Grounding an electric clock -or not?

Discussion in 'Electric Horology' started by RJSoftware, Apr 7, 2016.

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  1. RJSoftware

    RJSoftware Registered User

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    #1 RJSoftware, Apr 7, 2016
    Last edited: Apr 7, 2016
    Hello all.

    Finally finished the Telechron Banjo clock with Westminster chimes. Works great..!

    But as I was admiring the job I did a thought occurred to me.

    This 2 wire lamp chord fed clock, what if the coil where to catch fire?

    So I think maybe I should try to have an extra grounding wire. Hook the grounding wire to clock motor and other metal (basically bezel and motor) and have it connected to the 3rd wire of 3 wire electric chord.

    Now problem is that when I go to Lowes/Home Depot I see the selection of electric chords are rather thick-ish.

    Probably most are either 16 gauge or 14 gauge 3 wire (green for grounding).

    Thing is at my house I have a bunch of Telechron's running with the typical setup of a 2 wire lamp chord. I read typical 2 wire lamp chord is 18 gauge. In electrical 18 is smaller than 16 so 16 can handle more amperage which is better for safety.

    I have never had any problem with any of my Telechrons. At most they only seem slightly warm. Also I understand that they are not mechanically connected, but the coil, influences the rotor to turn via magnetic field. So a mechanical failure won't produce higher amperage.

    Still, one has to wonder of the problems that could be faced if for some reason the clock was to catch fire. For example the coil short out and for some reason not trip the breaker. Heat up and start a little fire action going.

    I wonder about the liability of a clock repairman who repairs an electric clock. Especially if in the repairs he does not upgrade the repair to present day electrical standards.

    Some luck I have and that is I find Home depot sells 16-4 wire ( 4 different colored wires each 16 gauge -all wrapped into one reasonably small chord that is black) and a chord cap that one can connect the wires and have a safe 3 prong plug which then includes grounding.

    My thinking is that I can double up on the grounding wire (using 2 of the set of 4) and the remaining 2 for the power (hot and neutral).

    So I bought some of the 16-4 and when I get home I discover that it's solid not stranded. Ok, would have been nicer and more durable to be stranded, but it is somewhat flexible. Still wondering if persistent handling down the years would eventually produce breaks. But it is flexible. Small strands of solid. Larger than the old phone wire. 16 gauge sold wires.

    Then later it occurs to me that I could just take and strip off one lamp chord wire of an extra lamp chord I have and tape it to the 2 wire. Then I have flexible wires and tied into the chord cap. So I could have the 3 wire grounding.

    I will also have to basically spend up a roll of black electrical tape to make the thing look like one chord. The 16-4 is already pretty so I am in a decision predicament.

    That and part of me says, heck with it. It's still working fine. This clock belongs to a lady who's grandfather owned it and wanted it restored as close to original as possible. But I did influence her to do it that way. Basically she wanted it to chime like the way she remembered it did when she was a kid.

    She has been very patient with me so I wish to do a good job.

    What would you do and why?

    Any applicable experiences/comments/opinions appreciated.

    All food for thought.

    RJ
     
  2. harold bain

    harold bain Registered User
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    RJ, I have often wondered why your typical plug in lamp is only two wire. I think adding a fuse (around 1 amp) might go further to safeguard your clock, but with only a two wire plug, it would be difficult to have it on the hot wire as it should be, as most two wire plugs can be plugged in either way. Perhaps two fuses, one on each wire, might be better:whistle:
     
  3. RJSoftware

    RJSoftware Registered User

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    #3 RJSoftware, Apr 7, 2016
    Last edited: Apr 7, 2016
    Another thought occurs to me. Maybe I should buy a small fuse block from automotive and put inline fuse on one of legs of the two wire lamp feeds.

    Hmmm, now we get back to the old electrical values of inductive resistance maybe. What is the starting percentage of overload or is this coil type arrangement different I think.

    I don't remember. An inductive load on other motors is related to starting amperage and I think I recall 180% of running amps. There is no capacitor on these.

    Sorry I get carried away. Momentary.

    But I don't think these types of motors are the same. Probably amperage is same no matter what. (excluding a short).

    Wonder what the typical amperage rating is. Also I had to replace the old rotor and coil with newer combo motor. It's more like a synchron than a Telechron as it has the coil loop but spins a single disc instead of Telechron rotor.

