Indicating AC polarity?

Discussion in 'General Electronics Chat' started by Mike33, Feb 26, 2009.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    349
    25
    Hey guys,

    I need to determine which wires of an old house (cloth insulation, no safety ground) are 'hot' and neutral. I figured I'd have to build a rectifier/filter arrangement and use an LED with a 10K resistor to operate at 120V. It's a pain (need higher rated cap than rat shack carries...). I tried using just a power rect. diode to cut the AC to rippling DC, but no dice, the LED still glows.
    Any suggestions for a simple circuit to accomplish this? Just making sure some outlets are wired correctly with respect to hot and neutral. Down the line safety grounds have to be added.
    Suppose I could get an outlet tester and just mate it to one of those adapters with the ground screw tab not connected....it'll show no ground but would indicate hot/neutral reversed....
    Thanks!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Buy a cheap tester screwdriver like this:

    http://www.4barmey.co.uk/cgi-bin/pr...67885,78456:216970,185206:78457,214486,323746

    This will illuminate only when its bit touches the hot wire.

    However, have in mind that in an ungrounded house you may see a small voltage on the neutral wire. This happens if the ground wire of the step down transformer is far from the house and a large amount of current flows through the neutral wire (a voltage is dropped across the neutral wire due to its resistance).
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I'm not too sure any tester can tell AC hot from neutral without a ground reference. As an example, use any means to determine which lead on a stepdown transformer is hot with respect to the other when both float.

    I would use a wire to a water pipe or a grounded point, and measure with a meter or neon tester to either outlet slot. It's possible for the old distribution panel to be so bad that both sides will cause the tester to show voltage or a glow, but that would mean the utility company's made ground outside the house is also toast. That is very bad to find.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Is it a knob & wire setup? Beenthere's suggestion of using a known ground for reference is about the only sure way to do it. However, cold water pipe grounds have occasionally been known to be 'hot', resulting in several deaths. Google has horror stories and how it happens. Using a field detector screwdriver will only indicate voltage relative between the outlet and you, so if current is flowing in the circuit, it will light up on both. It will also light up on both if the ground is truely floating (as beenthere stated above).

    I'd suggest pounding in a fresh 8 foot copper ground stake ($38 US) outside both for measurement and to get your main distribution point grounded. After that essentially start over with a new breaker box, meter, etc. That's what I ended up doing with the last house before moving. It was built in the late 40's, had knob wiring for everything, plus strung around fuse panels/splits as electricity was used for more things over the years, both "screw in" and "bar" fuses... blech! Some of that (panel/meter/etc) will need to be done by electric company or contractor (US), but a good portion of outlets can then be DIY if your locality allows it. Otherwise, using a contractor for everything is very expensive. However, compared to a fire, it adds perspective. I did a lot of it incrementally ater the breaker box replaced the fuseboxes, but in the past 15 years, laws have changed and I'm not sure what's "Allowed" to do to something you own anymore.

    New code makes changing electrical a bit less simple. Bedrooms/some other areas need to have have an Arc Fault interrupter in all bedrooms, in addition to Ground Fault interrupters for any rooms with water if installing any new outlets. Usually the AFCI is put in as a breaker, and the GFCI is at the first outlet in a chain. I am glad they created usable AFCIs, I just wish it wasn't forced.
     
  5. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    349
    25
    Thank you guys, it was as I suspected, then! The lack of ground reference is really a pain. The house is that of a friend, who is having me just replace old, worn 2 prong outlets. I'd say the wiring is about 80 years old! It is in fairly good condition, but still...I was curious about making sure all the outlets are "fat blade neutral/thin blade hot", but I may not be able to assure that. The owner is aware of the potential for an in-wall fire due to overloading circuits, etc. Who knows what's in there?? It's probably only a 75A box, and there are MANY outlets on single circuits, a real mess.

    A few outlets have been retro-fitted for a ground, but many have none. So far it seems that the ones they grounded were done correctly - I've experienced hot on a water pipe on other jobs, it's not fun!!

