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Author Topic: AC Circuit Load Tester?  (Read 2495 times)

Lyle Williams

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Re: AC Circuit Load Tester?
« Reply #20 on: December 22, 2013, 01:06:51 pm »

I find my NCVT to be one of the least objective test tools I own.  Lots of false positives and false negatives.  The results are very subjective; earth and neutral normally light the thing up, just a bit less than the real live.  Normally it takes quite a bit of waving and prodding and switching for me to be confident of a result.
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #21 on: December 22, 2013, 02:45:48 pm »

I find my NCVT to be one of the least objective test tools I own.  Lots of false positives and false negatives.  The results are very subjective; earth and neutral normally light the thing up, just a bit less than the real live.  Normally it takes quite a bit of waving and prodding and switching for me to be confident of a result.

I have at least six different brands/models of NCVT's on my bench, and find they're close to 100% accurate in indicating a hot-microphone, guitar, or mixing console with at least 40 volt AC. And all will beep/flash an inch or two away from a RPBG outlet due to the increased metal area biased to 120-volts. However, I do find that the original thing they were designed for (testing outlets or wires for power) is probably the worse thing they do, occasionally needing lots of waving and prodding. What brand/model do you have? I do know that the earlier models (more than 5 years old) were much worse at consistent testing. 

The argument that a NCVT shouldn't be used to confirm a panel is dead before reaching your hands inside is probably also a good one. I always use a meter to confirm a panel is off before grabbing anything potentially hot. But realize that even a DMM can fool you. I once used a voltmeter to confirm a power supply was dead, but as I was reaching my hand into the circuit I noticed that one of my meter leads had popped out and was laying on the test bench. I stopped in my tracks, plugged in the meter leads, then found the supply was indeed "hot". It's always good to double check before trusting ANY technology with your life. And I always scrape a knuckle on the wires that I've just confirmed were "dead". That way if it is live, your hand will jump away from the wire, not grab onto it and not being able to let go. Been there, done that...
« Last Edit: December 22, 2013, 02:49:22 pm by Mike Sokol »
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Tim McCulloch

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Re: AC Circuit Load Tester?
« Reply #22 on: December 22, 2013, 03:29:53 pm »



On second thought...

(big edit)

Every tester is a little different.  Knowing when you're looking at an invalid test takes time & experience.  Not knowing you're looking at an invalid test is why I say "test, test again, and have someone else verify".
« Last Edit: December 22, 2013, 03:35:42 pm by Tim McCulloch »
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #23 on: December 22, 2013, 04:58:10 pm »

Every tester is a little different.  Knowing when you're looking at an invalid test takes time & experience.  Not knowing you're looking at an invalid test is why I say "test, test again, and have someone else verify".

There are OLD electricians, and there are BOLD electricians, but few OLD & BOLD electricians. Something on the order of 1,100 people per year are electrocuted in the USA alone, most of them in the trades. One of the highest electrocution rates belongs to outside painters, who often get their aluminum ladders tangled with overhead power lines. Plant electricians take another big hit. Always remember you're working with deadly power whenever you're hooking up any sound or lighting gear.

Let's be careful out there...

Lyle Williams

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Re: AC Circuit Load Tester?
« Reply #24 on: December 22, 2013, 09:12:44 pm »

Fluke 2AC.

Australian power outlets don't have extra metal for ground - it's just another hole for a pin.

With this outlet geometry, I'm pretty sure I couldn't pick a RPBG with the tester.  Fortunately our electrical history has travelled a path that make this condition very rare here.
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #25 on: December 22, 2013, 10:01:57 pm »

Fluke 2AC.

Australian power outlets don't have extra metal for ground - it's just another hole for a pin.

With this outlet geometry, I'm pretty sure I couldn't pick a RPBG with the tester.  Fortunately our electrical history has traveled a path that make this condition very rare here.

Most interesting. So you're saying that there's no extra metal strap on your outlets? In the States there's generally a metal strap that extends across the entire NEMA 5-15 receptacle which provides mounting tabs on the ends. If that strap is electrified by a hot ground then any NCVT in the area will light up.

I've attached what I assume to be a picture our your outlets "down under" as well as the USA version. I believe that you're correct that it may be difficult to find a hot-ground in your outlet design since it doesn't have all the extra surface area of electrified metal. However, I think that most any NEMA 5-15 outlet in the states with a hot ground would make your Fluke 2AC beep and flash from an inch or more away.

Lyle Williams

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Re: AC Circuit Load Tester?
« Reply #26 on: December 23, 2013, 03:02:18 am »

Interestingly enough the Australian plug is actually a US plug that got borrowed in (I think) the 1920's but then faded into obscurity in the US.

The NCVT certainly lights up when it comes near a live Australian outlet.  Sometimes the ground pin side of the outlet produces a lesser response, but can sometimes produce a significant response especially when the wire in the wall leads downwards towards the floor.
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Steve M Smith

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Re: AC Circuit Load Tester?
« Reply #27 on: December 23, 2013, 06:45:28 am »

The UK socket also has no grounded metal parts exposed - except for the fixing screws.



However, we do also have metal versions nut they are not so common.



For stage use, this is the sort of thing which would be put out for musicians to plug into.




Steve.
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Jason Lavoie

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Re: AC Circuit Load Tester?
« Reply #28 on: December 23, 2013, 11:37:13 pm »

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. 


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.

I suspect that the NCVT works by using the user as a reference, which under normal circumstances should act like a ground of sorts.
I think if you were isolated and touching a hot chassis with one hand and used an NCVT with the other it wouldn't light up.

If I'm correct, then putting an NCVT into any device would require a known good ground for it to be useful, otherwise it isn't any better than a neon bulb.

I could be wrong..

Jason
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Mike Sokol

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Re: AC Circuit Load Tester?
« Reply #29 on: December 24, 2013, 02:37:58 am »

I suspect that the NCVT works by using the user as a reference, which under normal circumstances should act like a ground of sorts.
I think if you were isolated and touching a hot chassis with one hand and used an NCVT with the other it wouldn't light up.

If I'm correct, then putting an NCVT into any device would require a known good ground for it to be useful, otherwise it isn't any better than a neon bulb.

I could be wrong..

Jason

Jason, you are correct on all counts. A NCVT capacitively couples through your body to the earth, listening for the 60 Hz (or 50 Hz) hum. If you're standing inside a vehicle with its chassis elevated to 120-volts above earth potential (what the RV crowd calls a "hot-skin"), then a NCVT will not beep if you get near any of the metal within your Faraday cage. However, you can reach outside and point it at the ground and it will beep.

The NCVT manufacturers also warn that if you're standing on a tall fiberglass ladder you might not couple to the earth sufficiently and get a false negative. Some goes for testing inside a 3-phase power panel since it's possible (but I think  unlikely) to find a null spot in the middle of the 3-phases that will give a false negative.

But building a NCVT into a tester is pretty simple as long as you intend it to test while someone has his hand on the side of the tester to provide the earth ground capacitive reference. I also have a short video of me with a Amprobe NCVT taped on the side of my Sprinter van with a little light-pull chain handing down to a few inches above the earth. Whenever the chassis/skin of the van is electrified with AC voltage, the NCVT will start alarming without anyone touching it. That's because the body of the tester is coupled to the side of the van, while the sensing tip to coupled to the earth via a paper clip and a little chain.

More to experiment along these lines later, but it sure is interesting.
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