    Some of you know what I mean. It's a Telechron after market motor. No more Telechron coil assembly needed. The coil is in the motor like a synchron. The flag relies on the magnetic pull of the field coil of the Telechron to keep it from rolling over and showing the red area. When power drops out the flag rolls to red. That way if the owner sees the red flag s/he knows the time is probably incorrect.

    That did cause one small issue. The dial has a hole to show the red flag that shows when power went out. But having no Telechron coil anymore means no need for a flag. The Telechron movement she purchased for me to use did not have flag mechanism anyway.

    The only other issue is that the Banjo (Revere) has chime silence lever hole in dial with label.
    The replacement Telechron movement has a chime silence but is in the back. Still accessible.

    No flag or chime silence makes it obvious that it's a marriage of sorts, or I should say a restoration with wrong Telechron movement and the chime block I made to vertical hammer strikes to horizontal is fairly obvious too.

    Sill, I'm happy with it.


    Thoughts appreciated.

    RJ
     
  4. sundy58

    sundy58 Registered User

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    Any inductive load is going to have an inrush current. Not as sure about a capacitive load. You could swap in a fused outlet but that will probably be 15 A, way too big. I think your fuse block is the way to go. I think the ground wire would give you little or no protection or utility. As for sizing the fuse 180% to 2005 is a good starting point. If the housing of the clock is wood then the occupants are reasonably protected, there are no electronics that need a ground, the coil only flows electricity and needs no ground reference. For your peace of mind and perhaps some occupant protection a label warning that there is a dangerous voltage inside could be a good thing. For even more occupant protection put a connector in the line that comes unconnected if the back is removed, an interlock as we called them in the USN.
     
  5. David S

    David S Registered User
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    RJ a very interesting thread. You have mentioned a few things in your post. Some sort of over current protection, possibility of fire, adding a third wire ground, solid vs flexible, liability.

    First if liability is to be considered then all of the components you add or replace should have regulatory approvals whether it be UL or CSAus.

    I would not use 4 conductor solid wire. Normally if the cable is UL approved there will be printing on the outside with the applicable voltage rating.

    Regarding fire. Most of the incidents of fire that I have been involved with started not by direct shorts but rather "soft" shorts. Somewhere there is some sort of leakage resistance or bridging which causes a current to flow generating sufficient heat to start a fire. This current is less than require to trip the branch circuit over current protector. Harold's suggesting of installing a low current fuse would provide some protection in this case. However from a liability perspective then the fuse and fuse holder should be rated for 120vac and be UL approved. Most automotive fuse holders and fuses are rated for 32volts and often approved to an SAE standard.

    Grounding is normally provided for shock protection to protect people from an electrical shock should what is normally considered as accessible dead metal becomes energized. The ground wire would bond all of the internal dead metal to the third wire ground. But this is primarily a shock hazard protection.

    Now regarding two prong plugs. You can get "polarized" plugs with one plug blade being wider than the other, so that it can only go into the outlet in one orientation. For our North America electrical branch circuit outlets the wide prong is the neutral and the narrow is the hot. Of course this may not provide protection on very old houses with older branch circuit outlets.

    The best thing that you can do is to use good workmanship methods and approved materials.

    David
     
  6. RJSoftware

    RJSoftware Registered User

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    #6 RJSoftware, Apr 7, 2016
    Last edited: Apr 7, 2016
    Ok, I think the inline fuse is the way to go. (Thanks all..!)

    David. Drat..!!! I was hoping the SAE would have suffice. Certainly the glass type fuse seems good. The amperage is probably less than 1 amp so the 180 above would still be a small fuse. Probably 5 amps.

    Just curious, what would be difference as to 35 to 120 but maybe arcing/jumping potential..? Wonder as I think I have even seen SAE on regular electrical appliances. Not sure.

    Ok, noggin kicking in. We is talking more heat at higher voltage. E/R =I (aka heat). Volts goes up so does heat. Ok. So the SAE is made of softer plastic or something...?

    As to the purpose of the grounding I have always seen it as a safety wire, where doubling the current carrying capacity of the return gives greater assurance to trip the breaker. That and the added functionality of GFCI.

    When talking to some of the would be electricians at the Home Depot / Lowes I have to chuckle a little bit to myself when they can't explain the reasoning of the grounding wire vs neutral.

