-quart, definitely what you want when baking cookies). It has a three-prong plug and an all-metal enclosure. If the neutral and ground on its plug are both, by virtue of being plugged into a three-hole socket, electrically connected, how is that different from simply connecting the neutral pin on a two-wire plug to the mixer's enclosure?

(I have to say I feel really dumb, asking this. I've been an electronics hobbyist for a long time, but I admit I can see that "electronics" and "electricity" are not the same things. Regardless, I feel like I ought to know the answer to my question, but I just don't.)


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## Aaron Becker (May 16, 2016)

Steven,

I'll attempt to shed some light on this without crossing the dangerous "do not enter" territory of controlbooth I've seen before for liability reasons (mods, I'm new, so forgive me if this crosses the line)...

Neutral - in non-scientific terms (electrons and atoms), the neutral wire is the "return" path for the electrical current serving any given device.

Your house (and most residences in the US) have 240 volt service from the "street" (whatever means that may be), with 3 conductors - two "hot" and 1 "neutral". (4 if you count the ground). 

The purpose of a ground path to earth (ground) (in layman's terms) is if somewhere in the electrical path/device/wire/etc electricity is getting lost somewhere-and not making it's way back to the neutral/ground bus bar at the tie in, breaker panel, etc. It provides a safe path for electrons instead of taking the shortest path to ground, which is what, by nature (science, actually) they want to do. The shortest path could be a priest, performer, or technician, as mentioned above. The ground prevents this. Removing it, although solving a sound problem, creates many safety issues. The ground wire is also connected to the metal on the device (on your counter top mixer, any metal you can touch on the frame is grounded, on a sound board, the frame (if metal) is grounded to earth ground) to discharge stray electrons. The GFCI, as mentioned above, also helps by shutting off the flow of electricity in the event that electrons are not following the prescribed hot-neutral path within the circuit. 

Best of luck,

PS- those of us above who were inquiring about the ground on the outlet were not suggesting it SHOULD be removed, I think we were curious from a standpoint if that's why it "solved" the hum problem or if it was something bigger. 

PSS- the fact that the lights being at 50% created a louder hum could be a variety of issues. Grounding, unshielded lines run in long, close, proximity to AC lines or lighting lines, etc. Hard to diagnose without knowing more details.


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## RickR (May 16, 2016)

Mostly right except for wondering about ground. It's a safety thing and therefore critical in my book.

Theory says that a completely unloaded ground will/must have a very low impedance to earth ground. So when (not if) a fault occurs there is a very brief but large current to ground via a safe path. Large you say?? Yes, large enough to trip whatever breaker/fuse 'over current protection' and shut down the offending circuit. That's why the fault is brief. If anything gets in the way of that fault current it may not be large enough to make it brief enough to prevent damage. This is why GFIC and Arc-fault devices are still a good idea. Breakers do not work fast enough to protect people.


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## RickR (May 16, 2016)

I see Aaron chimed in just before I did. A different way of looking at it is always good.


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## Aaron Becker (May 16, 2016)

RickR said:


> I see Aaron chimed in just before I did. A different way of looking at it is always good.



I think we were getting at the same thing, grounds are important!


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## Stevens R. Miller (May 16, 2016)

Aaron Becker said:


> Neutral - in non-scientific terms (electrons and atoms), the neutral wire is the "return" path for the electrical current serving any given device.



Thanks. I did somre more reading on it and this closely matches that. I might say that the neutral wire complets the circuit, rather than that it is a return path, since this is AC we're talking about. Current flows both ways through the hot and neutral wires. What I didn't take into account is that, when all is as it should be, _no_ current flows through the ground wire, even though it is electrically connected to the neutral wire (but not in my mixer, be that audio or kitchen).


> The purpose of a ground path to earth (ground) (in layman's terms) is if somewhere in the electrical path/device/wire/etc electricity is getting lost somewhere-and not making it's way back to the neutral/ground bus bar at the tie in, breaker panel, etc. It provides a safe path for electrons instead of taking the shortest path to ground, which is what, by nature (science, actually) they want to do. The shortest path could be a priest, performer, or technician, as mentioned above.



