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Wiring Diagram for Potter&Brumfield PM-17AY-120???

Discussion in 'Electrical Engineering' started by John, Nov 26, 2003.

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  1. John

    John Guest

    Two things:
    1) I know that impedance and resistance are the same for a resistor, but am
    concerned it is not the same for the coil. It has been a very long time
    since my EE courses, so I am a little fuzzy on this. I suppose though that
    for 60hz, it doesn't matter what the voltage or current are, the impedance
    for the coil is constant, so a resistor that matches it in one circumstance
    will match it in all (for a constant hz)?

    2) If the coil draws 128ma at 120, it has an impedance of 1000ohms. No? So
    why are people telling me to use a 3000ohm resistor? That would cut the
    current at 240v down to 60ma, and might not be enough for the coil to work
    properly.

    Thanks
     
  2. John

    John Guest

    I guess I am being obstinate for a few reasons
    1) As I read the spec sheet, you can put a resistor in series with a DC
    coil, not an AC coil. I don't know if they have a good reason for that, or
    just want to sell 240v coils.
    2) If I were to use a resistor, it seems to me that it should be a 1k to
    double the impedance, not a 3k. The coil draws 128ma at 120v, so it's
    impedance is 1k, no? So, when people advise me to use a 3k resistor, I have
    to be skeptical. No one has explained why they say 3k, so I could be
    misunderstanding something here.
    3) While I am not saying it is a desirable practice to send current down a
    ground, my oven puts 7a down the ground (as do millions of other electric
    ovens) and it meets code and seems reasonably safe; so it just doesn't seem
    that terrifying.

    Perhaps a resistor is a better way to go, and if it is, I will do that.
    Before I had a wiring diagram for the starter (which is what I tried to get
    here) an industrial electrician told me that in general, going to ground was
    much better solution than a resistor. I will have to clarify with him why he
    said that.

    I appreciate your help though.
     
  3. John

    John Guest

    Well... not quite.
    The START switch is momentary contact. After the momentary close, current
    goes to the coil through one of the relay contacts. So, if the ground
    opens, the coil loses power, the relay opens, and there is no voltage to the
    coil or frame.
    I thought all START switchs worked like that; maybe not, but I know mine
    does.
    I said "almost impossible".
    Ever get a shock? I imagine everyone who works with electricity has had
    plenty of them. Every go into VF? I haven't, nor have I spoken to anyone
    who has. (Now of course I am talking about 120v; all the reported
    electrocutions I have ever hear about have been from 4000v or more.)
    The figures I have seen is that it takes 3 seconds at 100ma for VF, and that
    normal shock from a 120v hot is about 10ma because a person's resistance is
    pretty high. 3 seconds is a pretty long time to be in contact with a table
    saw while getting a shock, even if it were 128ma.
    Only this is not a normal shock from a 120v; this is a shock after the coil
    has caused a voltage drop, so it should be even less. Even if I were in
    salt water, I couldn't get a shock of more than maybe 60ma because of the
    coil.
    And this completely ignores the alternate ground directly through the frame
    to ground, which is certainly better than through me; for if I am in salt
    water, the frame has to be also.
    So, I probably wouldn't get any shock at all.

    Bear in mind that the same things have to 'align' for this to be dangerous
    as have to 'align' for my oven or dryer (which quite legally send current to
    ground) to be dangerous, and it just doesn't happen. (The oven and dryer
    are actually much more dangerous, as you are likely to have damp hands when
    using them, but there still are no deaths from millions of appliances. I
    have never used my saw with damp hands, nor have I ever seen anyone else do
    so.)

    Would you like me to ground the saw frame to the water pipe it is next to?
    A 128ma ground loop is not a big deal. (now attack me for that statement).
     
  4. John

    John Guest

    Actually I was just looking at lights today! I figure I need a 15w, and
    they have candellabra bases, which I couldn't find cheap sockets for. Any
    suggestions?
    That is correct. The ground connection breaking would have the same effect
    as pushing the STOP switch; cutting power to the coil.
     
