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Author Topic: AC Circuit Load Tester?  (Read 1554 times)

Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #10 on: December 17, 2013, 10:28:08 am »

A DMM alone isn't enough - you need to do a load test, and potentially a non-contact tester to test for reverse polarity bootleg grounds.
I would agree. A load test using any of the four Ground Loop Impedance Testers I mentioned will test the wiring for not only its ability to supply load current, but perhaps more importantly for the safety ground's ability to provide a low-impedance path for ground fault currents. That's because it will likely be obvious if your circuit has a corroded connection for the line path or you're drawing too much amperage, since a brown-out situation is pretty easy to detect (lights dimming, amps clipping too easily or going into alarm mode, circuit breakers tripping, etc...). But if the safety ground path fails you can have a situation where your entire backline or PA system has all the gear chassis elevated to 120-volts above earth potential. Then touching a mic or guitar strings and anything else will allow the fault current to flow through the meat puppet.

If you think this doesn't happen in the modern world, just look below for what happened to Chad Gilbert from New Found Glory a few months ago. I don't know exactly what caused it, but this was a close call that could have killed him. Of course if all wiring was up to code and properly maintained, then testing wouldn't be necessary. But considering that we put many 10's to 100's of thousands of dollars of sound gear on the line when we plug into an unknown venue, not to mention that humans will be grabbing metal objects connected to these sound systems, spending 5 minutes to verify the wiring voltage, polarity, and grounding is pretty cheap insurance. 

Update: Chad Gilbert is now out of hospital and has tweeted of his recent experience.

Last nights incident!

While recording the NFG live album at the chain reaction last night I was shocked by the microphone. In between songs I grabbed the mic to speak and when I did it felt like I couldnít let go. I was being shocked and it froze me in place and when I was able to release my hands off of the mic I collapsed. When I came to I had friends standing over me and it felt like I was asleep in one of those Sci Fi movies when they wake up after traveling for 3 years to fight an alien. My friends escorted me off the stage. I told the band to finish the set for the sake of the audio recording. At that point I still was in a daze. The ambulance came and picked me up and drove me to the nearest Emergency Room. After getting numerous tests off and on for about 12 hours from the friendly people at UC Irvine I was released. They checked my heart for most of that time to make sure the electrical current didnít make my heart beat irregularly. Everything came back positive. I am gonna play the show tonight but this time with a wireless to settle my paranoia of this happening again because man it hurt and sucked. Thanks to all my friends and fans around the globe for the sweet messages!

Chad

Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #11 on: December 17, 2013, 10:30:19 am »

This one look similar but quite a bit less expensive:
http://www.ebay.com/itm/190978962970
Looks like it could work and does the right things, but I've never tested one. I'll see if they'll send me a sample for review.

Lyle Williams

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Re: AC Circuit Load Tester?
« Reply #12 on: December 17, 2013, 02:31:01 pm »

Unfortunately, those of us in this industry need to start thinking like electricians, rather than ignorant consumers.  Knowing how to verify correct receptacle function is absolutely critical, particularly since we often use multiple circuits, which can amplify the dangers of wiring failures or errors.

There are two paths that I think are followed:

1. Knowledge about circuit behaviour and fault conditions leads to a desire to measure for the presence of these fault conditions.  With specific tests in mind, and an understanding of the limitations of those tests, a tester is purchased and operated.  Each test can detect the specific common faults we have though of but will not detect a smaller subset of more exotic dangerous conditions we haven't though of.  This is the thinking like an electrician path and it probably leads to greater safety.

2. A test appliance is purchased and operated with the hope that the test appliance can somehow make a safe/unsafe decision.  I think this path probably leads to greater danger.
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #13 on: December 17, 2013, 06:10:01 pm »

There are two paths that I think are followed:

1. Knowledge about circuit behaviour and fault conditions leads to a desire to measure for the presence of these fault conditions.  With specific tests in mind, and an understanding of the limitations of those tests, a tester is purchased and operated.  Each test can detect the specific common faults we have though of but will not detect a smaller subset of more exotic dangerous conditions we haven't though of.  This is the thinking like an electrician path and it probably leads to greater safety.

2. A test appliance is purchased and operated with the hope that the test appliance can somehow make a safe/unsafe decision.  I think this path probably leads to greater danger.

I try my best to do the #1 path and also set up failure scenarios on my test bench to measure and analyze what is actually happening under strange wiring situations. If you assume that everything is wired correctly and 100% to code, then you could be making a deadly mistake.  For instance, RPBG (Reverse Polarity Bootleg Ground) outlets should NEVER happen. But in old buildings wiring shortcuts are often taken either to save money, or simply because a DIY guy didn't know any better. That's what makes older church wiring so dangerous. Many times receptacles have been rewired by church volunteers who have no knowledge of electricity and certainly are not licensed to be doing electrical work.

A lot of my RV readers do the #2 path and simply want to plug in something like a SurgeGuard voltage protector to give them a confirmation that everything is wired correctly at a campground outlet. Unfortunately, there are no RV voltage protectors that will detect or disconnect an RV from the hot ground in an RPBG outlet. Still, these same readers will insist that they don't need to measure any campground outlets since they have a $300 black box on their shore power plug that protects them under all conditions. I also did a gig with a guitar player who claimed that his $100 surge strip would protect him from getting shocked, even though he had the ground pin broken off of its power plug. Nope, not a GFCI, just a fancy surge strip with RF noise filters. That's just crazy, but he believed it. Yikes!!!