I see. So, if the hot and neutral wires were always intact, and a complete path from one, through a device, to the other were always to exist (or, were not to exist because the path were to be safely interrupted, with, say, an "off" switch), the ground wire would never be needed. But, if the path is interrupted, and the hot wire makes contact with the chassis of a device, the ground wilre completes the path back to ground, and in a very low-impedance way, which: 1) means very little current will flow throw anyone touching the chassis and also in contact with another low-impedance path to ground, and 2) will pull so much current that it will trip a breaker.

From Richard Cadena's article, I gather that a GFCI adds two more protections: first, if the ground path itself is not effective, either by reason of it being open, or not low-impedance, a lot of current might flow through a person instead of back to either the neutral or hot wire (depending on which phase of the AC cycle we were in), it will detect this and open the circuit, something a breaker might not do if the total current being drawn is still below the breaker's limit and, second, will do it faster than a breaker would anyway.


> PSS- the fact that the lights being at 50% created a louder hum could be a variety of issues. Grounding, unshielded lines run in long, close, proximity to AC lines or lighting lines, etc. Hard to diagnose without knowing more details.



Yeah, one step at a time . Now that I know a bit more about what a ground wire does, it's probably time to gird my loins and study up on ground loops. The hum we're getting may be from one of those, or something else (or both). Our performance mixer plugs into a channel on a rack mixer that is quite close to the dimmers. I wouldn't be surprised if having audio lines that close the dimmers isn't, itself, a bad idea.

I read up recently on MADI, a synchronous digital audio protocol. Gosh, I'd love to work in a house that used _that_. I can see how it wouldn't cure every hum problem one might face, but it sure does seem to solve a lot of problems, hum and others, and has a certain logical elegance I like. Also, as it is a digital protocol, not an analog contraption, I can understand it better  .


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## Aaron Becker (May 16, 2016)

Looks like you did some research and found some matching answers! That's always good. 


Stevens R. Miller said:


> I see. So, if the hot and neutral wires were always intact, and a complete path from one, through a device, to the other were always to exist (or, were not to exist because the path were to be safely interrupted, with, say, an "off" switch), the ground wire would never be needed.
> .



Correct.

And yes, you are correct on the GFCI and circuit breaker protection, which *should* provide quick enough protection (fractions of a second, when dealing with a GFCI) to save a person from getting electrocuted. 

In an ideal world, audio and lighting power would be pulled from different "phases" and be isolated, which resolves a lot of problems in the professional arena, or even at the college/high school level. This may not be the case in the facility you are helping at. In general, I tend to keep lighting dimmers and sound rigs far, far away.  

Digital protocols are a good protection, but I wouldn't substitute them for a good ground system that is designed and implemented correctly. I would say that if the "marked" outlet in your case is providing you hum-free results, that's a good option, IF (and only if) the ground is still intact. (Have you purchased a tested from the local hardware store yet?)


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## Chris15 (May 16, 2016)

One thing to note apart from the fault shunting properties of the earthing system is the around metal chassised devices...
The Neutral will in normal operation be at a small but real voltage above ground, because it has finite impedance, and so with current flowing in it, will have a voltage drop across it.
If we bonded chassis to neutral, and you had say 2 circuits in a kitchen, you could have say 2 toasters with different voltage potentials on each and get a tingling touching one in each hand.

There is also a benefit for dissipation of EMF and static to ground through the chassis.

Talking of breakers and humans...
Let's be clear on what breakers are there to do, it's to stop the power flowing before the wire gets so hot it causes an electrical fire. They are NOT effective against shock protection in most circumstances.
It takes roughly 30 - 50 mA across the chest to cause ventral fibrillation, when that happens you need a hospital, let it continue for long and you'll need the morgue instead. Even 100 mA is NEVER going to be the different between a breaker tripping and holding.
The body has an impedance in the order of 100 k ohms with dry skin, but that can quickly drop to 1k or so with wet skin, so mostly we sit somewhere in between. That means normal mains voltages are enough to generate a problematic level of current.