  5. John

    John Guest

    I want to thank you both for the most thoughtful answers in this whole darn
    thread.
    I guess it is the in rush that is concerning me; I am afraid they had a good
    reason for suggesting resistors with DC but not with AC.
     
  6. Guest

    It doesn't say you cannot use a resistor on AC coils. They
    don't have a relay speced at 220 V DC, so they tell you
    how you can do it at that voltage with their 120 V DC relay.
    Agreed, with some comments. At one time, code allowed the
    connection of neutral and ground within the range. That is
    no longer allowed. In any event, if your oven is putting
    7A to ground, it was not wired to code at the time it was
    installed, or it is defective. It shouldn't even be putting
    7 A on neutral. The neutral to an oven serves controls
    and lights, which don't draw that kind of current. Code
    never allowed even the low current that lights or controls
    draw to be placed on ground. What it did allow was a
    circuit wired without an equipment grounding conductor for
    some specific appliances - ranges, ovens and dryers - and
    appliance manufacturers connected the neutral to the frame of
    the appliance inside that appliance. I agree with it not
    seeming that terrifying - but that's one reason we have
    the code. What you or I don't think of as terrifying can
    nevertheless kill us. The experts establish the code as
    a practical safeguard. The code doesn't allow intentionally
    putting current on the ground conductor. We may deal with
    installing 200 ampere services, wiring things hot of necessity
    while doing that or some similar situation. So we could sneer
    at 128 mA - but medical experts tell me it can kill me,
    and GFCI's are set to trip at ~5mA. So I figure I better
    follow the code. Besides, the last thing I want is a shock
    when I'm feeding some wood through the saw. My name's not
    Lefty, and I want to keep it that way.
     
  7. Ben Miller

    Ben Miller Guest

    John

    You are completely misinformed about electrical safety & electrical hazards.
    Many electrocutions occur at 120 volts (& 208, 240, 277, 480). There are no
    rules such as you described. The amount of current and the time required
    depend on many variables.

    Do not connect the relay to the machine frame. The hazard willl exist if the
    ground connection becomes a high resistance. It does not need to open
    completely. Others have explained this to you in detail.

    The other ideas, such as series resistors will work BUT you need to mount
    them, space them from wiring, and insulate or guard them appropriately.
    Unless you are educated in designing machine controls, I do not recommend
    this.

    I suggest that you buy a 240 volt relay or pull a neutral wire, and be done
    with it.

    Ben Miller
     
  8. Guest

    Right. The DC resistance of the coil is 120 ohms - but I
    did not use that figure. Instead, I used E = IZ to figure
    out what Z is. (It actually does not matter what Z is, you
    can figure out the size resistor needed without knowing
    Z. See below)
    Ummm - I got a little lost here. I didn't mention inrush
    or impedance energized vs de-energized in my other post,
    so I don't understand what you have in mind.

    Try it this way, it's simpler and you don't need to know
    the value of Z (coil impedance) in the original:
    The unknown Z draws 128 mA on a 120 volt circuit.
    We first double, then halve the voltage with this circuit:
    hot---Z----Resistor----hot

    Therefore, the resistor has to drop 120 volts, while the
    relay draws 128 mA. If we stuff a 1K resistor in there,
    it will drop 128 volts when .128 amps is drawn by the
    unknown Z. That's close enough.

    In my first post, I recommended putting a voltmeter
    across the coil on the new circuit to see if the
    voltage was around 120, because 128 mA is a nominal
    figure. We don't know what the current draw really is,
    so some changes might be needed.
     
  9. John

    John Guest

    Everything you say is reasonable, except for one fact.
    My GE stove does feed 7a through the neutral/ground. When I first discovered
    it I thought the stove was broken, but GE sent me a wiring diagram and that
    is how it is designed.
    I don't like it, because you are quite likely to have wet hands when using
    the stove, but I am sure GE made lots of them.
    Just for fun I tested my 1 year old dryer. There was 4.1a in the ground. I
    haven't looked at the wiring diagram, but I expect the motor is 120v.