I did a survey 2 years ago on PSW which revealed that 70% of the 3,000 musicians and technicians who answered had been shocked on stage or by sound gear at some time in their career. We tend to accept getting shocked as just part of the business. But every shock is an indication that something is seriously wrong with the wiring somewhere and it's time to get out the meter for a closer look. Of course it would be far better to be proactive and find wiring problems BEFORE they shock and possibly kill someone.

Keep fighting the good fight for electrical safety and understanding. And NEVER accept getting shocked as normal.

Lyle Williams

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Re: AC Circuit Load Tester?
« Reply #14 on: December 17, 2013, 11:14:09 pm »

Amen brother.
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frank kayser

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Re: AC Circuit Load Tester?
« Reply #15 on: December 21, 2013, 04:44:11 pm »

I also jumped on the Extech CT70.  Of course I want a "plug in magic black box" to give me an irrefutable go-nogo power status.  I have been reading here and am learning that particular holy grail does not exist. 

My question - probably due to my misreading something and assuming I had it correct, I was under the notion that the the trouble with detecting a RPBG connection was the inability of most meters (all contact meters) to figure whether hot and neutral are exchanged. 

Never mind.  I re-read EC&M article on RPBG.  The key I missed was testers can't detect fault when both neutral AND ground are hot - they can detect -OR- just fine.

Even with my new toy (euphemism) I will continue to test with the non-contact tester as described in addition to the Extech. 

Trust no one, test, and think like an electrician.
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #16 on: December 21, 2013, 07:15:21 pm »

Never mind.  I re-read EC&M article on RPBG.  The key I missed was testers can't detect fault when both neutral AND ground are hot - they can detect -OR- just fine.

Yup, that was exactly my discovery. And after figuring this out I contacted all the Tester manufacturers (Amprobe, Extech, SureTest, Triplette, etc...) and every one of them argued with me that their testers would easily find an RPBG. But after I sent them a simple diagram and asked them to repeat my experiment, they all called me back and confirmed they had missed that mis-wiring possibility.

So how do RPBGs happen? Well in a perfectly wired world they shouldn't exist at all. However, if you're plugging into a building originally wired before around 1968, then there's a real possibility of one existing in a grounded receptacle. It's way too easy for an electrician or DIY guy to simply jumper from the neutral screw to the ground screw, instead of running a new wire with a ground back to the service panel. See diagram below. If someone in the past also swapped the hot and neutral wires in the wall, then it's an RPBG which can't be detected by any normal test methods, including 3-light testers, Ground Loop Impedance Testers, or even measuring H-N, H-G and N-G with a voltmeter. While you could run a separate wire to a ground stake outside and measure between it and safety ground wire  on the receptacle, a far simpler way is to simply point a NCVT near the ground slot on the receptacle. It it beeps at you, then the ground is at an elevated voltage above earth potential. Plus you can use the same NCVT to do a quick stage check for hot-chassis guitar amps or microphones. Believe me, the first time you point one at a mic or guitar and it lights and beeps, that will get your attention fast. 

Quote
Trust no one, test, and think like an electrician.

So true....
« Last Edit: December 21, 2013, 07:59:05 pm by Mike Sokol »
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Philip Roberts

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Re: AC Circuit Load Tester?
« Reply #17 on: December 21, 2013, 09:51:03 pm »

Is there a reason that a NCVT couldn't be built into an outlet tester? Have it be "non contact" to wiring inside the tester once it's plugged into the outlet but before engaging other test circuitry.

Philip
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #18 on: December 22, 2013, 08:03:59 am »

Is there a reason that a NCVT couldn't be built into an outlet tester? Have it be "non contact" to wiring inside the tester once it's plugged into the outlet but before engaging other test circuitry.

I've made this very suggestion to the key players in the outlet testing industry. It would be pretty simple to kludge together a 3-light cube or Ground Loop Impedance Tester with a NCVT. In fact, you can buy a current model DMM with a built-in NCVT on the top of the case. However, these DMM models will only test a wide area to see if the outlet is powered up, not a small enough spot to differentiate between the hot and neutral sides. But it could be used to check for a hot-chassis on a mic, guitar or mixing console.

So for now, at least, I still think it's the best policy to use a NCVT to double-check outlets and stage gear for hot-ground conditions in addition to a voltage and polarity test.

John Roberts {JR}

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Re: AC Circuit Load Tester?
« Reply #19 on: December 22, 2013, 11:56:04 am »

I haven't thought about this for a while, but it seems like it shouldn't be too heavy lifting to design a smart outlet strip with NCVT that automatically detects hot on ground and opens that connection path. In combination with GFCI the meat puppets could be protected and show could go on. Even if RPBG outlet is all the power that is available. It could flash an irritating red light (maybe even built into the plug) when ground is corrupted to annoy everyone and inform them that something is horribly wrong with that outlet. 

Anyone these days who is not using GFCI on back line is taunting the grim reaper. It seems GFCI should trip and prevent the shock from continuing, but will stop the show... a smart NCVT outlet strip could keep the show going, while it is debatable if that is worthwhile, or merchantable. One can easily argue the RPBG outlets should not be tolerated under any situation, and the market for these would probably be tiny (market that would buy them, not market that could use them).   

JR

PS: The smart outlet strip might even remap the hot and neutral but that adds more cost for perhaps only modest benefit (in combination with GFCI), The hot ground seems like the dominant danger so that could be opened with one power switching device. 
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Tune it or don't play it... please
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