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[Keith Humphrey]

Mike,
Just would like to thank you for the additional use of the non-contact voltage tester. I have used one for a fairly long time to trace wiring circuits. Like most folks I've always relied on the 3 light tester and a voltmeter when going into unknown places.

Not to take too much of a tangent but I have run into situations in some older buildings where circuits are on separate building grounds that are not bonded. Is there a fairly easy way to determine this without tracing the circuits and opening up the breaker boxes? 

[Mike Sokol]

Mike,
Just would like to thank you for the additional use of the non-contact voltage tester. I have used one for a fairly long time to trace wiring circuits. Like most folks I've always relied on the 3 light tester and a voltmeter when going into unknown places.

Not to take too much of a tangent but I have run into situations in some older buildings where circuits are on separate building grounds that are not bonded. Is there a fairly easy way to determine this without tracing the circuits and opening up the breaker boxes?

I've had something similar happen at a University graduation where they pulled power from multiple buildings, each of which had its own ground rod. They had the GFCI breakers on the building exteriors trip randomly, which spoiled the ceremony. I identified the primary reason for the GFCI tripping was a TRS to XLR adapter feeding 500 feet of mic cable to the video trailer, which was powered from the other building. Since there was a direct shield connection between the two mixers that were powered from different buildings, there were all kinds of ground loop currents flowing, which tripped the GFCI's at the worst possible time. I added a Whirlwind IMP-2 DI box on the output of the first mixer feeding the mic input on the video sound mixer. By lifting the ground switch on the IMP-2, I was able to disconnect the shield ground and stop the ground loops. That has eliminated the GFCI tripping for the past 3 years I've done the graduation. 

So let's consider your situation. You want to know how to identify which outlets are being fed by separate building grounds, probably to stop hums or other artifacts. You can either choose to live with the different ground potentials and use audio isolation transformers between all gear. Or you could get someone to run a very heavy grounding strap between the different power panels (but check with local and national electrical codes first, since I don't know the legal aspects of this sort of bond, but I suspect it's OK to do).

As far as testing for separate ground rod connections in various outlets, it could be as simple as running a test wire between the ground contacts on various outlets and monitoring the voltage, which should be a fraction of a volt at most. I have a test setup using ProStat Ground Qubes and 100 ft wires with banana plugs hooked into a digital voltmeter. See http://www.prweb.com/releases/2011/03/prweb5140894.htm The Qubes are only $5 each and protect you from accidentally hooking into a hot wire. But remember to check the outlet grounds for line voltage using VoltAlert to be sure nothing has been RPBG connected.

Let me think on this a bit more, but that's what I can think of off the top of my head.

Mike

[Steve Hurt]

Trying to boil this down and make it simple enough to retain....

I have a Greenlee GT-11 voltage detector which lights up only when held next to the hot wire.
I assume it is Greenlee's version of the tester you mention.  I also have a number of the little yellow 3-light testers.

If I:
1) Test all outlets with the Greenlee tester and on all sockets, the only hot wire is the correct one (the smaller of the 2 blades),
and
2) Test each outlet with my yellow 3 light tester that tests for "open ground/open neutral/hotground reverse/hot neutral reverse/wired correct" and get the "wired correct" lights

Then, I'm good to go?

(Actually, it sounds like I need only to check the ground leg with the Greenlee tester)

[Mike Sokol]

Trying to boil this down and make it simple enough to retain....

I have a Greenlee GT-11 voltage detector which lights up only when held next to the hot wire.
I assume it is Greenlee's version of the tester you mention.  I also have a number of the little yellow 3-light testers.

If I:
1) Test all outlets with the Greenlee tester and on all sockets, the only hot wire is the correct one (the smaller of the 2 blades),
and
2) Test each outlet with my yellow 3 light tester that tests for "open ground/open neutral/hotground reverse/hot neutral reverse/wired correct" and get the "wired correct" lights

Then, I'm good to go?

(Actually, it sounds like I need only to check the ground leg with the Greenlee tester)

You are correct, and even though in theory you only need to check the U-shaped ground with a non-contact tester, you should really confirm that line level voltage lights up the tester every time. That's the best way to confirm that your battery hasn't just gone dead in your non-contact tester, which would lull you into a false sense of security. So I generally test from Hot to Neutral to Ground to Hot, in a circle. Which should of course be On - Off - Off - On. You will find that if the Ground is wired at line level due to a RPBG, your tester will light up anywhere near the front of the afflicted outlet. That's because there's a metal strap going up the center of the outlet which will indeed radiate voltage and be capacitively coupled to the tester (yes, that's how they work). Then just plug in your 3-light ground tester to confirm everything else, and you're good to go.