    Here in the U.S., I believe a homeowner can do all the work up to the box, but it has to be connected and final inspected by a licensed electrician. Old homes like this are grandfathered (no requirement to proactively change everything), but common sense dictates getting on that as soon as you can! It's a real problem having to run new Romex, but if it were my house that is what I'd do...
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I have seen an "upgrade" where romex was spliced onto knot and tube wiring. The insulation was scraped off and the romex soldered on, but with acid core solder. The joints were big balls of white corrosion. Scary to look at.
     
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Number of outlets does not matter. Current draw does matter. Good fuses or circuit breakers are a must. Knob and tube wiring can be very good, even if wire insulation fails the wires are physically seperated. Touching a wire is bad. Lack of ground is a minus ( get that a not 0 is a minus )
     
  8. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    A neon screwdriver will work fine. Even if the neutral is not earthed at the house it will be earthed somewhere by the utility company so the neon should only light up when you touch the active line.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    This depends on where one is, as local requirements vary a lot. Some jurisdictions require permits be purchased. Others require inspection by a county inspector rather than by any old licensed electrician. Check with your local building codes division for details about your area.
     
  10. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    349
    25
    Yes, you're correct, Thingmaker...I live in the rural Northeast, Very Rural! As in, no building permits needed for anything. Inspection, in this case, would be for liability and peace of mind. But I grew up in MA, where ANYTHING needs 5 or 6 inspections, just like in CA. So that's why I will only limit myself to changing a couple of floppy outlets. And reporting that "hey, you need to get an elec. in here to upgrade this system!". LOL

    Good point on # of outlets, Russ...just pointing out that when everything on the 1st and 2nd floor is on 1 circuit, you will then likely approach the draw limit and start tripping breakers (as they do at times...). The knob and tube wiring IS ok in that the wires are separated, just that they weren't designed for us moderns who put on 10 lights and run microwaves all at the same time ;o) Over-use.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    If he keeps the knob wiring, He should at least change to an all AFCI breaker box. Cross wire/frayed plugs/wire to ground shorts are what they were designed to break on before fire happens. They managed to get the tolerance to allow non-isolated arcing motors to run, and still trip on similar low-grade arcing not appliance related.

    Too bad they were invented about 60 years too late. Modern standards for wiring pretty much fix the major fire issues, but for existing wiring, it is a recommended upgrade, although a bit over double the cost of a "normal" breaker"
     
  12. ron_o

    New Member

    Mar 1, 2009
    6
    0
    Get an AC sensor that uses the "Hall Effect" to determine which side is hot or not, similar to this:
    http://www.exair.com/en-US/Primary Navigation/Products/Static Eliminators/Pages/AC Sensor.aspx

    IMHO, don't mess with the house and just have him upgrade the electrical system. I'm sure it's Knob & Tube is what you are referring to. With it you can't even insulate around it. Also if you are going to keep it then at least GFCI all receptacles to effect a ground like condition.

    I bet the house is built with "Balloon Construction". Actually, wiring this house should make it easier on you if it's that type of residential construction. Just find out where the bracing in the walls are (they are positioned diagnally) and you are good to go.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That might be true in Australia, but it doesn't seem to be the general case in the USA.

    Here, a residential service is a center-tapped 240v secondary, with the center tap being Neutral, the ends of the secondary winding being L1 and L2. Measuring from Neutral to L1 or L2 gives 120VAC.

    Neutral is tied to earth ground at the electrical service panel. Outside of the service panel, the Neutral line is the return path for power. Ground conductors should never carry current; they are for safety in the event of an electrical fault.
     
  14. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    I really like the idea of upgrading all the circuit brakers. Not sure if gfi outlets will work correctly without a ground. Upgrading the breakers and leaving the old stuff in place and then adding selective additonal circuits, up to modern standards might be a good compromise.
     
  15. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    A GFI receptacle does not require a grounding conductor to work properly. One should label the outlet as ungrounded, of course.
     
Loading...