    I know you understand as you say "primarily" a shock hazard prevention. I also appreciate your knowledge about the "soft shorts" very interesting. Never considered that but did experience it with a nail accidentally hammered into wall where romex was installed.

    Next step where to find 120v rated inline fuse. I think this will be adequate enough.

    Being that it is AC one leg coverage should be adequate hey...! Open circuit equals no current.

    Ok, maybe a polarized plug in combo with inline fuse. Even though an open circuit is problem solved. Having the Hot cut off from any contact is more shock protection.

    RJ
     
  7. harold bain

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    Older time recorders using both telechron and synchron motors were usually fused at 1/2 amp, when they did have a fuse. I seldom had to replace that fuse.
     
  8. David S

    David S Registered User
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    "When talking to some of the would be electricians at the Home Depot / Lowes I have to chuckle a little bit to myself when they can't explain the reasoning of the grounding wire vs neutral."

    RJ I am not sure what you mean by that. It could refer to grounded three wire vs double insulated which is a two wire application. A great many if not most power tools today are double insulated. This has replaced grounded tools which used to be the norm, especially when most power tools had metal housings.

    Double insulated tools refer to a totally different construction that tries to ensure that any electrical break down between internal electrical parts cannot present an electrical hazard to someone holding the tool.

    While both tools as they exist as stand alone tools are safe. In the real world I would chose a double insulated tool over a grounded one.

    David
     
  9. novicetimekeeper

    novicetimekeeper Registered User
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    The more I read about other wiring systems the more I think we have it right in the UK.
     
  10. David S

    David S Registered User
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    Hey Nick, I am only familiar with your system from my colleagues in the UK about your "ring main" system. Can you share some more information. Of course your "normal" voltage is 240..or 230, or 220 depending on how harmonized the various European countries are, but it is still double our "normal" every day appliance voltage.

    I am interested in your thoughts.

    David
     
  11. eskmill

    eskmill Registered User
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    #11 eskmill, Apr 7, 2016
    Last edited: Apr 7, 2016
    RJ and other contributors: The subject electric banjo clock was made for indoor use and was considered by the electrical codes in the US as a "small household appliance."

    It and similar small household appliances were manufactured so as to comply with the applicable electrical code. The approving agency in the US is "The National Board Of Fire Underwriters" and its Electrical Code (NFPA 75 or "National Fire Protection Association's code 75"} The UL would have supplied the manufacturer with a label (UL) indicating that the Underwriters Laboratory had approved of the clock or appliance when used in the intended manner for which its was approved.

    In Canada, the CSA has similar standards and approvals as do most countries in the world. In Germany TUV pronounced as "tuff" in English language is the authority.

    The problem that faces the clock repairer when he repairs or replaces the electrical cord or electrical parts of the clock is his certification as an electrician or proving that the repair was made in a safe manner and in accordance with the provisions of the applicable codes.

    If the clock was originally supplied without an inline fuse or three wire attachment cord, then it is not considered needed.

    On the other hand, if you have modified the clock in some way or manner in which you consider it to constitute a fire or electrical shock hazard to the user, then you should consult a certified electrician who is qualified to supervise and guide your work.

    I think we would all agree that if the clock is older and was made without the approval labels, then there is liability if and when you make electrical repairs or modify the circuit.

    Specifically, some of the electric clocks that were made prior to about WWII in the US and in particular some made by lesser known merchandisers were housed in metal cases and would constitute a shock hazard.
     
  12. novicetimekeeper

    novicetimekeeper Registered User
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    We don't have two pin sockets, except special shaver sockets which are isolated, so all two lead appliances are fused on the live side.

    I believe the deal with the voltage is that mainland Europe was 220 and UK was 240 so it was agreed to settle on 230 plus or minus 10 which made everybody happy. Most houses here now have MCBs and RCDs and more recently RCBOs.
     
  13. sundy58

    sundy58 Registered User

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    eskmill all excellent points.
     
  14. RJSoftware

    RJSoftware Registered User

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    Hey Eskmill.

    The clockworks is basically two components. The electrical motor/armature and the remaining gear assembly.

    The original Banjo clockworks was lost some time prior.
    A substitute clockworks was taken out of a different clock (donar clock) by same maker and installed in the Banjo.