I can't speak to US GFCIs, but I can talk about RCDs. If the vector sum of all current carrying conductors on a circuit is >30mA, that sucker will trip within 100 mS to meet standards. In certain applications, it reduces to 10mA to trip (healthcare areas being one such example). There is ONE fault that an RCD won't save you from... Making a circuit between any of the current carrying conductors. If you end up with active in one hand and neutral in the other with rubber shoes, no RCD will save you, it sees the currents coming back just fine. Ditto between actives in a 3 phase system. About now's where the reminder about one hand behind your back when dealing with live anything comes into play...

If you want to talk MADI vs analog audio, let's do that in the sound forum, if only it was as simple and nice as you like to think it is...


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## FMEng (May 17, 2016)

It also possible to avoid hum issues in analog interconnections without doing anything special with AC grounds or power "cleanliness." Whole books have been written about this, but here are a few tips for making connections between equipment separated by significant distance or where both ends are powered from different AC sources:

1. Make sure that the outputs and inputs are truly balanced. Unbalanced audio and impedance balanced inputs and outputs need to be avoided. Manufacturers who make such equipment should be shot.

2. On line-level circuits, disconnect and isolate the shield at one end of the cable. This is typically done at the input side. Commercially made, XLR ground lifters should be in every audio kit.

3. An audio transformer, with a Faraday shield, can provide extra ground current isolation and exceptional common mode noise rejection. The transformer usually works best at the input.

Consider that the public telephone network has made noise free audio connections over miles of distances using balanced audio pairs and audio transformers, and they don't use any shielding. Not only that, but their cables run alongside power lines. These two concepts are very powerful.


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## Aaron Becker (May 17, 2016)

FMEng said:


> 2. On line-level circuits, disconnect and isolate the shield at one end of the cable. This is typically done at the input side. Commercially made, XLR ground lifters should be in every audio kit.



Truth. This is actually more common than people think. I've come across several installs where the DSP was located near the mix location (usually during the initial install), and the ground was disconnected at the DSP from every run from there to the amp rack, backstage in a loft. The system is quiet, all things considered, and the power likely isn't isolated or split on a specific phase. 

Good points.


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## BillConnerFASTC (May 17, 2016)

One reason for a ground and a neutral, it's not unheard of for the hot and neutral to be reversed. It's about things going wrong, not when everything is hunky dunky dory.


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## Stevens R. Miller (May 17, 2016)

FMEng said:


> Consider that the public telephone network has made noise free audio connections over miles of distances using balanced audio pairs and audio transformers, and they don't use any shielding.



How much of their success might be due to band-pass filtration?


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## Stevens R. Miller (May 17, 2016)

BillConnerASTC said:


> One reason for a ground and a neutral, it's not unheard of for the hot and neutral to be reversed.



Good point.


> It's about things going wrong, not when everything is hunky dunky dory.



Yeah, that's been a big key to my understanding this. The ground line may save you _if_ something goes wrong. Until then, it's just metal.


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## Stevens R. Miller (May 17, 2016)

Chris15 said:


> One thing to note apart from the fault shunting properties of the earthing system is the around metal chassised devices...
> The Neutral will in normal operation be at a small but real voltage above ground, because it has finite impedance, and so with current flowing in it, will have a voltage drop across it.
> If we bonded chassis to neutral, and you had say 2 circuits in a kitchen, you could have say 2 toasters with different voltage potentials on each and get a tingling touching one in each hand.



Ah, maybe this is one reason why there aren't just two neutrals (or two grounds) in parallel, each serving as a backup to the other.

Such an approach (or just bonding chassis to neutral) also has current flowing through the chassis, doesn't it? If it were possible to disassemble a chassis, one might open the circuit it was a part of, with one's left hand on one side of that circuit, and one's right hand on the other...


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## FMEng (May 17, 2016)

Stevens R. Miller said:


> How much of their success might be due to band-pass filtration?


None. Broadcasters used wide-band telephone circuits for 70 years. Those circuits were conditioned to be flat from 30 Hz to 15 kHz, and they passed strict specs for noise. On plain old telephone circuits, the harmonics of 60 cycle hum is still readily apparent, so users would definitely complain.


