Interesting mains "phenomenon"?

Discussion in 'General Electronics Chat' started by Mike33, Nov 7, 2013.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Ok, I've heard it all...was working on a job where the cover was left off a 240V sub-box. So the busses were exposed - a potentially lethal setup.

    Me and the GC took a look at the main panel in the basement...a breaker said to serve that sub-panel (with appropriate red & black wires) was pulled from the bus there, and also "off".

    But, my GB proximity indicator ("non contact voltage tester") went off and showed the sub box as "live". The electrician stated that sometimes a "static charge" can build up in the sub box and give a false positive when using a proximity detector.

    I want to call bull on this. Capacitive re-charge? To the 50V necessary to set the thing off?? And - I believe these proximity detectors require AC CURRENT FLOW to go off???

    Either that or we've just encountered a really cool but little discussed phenomenon. I don't want to get the electrician into hot water, but I also don't like to be made to look the fool, so I haven't said anything.

    Thoughts?
     
  2. tcmtech

    Well-Known Member

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    Personally I prefer to not use those non contact testers because they tend to give false reading to often.

    What you are saying is not all that of uncommon thing to see happen with those testers on systems with higher working voltages.
     
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  3. Mike33

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    Feb 4, 2005
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    So, they are known to give false positives, repeatedly, even when you repeat the test on a bulb and turn it on and off, with expected results? (bulb on, beeps....switch off, beeps stop, etc)

    I tested those busses 2 or 3 times, and that sucker was going off...I really wish I'd had a DMM with me!
     
  4. poopscoop

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    Dec 12, 2012
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    Not sure from your description, but that sounds like a "Phantom Voltage", which is an effect of capacitive coupling between AC wires.

    This can also fool a DMM if it doesn't have a sufficiently low impedance.
     
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  5. wayneh

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    Sep 9, 2010
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    +1 Definitely a possibility. Attaching a small load (e.g. a small wattage bulb) will pull the voltage down IF it's just "phantom" coupling. Inductive, I think, not capacitive.

    I've just been working on my refrigerator, and measured a neutral wire at 120V with my meter. But it was a phantom, caused by the other end of the wire failing to open. The orphaned length of wire parallels the hot wire, and that was enough to give the phantom voltage.
     
    Last edited: Nov 7, 2013
  6. BillB3857

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    Feb 28, 2009
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    That is the reason a solenoid type voltage tester was on the required tool list when I hired in as an electrician at an aircraft plant. It will draw enough current to kill any false readings.

    http://en.wikipedia.org/wiki/Solenoid_voltmeter
     
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  7. #12

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    Nov 30, 2010
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    My first instinct is that a sub-box can not measure as, "live" because it is connected to bond. Faulty wording? Faulty assumption on my part? Faulty bond connection?

    I want to report a false negative while I'm here. I was sniffing a broken light bulb with my non-contact tester and it showed "dead". A metal reflector for the lumens was shielding my voltage sniffer from the live wire.

    Never bet your life on a single layer of safety.
     
  8. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Nope, never would trust such a rudimentary thing. I'm just sorry I didn't bring my decent DMM with me, which would have provided definitive results. I am suspicious about the inductive 'recharging' thing, altho I have seen it happen (the opposite actually, capacitive-type, with caps that have been discharged in tube amps coming 'back to life' a bit. But only a few volts).

    This 'tester' reads from 50-600VAC, and has worked flawlessly for me. So I am thinking minimum of 50VAC was present. I still actually read the wires if I'm doing any work, but to move the tester away as I did, press the button and approach the box, get the hit near the busses, and repeat this like 10x makes me suspicious. I think the electrician is covering his butt.

    He left the cover off the main panel in the cellar, for reference....one tripped breaker & curious homeowner and ZAP. He told GC he was in today and made sure it was dead, so no need to carry it further.

    Altho I'll have my DMM with me in the truck tomorrow. :D
     
  9. tcmtech

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    Well if your electrical contractors are anything like ours I wouldn't trust him to tell you if the sun was shining while you both were standing outside. :rolleyes:

    Lying bunch of crooks is what we have around here and never corner one about such trivial things as what the NEC code books says relating to what they are doing.

    "I'm a licensed contractor so I don't need the book" is the first reply they give. :mad:
     
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  10. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
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    Yup. That, plus the offhand wisecrack: "oh, guys, that box is live, don't touch!" before leaving a few days ago.

    Suddenly, "no, no, that's fine! Your guy with the little proximity tester must be wrong.". After having gone and disconnected the box....

    An any rate, if that WAS the case, it was the single worst safety thing I've ever seen in over 20 years working in the trades.
     
  11. PackratKing

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    Just peeked in to add... It bugs the bejeebers out of me when I run onto a contractor that somehow thinks they have to resort to scamming to pad the bottom line...

    I have always noted, that if you are paying attention, and do your best, word-of-mouth advertising will keep you in more honest work than you can handle anyway, and there is absolutely NO need to sell your reputation for a fast buck...:D ergo my sig.
     