    However, the substitute clockworks of the donar clock did not have an original movement but a later model movement built to be adaptable to that particular clockworks.

    Both clocks only used a 2 wire lamp type chord (standard 2 wire chord). Both where approved at the time they where made. It is a common thing to see these motors installed in Telechron clocks.

    There was no modification made to the now clock motor. It is as it was made by manufacture.

    I believe adding an inline fuse and a polarized 2 prong plug (fat neutral) to only be beneficial.

    Maybe unnecessary but nice. A clear advantage.

    RJ



     
  15. RJSoftware

    RJSoftware Registered User

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    Ok, maybe we don't see eye to eye. :) (not an issue ok)

    What is the difference between a "grounded" conductor and "grounding" conductor..?

    (Has nothing to do with double insulated -different method of protection).

    RJ

     
  16. davefr

    davefr Registered User
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    #16 davefr, Apr 7, 2016
    Last edited: Apr 7, 2016
    The biggest risk with these clocks is the connection breaking loose from the coil due to strain/flex combined with old deteriorated rubber insulated wire. Coils don't sort out internally. A loose/floating live wire/terminal is where the risk is. (more electrocution risk then fire risk IMHO)

    The biggest improvements you can make in order of priority are:
    1. Replace internal wiring with modern THHN and tie wrap it so there's absolutely no flex/strain.
    2. Rewrap the outer core of the coil with 3M #27 glass cloth tape (not electricians tape)
    3. Use a 3 wire cord and connect ground to the iron frame of the coil frame. At 4 watts, you could probably go down to AWG 22 electrically.
    4. Use a GFI protected outlet
    5. Insert a fuse

    For my own clocks I do #1, #2 and usually #4.
     
  17. Willie X

    Willie X Registered User

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    RJ,
    I've never seen a home clock with a fuse or a 3 wire earth ground cord. And, I've never seen a comercial clock drive without a 3 wire earth ground cord, except very old drives.
    Many of the commercial drives have a 1/2 to 1 amp fuse in conjunction with a MOV, or some other type of surge/lightning protection, for the larger synchronous motors which are pricey.
    An 18 guage non polarized light weight lamp cord is what I like and they can be hard to find. You can get them online from lamp repair suppliers. Not much need for a fuse in anything with a winding so fine. If anything shorts to ground the fault will be cleared in an instant and the home breaker won't even notice it.
    I always replace like for like. As already mentioned, if you start doing mods and something flukey happens you might get the blame.
    Oh, the cord should never ever get the least bit warm, unless you are close to the coil or motor/rotor which do normally get warm.
    Willie X
     
  18. Tinker Dwight

    Tinker Dwight Registered User

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    MOV's are a close second iv starting fires.
    They do protect equipment that they are not in.
    Never mount one inside of a piece of equipment
    you care about.
    A ground wire is only for the protection of people
    from electric shock. A GFI socket is better.
    JMPO
    Tinker Dwight
     
  19. eskmill

    eskmill Registered User
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    RJ asks in #15: What is the difference between a "grounded" conductor and "grounding" conductor..?

    Specifically, a "Grounded Conductor" in the common US household circuit is supposed to be colored white and is the neutral part of the two wire circuit.

    A "Grounding Conductor" is often included in many US household appliance attachment cords. It must have green colored insulation, and longer than the other conductors so that it would be the last to sever in case of strain on the cord. It's termination in the appliance as a wire lug must be secured to the metal appliance chassis using a "metal moving" pressure technique usually with a "star" lock washer of the type intended for that use.

    I believe the above still stands as it was when I retired from the business in 1991.

    Les Lesovský in Alhambra California
     
  20. RJSoftware

    RJSoftware Registered User

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    #20 RJSoftware, Apr 8, 2016
    Last edited: Apr 8, 2016
    Hey Willie.

    Yes, this I would definitely believe. That the hair thin wire would just evaporate in a short so instantly that the breaker wouldn't sense the differential for more than a micro second. I have the polarized light weight lamp chord 18 gauge. But what is the point of using that if there is no fusing. A polarizing chord would guarantee that I could fuse the hot leg as it plugs into wall socket in only one direction. But non polarizing is same if no discrimination between hot and neutral is required. After all it's just a coil of hair fine wire, so both leads to the motor have no contact with anything but that wire.