I would be remiss if I didn't mention GLITs (Ground Loop Impedance Testers), which actually fire a single-cycle 20-amp pulse between the Hot and Ground connections. It then measures the voltage drop on the ground line and comes up with an impedance number in ohms. A solid ground connection should be less than 1 ohms, while a corroded connection will open up and show high impedance. I bought my first Woodhead GLIT over 30 years ago when I was an IE for Corning Glass, but now use an Amprobe INSP-3 which provides all sorts of additional testing.

I think there's some money to be made by electricians who could check clubs, performance stages, and churches for proper electrical outlets. As you now know from this thread, proper power for sound systems goes WAY beyond just plugging in a 3-light tester. I think all electrical outlets should assayed for proper voltage, ground impedance, ground loops, voltage drop, and a bunch of other things.

So you see, I've been worrying about this sort of thing for over 30 years, but really started studying it in detail when I started teaching my sound installer classes. If the moderator doesn't mind me plugging my seminars, here's the link to my ASSIST seminar at www.howtosound.com/ASSIST which includes everything I've been writing about here on sound system electrical safety using live demonstrations before your very eyes.

Mike Sokol

[[email protected]]

Mike thanks for hammering this home... I hope you get royalties on all the testers that were bought in the past few days. But I'm curious - how do you ever put the pen tester down, with so much danger at every outlet?

[Mike Sokol]

Mike thanks for hammering this home... I hope you get royalties on all the testers that were bought in the past few days. But I'm curious - how do you ever put the pen tester down, with so much danger at every outlet?

No, sorry to say I can't get a single test gear manufacturer (or even audio gear manufacturer) to support my electrical safety experiments or training in any way. I do get a few free testers from them, but I think it's mostly out of morbid curiosity to see what I'll come up with for tests. That said, I'm still working away at this with my blog at www.noshockzone.org and video channel at www.youtube.com/howtoseminars but really need sponsorship support so I can do this seriously.

Of course, I don't walk around with a non-contact tester wherever I go, but do check any new situation I'm in to protect myself and my seminar gear. And of course you want to use common sense. If you blow up a piece of gear or feel a shock, then it's time to bring out the big guns and really figure out what's wrong. I think many of these shock hazards have existed for decades but it wasn't until we started pulling power from multiple outlets with modern sound systems that the extent of the problem has become evident.

If anybody has any sponsor ideas, I'm all ears....

Mike

[Jonathan Johnson]

I think many of these shock hazards have existed for decades but it wasn't until we started pulling power from multiple outlets with modern sound systems that the extent of the problem has become evident.

I think the big obstacle here is that professional audio & video is about the only application where you would pull power from multiple sources and interconnect the grounds from those sources -- without even realizing that you are interconnecting the grounds. As a result of being the only application, most electrical device manufacturers will consider it to be a "minor" problem that doesn't apply to most commercial and residential applications. Further, it is so ingrained in electricians' and engineers' minds that all electrical systems should be set up in a star pattern, that they fail to consider that the end user might create a cross connection (loop) outside the installed system.

Even in the audio world, we've long known that a difference in potential between grounds will cause hum, but we have always assumed that the ground isn't hot. We've failed to realize the potential danger of a hot ground, and we've failed to realize the real possibility that a miswired receptacle or cord could result in that condition.

I sense some "not my problem" going on here. The AV manufacturers suggest that it's a problem best dealt with by the house electrician, and the electricians say the AV manufacturers shouldn't create a ground loop (star only).

Someone earlier mentioned power conditioners. Manufacturers could integrate GFCI technology (or even non-contact voltage sensor technology) to detect current on the ground. If detected, it would disconnect all three conductors and indicate the fault. (To avoid nuisance tripping due to small leakages it could have a higher fault current limit than typical residential GFCIs.

You're far less likely to see Reverse Polarity / Bootleg Ground in new construction. It seems to me that you're more likely to see it in remodels, where a receptacle may be fed by an older two-wire circuit, or where installations may be done by poorly trained volunteers.

Again, RPBG is the sort of thing that most people won't even detect, because most appliances are single-source and won't share a common safety ground with another appliance, and most small appliances now don't even require a safety ground since they have nonconductive shells. As you talk to manufacturers, you need to highlight that interconnected grounds are a common (and necessary) occurrence, that RPBG is a very real problem, and that ground loop hum is not just an annoyance but an indicator of a dangerous situation.

In other words, "if it hums, it could kill you."

We need to educate the professional audio folks to make them aware of this issue and teach them to test for and recognize it. One voice in a crowd doesn't count for much, but if you get thousands of AV professionals talking about it, the manufacturers will take it seriously.

[Geoff Doane]

I've had something similar happen at a University graduation where they pulled power from multiple buildings, each of which had its own ground rod. They had the GFCI breakers on the building exteriors trip randomly, which spoiled the ceremony. I identified the primary reason for the GFCI tripping was a TRS to XLR adapter feeding 500 feet of mic cable to the video trailer, which was powered from the other building. Since there was a direct shield connection between the two mixers that were powered from different buildings, there were all kinds of ground loop currents flowing, which tripped the GFCI's at the worst possible time.