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## RickR (May 17, 2016)

While we are in the weeds of neutrals, grounding and safety it's worth noting that most power switches do not switch the neutral. Perhaps they should for safety, protecting from that minor voltage when other devices are in use. I suspect it's due to single pole switches being cheaper.


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## Aaron Becker (May 17, 2016)

RickR said:


> While we are in the weeds of neutrals, grounding and safety it's worth noting that most power switches do not switch the neutral. Perhaps they should for safety, protecting from that minor voltage when other devices are in use. I suspect it's due to single pole switches being cheaper.



Also important to note this is exactly why switches should not be used for power-disconnects when doing service work, especially in a somewhat unlikely yet very dangerous situation where the neutral is (incorrectly) switched instead of the hot side. Not to mention switch loops, three way circuits, and all sorts of other legitimate but potentially unsafe configurations.


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## derekleffew (May 17, 2016)

@RickR , @Aaron Becker , not exactly. 2011 NEC 240.22 forbids interrupting the grounded conductor. See also http://www.ecmag.com/section/codes-standards/switching-neutral-ok .


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## RickR (May 17, 2016)

derekleffew said:


> @RickR , @Aaron Becker , not exactly. 2011 NEC 240.22 forbids interrupting the grounded conductor. See also http://www.ecmag.com/section/codes-standards/switching-neutral-ok .


'Forbids' with a few key exceptions, such as switching all other conductors simultaneously. 

That's the only way I've seen it done.

Sent from my SM-G900V using Tapatalk


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## Dionysus (May 19, 2016)

RickR said:


> 'Forbids' with a few key exceptions, such as switching all other conductors simultaneously.
> 
> That's the only way I've seen it done.
> 
> Sent from my SM-G900V using Tapatalk



Indeed, a 'neutral' (grounded conductor) may only be switched or disconnected until ALL "hot" conductors are disconnected. They may also be disconnected or switched simultaneously.

How many times I have seen "switched neutrals", especially in lighting circuits or switched receptacles is quite disturbing however (and very dangerous). With the 'neutral' disconnected there is still power there. I've seen lights switched off glowing, or not turn off at all. Also seen someone turn off a switch and then BOOM goes the box when you go to work on it without verifying that power is truly disconnected.

This is why the MOST IMPORTANT tool for someone doing electrical work is their assorted *METERS* (mainly multimeter). Always verify.

I would take my multimeter to both the receptacle that gave the hum, and the one that does not and "poke around" to see what is what pretty much immediately. HONESTLY IMO you should *ALWAYS meter your power BEFORE* hooking up any audio equipment. Too many buildings have bad power/wiring.

Test Hot to Ground (in North America) = about 120v
Test Hot to Neutral ("") = about 120v, and similar as the above, if not the same.
Test Neutral to Ground = about 0v (less than a volt or so is fine, any more is a concern)


EDIT:

The meters I ALWAYS have in my car are:

True RMS Multi-meter (CAT III at least, mine is CAT IV)
Non-Contact Voltage Tester (NEVER trust it, but it is great for quick checks and such)
Receptacle Tester (most of mine have GFCI test functions, one with AFCI test)
Network Tester (doesn't apply to this, but I always have it handy)
XLR "Rat Sniffer" Tester (how often this saves your butt on some gigs)


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## BillConnerFASTC (May 19, 2016)

Dionysus said:


> I would take my multimeter to both the receptacle that gave the hum, and the one that does not and "poke around" to see what is what pretty much immediately. HONESTLY IMO you should *ALWAYS meter your power BEFORE* hooking up any audio equipment. Too many buildings have bad power/wiring.
> 
> Test Hot to Ground (in North America) = about 120v
> Test Hot to Neutral ("") = about 120v, and similar as the above, if not the same.
> Test Neutral to Ground = about 0v (less than a volt or so is fine, any more is a concern)



In the OP's case, I'd meter between the two receptacles - all combinations.