    I think also that the main concern is over fire. I suppose electric shock is to be considered. But in my mind a person would get tingled and drop the clock. Hopefully they would not be standing in a bathtub full of water or some similar situation...

    The grounding would serve 3 purposes. 1. Prevent electric shock by metal parts exposed of the clock and 2. assure that a dead short would trip the breaker in case of faulty neutral not withstanding load. 3. Provide the capability of GFCI (Ground Fault Circuit Interuption ) to be employed. Keeping in mind that GFCI can not be employed without a grounding wire. (3 wire).

    That being said, along with getting the blame for any perceived "modification" that an inline fuse would be interpreted as being maybe I should just leave it fed via two wire as is.

    You know how it is, lawyers....

    At present the clock is fed by typical non polarized 2 wire lamp chord and the chord is tied in a knot at clock entry to prevent pulling on the electrical chord from straining the internal connection. (Two wire nuts) and that is further strapped inside of clock case to prevent the wires from dangling into the chiming levers etc.

    I guess you can imagine this will be the last electric clock I will repair for anyone other than me. If someone buys a clock I repair that is electric I am protected by "caveat emptor".

    Even though not much could probably happen and nothing has been modified by the replacement of the motor (the replacement motor not modified) there may be some legal technicality or some willing lawyer wishing to squeeze blood from a turnip for his angry clients. etc. etc...

    But the question is now, is it a modification to fuse the supply side of the clock as this does not modify the electric motor?

    In practical terms I think no. In legal terms.... not sure.

    Another thing about it is how I feel about it. All this talk could be for nothing as decades roll by and nothing happens. So while the thought passes through my conscious now and then I will only momentarily suffer pangs of doubt... Picturing the knock on the door as I am served court summons. Hmmm... As more time passes and nothing happens it should dissipate.

    I know, too much drama. But it makes for interesting debate. If the stomach can handle it.

    RJ




     
  21. RJSoftware

    RJSoftware Registered User

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    My main fear is over fire.

    One day I was sitting at home watching TV.

    I began to hear something hissing. Wondering what that noise was. I look over and I see that my fish tank heater has caught fire and roasting away. A small but impressive flame rising above the fish tank.

    Lucky nothing else caught on fire but I was glad that I was there to actually witness it and put it out. No more fish tank heaters...!

    Now thinking about this and the clock I realize that a heating element within the fish tank heater is a different kind of load. The clock I think is much less hazard prone than the heating element is. As the clock coils only taps into a small amount of current at a steady rate. A heating element being a resistive load is probably more risky because it deals with heat and heat is destructive.

    So the fish tank heater probably developed a short over time. But a clock coil of fine wire if it where to develop a short would probably burn into an open circuit condition. As no matter where the fine wire touched to be shorted it would burn away from amperage.

    The clock is definitely a better situation than the fish tank heater. So I tell myself anyway.

    RJ
     
  22. novicetimekeeper

    novicetimekeeper Registered User
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    Fuses exist to prevent damage to cabling, they are rated to fail before the cable is damaged. They won't necessarily prevent fire because what you need for fire is heat fuel and oxygen. That heat can be generated without blowing even quite a small fuse.
     
  23. RJSoftware

    RJSoftware Registered User

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    Hey Ntk. (long name :) )

    Yep. Plus the motor has a protective metal shield around the coil. That and the modification issue coupled with your statement of a probability of a fuse not totally eliminating the circumstance makes me think "as is".

    The donar clock was purchased by her and I installed it "as is". I hope this is enough to cover reasonable expectations.

    The replacement motor was identical to the donar original. Same make and model. The chord 2 wire non polarized lamp chord as same as original.

    RJ

     
  24. RJSoftware

    RJSoftware Registered User

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    #24 RJSoftware, Apr 8, 2016
    Last edited: Apr 8, 2016
    My final decision is to do like for like. Same motor as original fed by same type 2 wire non polarized lamp chord. No motor modification "as is" from factory.

    I was wondering what you guys where talking about when discussing a MOV.

    I read

    The Metal Oxide Varistor or MOV is a voltage dependent, nonlinear device that provides excellent transient voltage suppression. The Metal Oxide Varistor is designed to protect various types of electronic devices and semiconductor elements from switching and induced lightning surges.
    ---

    I don't think this applies as we are not discussing electronics but a coil magnetic field influencing electromotive motion. Not even a diode involved in this transaction hey..!