The GFCIs don't trip from ground current, they trip because of an imbalance in current between the hot and neutral. (A GFCI doesn't even need a ground to work, and it's one way to deal with 2-wire outlets that can't easily have a ground added, rather than making a bootleg ground.)  In your situation, I suspect there was more going on than simply a difference in ground potentials.  My guess is that there was already slight leakage downstream from the GFCIs, and either the ground wasn't good enough to draw enough current to exceed the GFCI's threshold, or the two grounds were in opposite polarity, thereby going past the trip point.

Pulling power from different services for the same system is fraught with danger.  I've heard some horror stories of extension cords run between houses.  All this discussion makes me even more determined to use my own distro as much as possible, since there will be only one tie-in point, and I know that no shortcuts have been taken with the construction.

Thanks Mike, for bringing this RPBG subject to the forum.  It's not a scenario I had considered before, although it's all too easy to see how it could happen once you pointed it out.  I did a bit of googling after you brought it up, and it is all too common in renovations it seems.  Apparently the NEC bans it (the bootleg ground, even with the correct polarity), but it's a cheap way around the problem that isn't easily detected by inspectors.  From what I could find, if a ground cannot be added to the circuit, a GFCI is an acceptable substitute, but the GFCI and all outlets downstream must be labeled "NO GROUND".

GTD

[Mike Sokol]

The GFCIs don't trip from ground current, they trip because of an imbalance in current between the hot and neutral. (A GFCI doesn't even need a ground to work, and it's one way to deal with 2-wire outlets that can't easily have a ground added, rather than making a bootleg ground.)  In your situation, I suspect there was more going on than simply a difference in ground potentials.  My guess is that there was already slight leakage downstream from the GFCIs, and either the ground wasn't good enough to draw enough current to exceed the GFCI's threshold, or the two grounds were in opposite polarity, thereby going past the trip point.

Perhaps you're correct.... but to top it off I believe that at least one of the GFCI's in question was wired with a bootleg ground (proper polarity). In that case the make-up ground loop current from the remote circuit would unbalance the H-N circuit at the GFCI and cause it to trip.  I didn't get a chance to analyze the actual failure since much of it was here-say and there was no way to duplicate the failure. As long as it worked properly the following year, everybody was happy. 

Now why in the world they would bootleg ground a GFCI I just don't know. But since all the other outlets in the old building were also bootleg grounded, I think it was inertia. It's just one of those things with old buildings and maintenance personal. I think they've now laid new conduit and proper 3-wire circuits in most of the buildings, but I'm still looking for trouble. 

Get's the old brain going, doesn't it???

Mike

[Mike Sokol]

I think the big obstacle here is that professional audio & video is about the only application where you would pull power from multiple sources and interconnect the grounds from those sources -- without even realizing that you are interconnecting the grounds. As a result of being the only application, most electrical device manufacturers will consider it to be a "minor" problem that doesn't apply to most commercial and residential applications. Further, it is so ingrained in electricians' and engineers' minds that all electrical systems should be set up in a star pattern, that they fail to consider that the end user might create a cross connection (loop) outside the installed system.

Even in the audio world, we've long known that a difference in potential between grounds will cause hum, but we have always assumed that the ground isn't hot. We've failed to realize the potential danger of a hot ground, and we've failed to realize the real possibility that a miswired receptacle or cord could result in that condition.

I sense some "not my problem" going on here. The AV manufacturers suggest that it's a problem best dealt with by the house electrician, and the electricians say the AV manufacturers shouldn't create a ground loop (star only).

Someone earlier mentioned power conditioners. Manufacturers could integrate GFCI technology (or even non-contact voltage sensor technology) to detect current on the ground. If detected, it would disconnect all three conductors and indicate the fault. (To avoid nuisance tripping due to small leakages it could have a higher fault current limit than typical residential GFCIs.

You're far less likely to see Reverse Polarity / Bootleg Ground in new construction. It seems to me that you're more likely to see it in remodels, where a receptacle may be fed by an older two-wire circuit, or where installations may be done by poorly trained volunteers.

Again, RPBG is the sort of thing that most people won't even detect, because most appliances are single-source and won't share a common safety ground with another appliance, and most small appliances now don't even require a safety ground since they have nonconductive shells. As you talk to manufacturers, you need to highlight that interconnected grounds are a common (and necessary) occurrence, that RPBG is a very real problem, and that ground loop hum is not just an annoyance but an indicator of a dangerous situation.

In other words, "if it hums, it could kill you."

We need to educate the professional audio folks to make them aware of this issue and teach them to test for and recognize it. One voice in a crowd doesn't count for much, but if you get thousands of AV professionals talking about it, the manufacturers will take it seriously.

All good points. As far as the last one, I'm looking for any potential soapboxes both in the pro-audio community and the renovators community. If anyone has suggestions for either, I'm interested.

Mike Sokol 

[ProSoundWeb Community]