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## Moose Hatrack (May 19, 2016)

On a Saturday morning trip to Menards I ran into a local handyman. In my basket were about a dozen white 15 amp receptacles. The handyman asked, "Why are you buying them without ground?"
I replied, "They're going in to my mother-in-law's house. Cosmetic change... after 50 years she's tired of ivory."
Handyman sniffed and said, "Well you just hook the ground up to the neutral... they both go to the same place in the breaker box anyway"
I wish I had thought fast enough to ask, "What about the homeowner or the next handyman who flops back and white in an upstream box?" (I did fish new Romex to specific locations where she had been using a ground lifter. I remember how popular "cheater plugs" were when grounding became code in what was it, 68?)


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## Dionysus (May 19, 2016)

Moose Hatrack said:


> On a Saturday morning trip to Menards I ran into a local handyman. In my basket were about a dozen white 15 amp receptacles. The handyman asked, "Why are you buying them without ground?"
> I replied, "They're going in to my mother-in-law's house. Cosmetic change... after 50 years she's tired of ivory."
> Handyman sniffed and said, "Well you just hook the ground up to the neutral... they both go to the same place in the breaker box anyway"
> I wish I had thought fast enough to ask, "What about the homeowner or the next handyman who flops back and white in an upstream box?" (I did fish new Romex to specific locations where she had been using a ground lifter. I remember how popular "cheater plugs" were when grounding became code in what was it, 68?)



That is indeed AGAINST CODE. Actually these days to pass an inspection (here anyways) "un-grounded" receptacles (especially those with a ground 'pin') must be protected by a GFCI as there is no bond connection for safety. Done that in many a house where it is not practical to run new wire at the time (only for those; if it is practical new wire with a ground is pulled). Of course there were the ones in the intermediate times where the receptacle had no ground, but the box was bonded; in which case its EASY.

I actually have a fun one coming up where I have to fish new wires as all the circuits on the upper floor are sharing the same ground wire. 14awg. yeah, no.

And I agree with Bill, I'd also check between the two. I would also attempt to trace the circuits to their breaker, and panel.


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## Catwalker (May 20, 2016)

I apologize in advance for derailing this thread further, but...

What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?


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## Wheezy (May 20, 2016)

Catwalker said:


> I apologize in advance for derailing this thread further, but...
> 
> What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?




To the load itself, there is little difference. The issue arises when you have an ungrounded cord, such as a lamp. With a hot neutral, the case of the light bulb is energized, rather than the center electrode. It will find a ground, such as you when you go to change the bulb. Additionally, if the neutral is switched, rather than the hot, you could still have voltage in a receptacle even with the switch off.


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## FMEng (May 20, 2016)

Catwalker said:


> I apologize in advance for derailing this thread further, but...
> 
> What happens when the live and neutral wire are switched? I have never seen the effects of this, and am curious as to what would happen. Also, in another thread (about video wall power supply, I believe) it was mentioned that what really mattered was not which wire the current was on, but the potential difference between the two. (Leading to the "is two 60V lines possible" question). How does that bit of information relate to this situation?



Without current flowing, there is no voltage drop from the hot through the load. When a switch in the neutral line is open, everything from the load to the switch stays at 120 V potential. That increases the chance for exposure to lethal voltages, which explains why the NEC chose to never switch the neutral.


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## FMEng (May 20, 2016)

We've drifted off topic. How the AC outlet is grounded contributes to the audio hum problem, but it is just one factor. The root cause is how the system of audio components are wired together, essentially making a giant, single turn transformer out of the cable shields. 

I can walk into any building, plug two audio components into any two AC outlets, and feed audio between them without hum. The problem is most people, including many industry professionals, don't know the tricks and tools to do it. The manufacturers also set us up for failures by designing equipment with inputs and outputs that are prone to the hum problem without special care. I will say that manufacturers are doing better than they were 20 years ago, but compromises are made to hold prices down.

Dealing with the problem at the AC outlet is sometimes easier than other methods, and it has the added benefit of reducing the chance of having hum problems show up unexpectedly when wiring or equipment changes are made in the future. This is why the finger gets pointed at the AC outlet, and why certain myths about AC grounding spread like gospel.


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