    But thoughts appreciated. Good to know also that MOV's catch fire.

    RJ
     
  25. novicetimekeeper

    novicetimekeeper Registered User
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    unrelated, but earlier in the year we finally bought a new tumble dryer. The old one had a worn bearing and for a long time we had only left it running when we were present for fear of fire.

    New one installed for two weeks before it caught fire though burning ceased once a capacitor exploded.
     
  26. RJSoftware

    RJSoftware Registered User

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    #26 RJSoftware, Apr 8, 2016
    Last edited: Apr 8, 2016
    Ntk.

    That reminded me of when my air conditioner went kapoot.

    I was watching TV again. When suddenly I hear "bang bang bang..!"

    It sounded exactly like gunshots. I thought maybe somebody was running from the cops and took refuge out in my car garage (where my ac unit is).

    This is a frightening thing especially when you live in a mobile home. Bullets could go through the walls like a knife would go through a popped can of Jiffy pop. Think aluminum foil tent. "Ping -zing ping" through my morning box of Captain Crunch no less...

    When I finally got the courage to open the door and see what was going on, I open the door and yelled "Hey what the H#ll is going on?" just then I see a billow of smoke roll by under the carport. Then another "bang" and I see it's the AC. I was trying my best to peek around the corner of the door and not get shot.

    No, no gang banger thugs. It was the AC capacitor. It caught fire and burnt the control board too.

    Ah well...

    RJ
     
  27. flynwill

    flynwill Registered User
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    The IEC uses the term "Grounded conductor" to refer to what most here in the US call the "neutral".

    I would probably just put a good quality 2-wire 18 gauge cord on the clock and call it a day. Pay attention to the splices and make sure they are tight. A fuse is not going to offer much protection against fire hazard, the hazards are probably your splices getting hot, the coil developing a short, or insulation failure leading to a low current arc as mentioned by David.

    A polarized plug will provide benefit if there is any significant difference potential hazard of the two conductors. On your table lamp the larger screw portion of the socket should be on the neutral side since it is a lot easier to touch with your fingers. But if in your clock it's just a synchronous motor coil then I doubt there's any benefit.
     
  28. davefr

    davefr Registered User
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    That's not correct.

    GFCI's are an acceptable way to provide protection on 2 wire ungrounded systems. The GFCI's do not need a ground to be functional. They measure minute differences between the hot and neutral currents. If a certain difference is recognized, it "assumes" a leak I.E. ground fault and trips the circuit.
     
  29. Tinker Dwight

    Tinker Dwight Registered User

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    GFCI sockets all have a ground pin but the device plugged into
    a GFCI doesn't require a ground wire or even a polarized plug.
    They work by detecting an imbalance in current on the hot and
    neutral wires. Any imbalance will trip them.
    Only the socket needs a ground.
    Tinker Dwight

    Oops didn't see Dave's post yet.
    Tinker Dwight
     
  30. RJSoftware

    RJSoftware Registered User

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    Hmm, learn something new every day. My understanding was (for years now) that GFCI tripped at any voltage between ungrounded (hot) and the grounding system.

    Thinking about it, current in vs current out with a difference of 5 miliamps. Running a typical drill outside on GFCI would vary the current 5 miliamps. You would think hey.

    Trying to understand the logic of it. I guess the current varies on the line side too during normal operation of the drill. So that even loading the motor heavily does not trip the GFCI. So in normal operation the load side stays constant with the line side during variation. I suppose then a dead short is possible (hot to neutral). So it must be detecting FAST changes and ignoring slow ones..., true?


    I guess those days as an electrician I never had to think about it. The grounding wire was always required anyway. The only thing I remember before they used GFCI recepticals they used GFCI breakers and I think I recall that they required a separate neutral sometimes. When they changed to GFCI recepticals it simplified things.

    But your right a GFCI will cover a two wire circuit as I understand it now.

    When your right your right -hey...!