  12. tcmtech

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    The other week I was rebuilding some metal halide flood lights for my dad and found that I had several ballast donors that had bad capacitors on them. 440 VAC 20 uf.

    I went around town to see if I could get any locally at our motor rewind shop but unfortunately they didn't carry that size in that voltage.

    Next stop was the HVAC place. (I hate them) They had them but wanted $25 each for cheap off brand chinese. I don't mind that brand but I told them no thanks. I can buy them on line for about $5 each if I don't mind waiting a few days. (To save $100 or better I can wait a month to be honest.)

    As a last resort I stopped by one of our bigger electrical contractors that I see doing repair work on our city street lights all the time.

    I had to explain myself three times before anyone figured out what I wanted. Sure enough they had a whole shelf of used take off to pick from! I grabbed 4 thinking I would be able to get them for cheap being they were clearly well used.

    Nope. Not even close. the wanted $100 for the four. I told them no thanks I can get these for around $5 online and walked out leaving them on the guys desk. The guy told me he really doubted that. :mad:

    Anyway I went home and found the exact same ones through Mouser Electronics for $4.39 each including the shipping! I ordered 10 even though I only needed 3. ;)
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I have problems at home locating mains wire runs using a proximity tester. The idiot house builders used foil-lined wall-board on some internal walls. That inductively/capacitively picks up phantom voltages which register as 'hot'.
     
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  14. ian field

    Distinguished Member

    Oct 27, 2012
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    The one I have is a reputable make (Fluke) - it can be rather erratic when the battery is low, with a good battery it seems reliable enough for a first check - but I'd always use a contact type tester before sticking my fingers in the gubbins!
     
  15. #12

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    I'm a licensed contractor, and I carry the book in my truck.
    First problem? I'm not a licensed electrician, I'm a licensed HVAC contractor.
    Second problem? I am not qualified to take the test to get a license for electrical work.

    That's what bothers me. I am qualified to work on satcom radios for nuclear submarines but I'm not qualified to take the test for an electricians license. That is because it is a throwback to the union days when the buzzword was, "job security". Many of the trades (if not all) require years of apprenticeship before you are allowed to take the test for a license. I was tracing TV control panels with more than 50 wires, over 40 years ago, but I'm not qualified to even take the test for the red wire, the black wire, and the green wire. Give me a break!:mad:

    ps, I buy 10uf/370V caps for $3. Those HVAC guys were doing exactly what you thought they were doing.
     
  16. tcmtech

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    Similar issues here as well. knowledge wise I have found it very clear I can literally run circles around them on all practical applications of single, three phase and DC or mixed systems tech.

    The only thing I can't do is give a person a straight face bid of $20K for a $4K job that would need less than $2k in new materials and 25 hours of labor to do by myself and then use all salvaged stuff I scrapped from bigger jobs to do it with.

    Same with doing heating and air conditioning work as well. There isn't any system I can't figure out if given half a chance.

    Right now I am building a custom outdoor boiler for my brother so that he can eventually heat his house and up to three other buildings with. :cool:
    I have it about half welded but working with 3/16" Hardox 450 plate gets tiring after awhile. :rolleyes:

    Try talking shop with any of our local HVAC guys about a project like that and its all deer in the headlight looks followed by a load of whos going to be liable if XYZ happens BS.
     
  17. killivolt

    Active Member

    Jan 10, 2010
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    I've got a stud finder that seems to do a pretty good job locating live wire.

    I could have used my proximity tester the day I decided to cut an empty conduit open to run some audio wire under a floor, I didn't think to bring it.

    That one cost me $400 that day for an Electrician, if I'm not a licensed and bonded Electrician I'm not doing it. I will do short runs at home, after calculating and staying to code.
     
  18. killivolt

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    Jan 10, 2010
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    I gave up HVAC 25 years ago. I'm getting to old for it.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    I hope you seem SOME value in talking about boiler safety/liability. Boiler explosions have claimed a lot of lives through the years and unrated boilers are illegal pretty much everywhere in North America.
     
  20. #12

    Expert

    Nov 30, 2010
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    You need to realize that I'm the exception, not the rule. Most "trades" people barely finished high school (or a GED) so they could qualify to take the test for a license, and they chose trade school because they thought it was easier than, "real" school.

    I, on the other hand, worked as an analog circuit designer before I started taking HVAC seriously. Electronics is my "ace in the hole". When a new design arrives, I just read the schematic. When you try to talk shop with the local tradesmen, half of them need a "factory rep" training class to fit a hard start kit.

    I'm not saying they are idiots. They do seriously good work, and their list of skills is as long as your arm, but they are not designers. In most cases, it is forbidden to change a factory design because of government regulations and liability issues. They don't have experience as designers, and they aren't going to get any. They are boxed in and that box is as far as most of them can think. The days when we built whatever was needed are over. It is actually illegal for me to oversize an evaporator to get higher efficiency because I can't afford the tens of thousands of dollars worth of government red tape to certify the efficiency rating of one air conditioner. That's it in a nutshell. It is basically illegal to do design work on a case by case basis. There is nothing left but good craftsmanship and replacing black boxes.
     
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