    RJ
     
  31. Tinker Dwight

    Tinker Dwight Registered User

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    Electrons are kind of like water in a pipe.
    If you put electrons in one end, it has to come out the other
    end.
    That is, unless there is a hole in the pipe someplace.
    Wall socket GFCI will not protect against shorts, neutral
    to hot. That takes a breaker.
    It will detect a short, hot to ground, causing an imbalance
    of current. ( my hole in the pipe analogy )
    Two different kinds of shorts.
    Usually, a ground connection is used for the test circuit.
    Tinker Dwight
     
  32. RJSoftware

    RJSoftware Registered User

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    What mechanism detects imbalance, is this another instance of MOV employment?
    How does it differentiate between a varying load and a short if no grounding?

    There is a bit of a mishmash in comprehending line/load supply/demand of current as even if one is to view it as one direction flow of electrons (hot to neutral) even though AC is back and forth, the current usage is determined at the load and is given by the supply. So how does that even apply to monitoring a fault when the load could vary anyway?

    The only thing I can think would make sense as to discriminating a fault would be the element of time. Where a FAST/INSTANT rise in current of over a certain mili-ampers higher than a given expected threshold.

    As a man drilling into the wood hits the knot in the pinewood the drill strains, but it's gradual load increase not in microseconds.

    When his helper accidentally pushes his shovel in the power chord and "ping" the 2 wire fed GFCI trips detecting the instantaneous load increase of the short.

    No grounding needed.

    Only guessing.

    RJ

     
  33. novicetimekeeper

    novicetimekeeper Registered User
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    The point of it is that with current what goes in must come out. If what goes in doesn't equal what comes out then it must be coming out somewhere it should not and that's why it is treated as a fault. The current can fluctuate but it will always be the same at the same time in the circuit UNLESS it has been split with some going elsewhere which is then a fault and potentially hazardous.

    OK the physics is rather fun at a quantum level, but basically that rule has to be obeyed regardless of the fun.
     
  34. flynwill

    flynwill Registered User
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    The sense element is just a small transformer with 3 windings. Two large gauge windings of a few turns in series with the hot and netural such that the normal load current cancels out, and a small gauge multi-turn winding feeding the sense circuit. If you've ever used a clamp over the wire current meter, its much the same idea except that it's clamped around both the hot and neutral.
     
  35. Tinker Dwight

    Tinker Dwight Registered User

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    Flynwill has the basic idea.
    It has a transformer with three windings.
    Two are just a turn or two but have cancelling
    fields. One is the neutral and the other is the
    hot. Any difference in current will cause a voltage
    on the third winding. When it reaches a particular
    threshold it disconnects.
    Part of the problem is that you are thinking of
    the current coming from the hot lead, going to
    the neutral.
    When the load changes, the current flow happens
    at the same time on either side of the load, as long as
    there isn't an alternate path to ground or neutral.
    Both wires in the sense transformer see the change
    in load at the same time ( except for a tiny imbalance caused
    by the capacitance of the tool to ground ).
    For all practical purposes both hot and neutral see instantaneous
    changes. The sense transformer never see the change in load
    of the drill, both sense turns cancel.
    Tinker Dwight
     
  36. RJSoftware

    RJSoftware Registered User

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    Ah, makes good sense. (accidental pun :) )
    Thanks all -good info!
    RJ
     
  37. Cheezhead

    Cheezhead Registered User

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    #37 Cheezhead, Apr 11, 2016
    Last edited: Apr 11, 2016
    After working with UL for several years in another product area I believe that I am qualified to add the following:

    There is no concept called "UL Approved". UL Lists products or UL Recognizes certain components. If your clock is marked "UL Listed" then it has been tested either in the mfr's. UL sanctioned facility overseen by UL or at a UL facility. If UL used the words "UL Approved", that could expose them to liability if the failure of a device resulted in damage to people or property. UL has reasonably made themselves immune against lawsuits to real or imagined shortcomings in their standards and as needed will upgrade their standards to provide safety for people and property.

    A UL Listed product has been tested when new to determine that a failure will not result in damage to people or property.

    A failure such as a high impedance coil burnout of a UL Listed 120 volt clock has been determined by UL to be safely contained inside the clock. If this were not the case, then UL could encourage the clock's designer to make that so or else provide an enclosure that can safely contain the failure.

    A fuse inside the clock on the hot side wire with a polarized wall plug could help further ensure that a clock failure will not make a problem but if a housefire or electrocution is caused by the modified clock, that may give rise to insurance liability questions. If the clock is not UL Listed, then what I said may not apply.
     

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