    The saw is next to a water pipe that shows low reistance to true ground. If
    I temporarily went the ground route, would it be reasonable to ground the
    frame to the pipe? I know ground loops are discouraged, but it is only 64ma
    at most.
     
  10. Guest

    You can ask the question 9 thousand different ways, the answer
    is still the same. Here's what you've been told, copied from
    replies to you in the thread.

    "I would NOT wire the relay coil circuit from one hot leg to
    ground"

    "Don't wire it to ground."

    "DO NOT use the ground wire under A NY circumstances"

    "If the coil can pass 128 mA, then you most certainly *can*
    be dead."

    "Do not connect the relay to the machine frame."

    "Current should never be on ground intentionally."

    "DO NOT WIRE THE RELAY COIL TO GROUND!!!!!!"


    I'm sorry you don't get it. I don't know how to make it
    any clearer.
     
  11. daestrom

    daestrom Guest

    This is where you are mistaken. The relay contacts opening will not
    I thought about the 'momentary' type of setup after I wrote before. But
    there is *still* a problem with an open ground when you use the ground for
    the coil. Say the unit is running when the ground opens. As you say, the
    coil drops out and the unit stops. So far, so good. What's probably the
    *first* thing you will try?? Push the start button again. Puts 120V 'hot'
    on the coil, through coil, to frame. If you're holding frame.....
    There are 'maintaining' style that use light duty on/off switch to control
    relay that in turn controls power circuits. Interlock/safety devices just
    wired in series with on/off switch so any one can open circuit to control
    relay and shutdown unit. Seen 'em both. As this was not a large, complex
    machine, I guessed it could go either way.
    Oh woe unto thee....

    There are probably just as many, if not more electrocutions caused by 120V
    than any other. The wide spread use by lay persons is a major factor.
    People that work on 4160 and up almost *always* receive proper training in
    such things (of course, accidents can happen anywhere).
    What you say about the coil being in series with your body is true. But
    remember, blood in the human body has about the same salinity as seawater.
    The only part of your body that has a high resistance is your skin. Cover
    that with some honest sweat and you'd be surprized how low the total
    resistance can get. One old study showed a hot sweaty sailer working in an
    engine room can have as little as 300 ohms from one hand to the other (right
    across the chest). The Navy had some old training film, 'The Deadly
    Shipmate' that demonstrated how *many* on board electrocutions where caused
    by 'good ole 120VAC'
    Do you *really* want to bet your life on this? Rubber feet on the legs?
    Plastic castors to make it roll around easy? You 'just happen' to be
    holding another power tool that has a metal case and a good ground?? So many
    possibilities.....

    I may be over stating things, (I do get teased about being 'Mr. Safety'
    sometimes), but I just wouldn't want anyone to get hurt. You do what you
    feel is right. I just wanted to give you some 'food for thought'.

    As far as 128ma ground loop.... No, in this sort of application it isn't a
    big deal by itself. If it were in process controls or networking, that
    would be different. I was only concerned that you may not understand some
    of the safety implications of this code issue.

    daestrom
     
  12. daestrom

    daestrom Guest

    I find that 'incredible'. The motor probably *is* 120V. In that case, the
    motor current should be from one 'hot' to the *neutral*, not the equipment
    grounding conductor. If you see 4.1A through the green, equipment grounding
    conductor, then you wired *that* up wrong as well.

    A typical electric dryer will have *four* wires, two 'hot' from a two-pole
    supply breaker, a neutral for carrying the motor, timer, interior lamp
    currents, and an equipment-grounding-conductor (often green) for bonding the
    frame back to the service panel. Sounds like you only have three wires and
    are using the 'middle' as both 'neutral' and equipment-grounding-conductor.
    I don't think code has allowed this for many years now.

    When you got the new dryer, you probably used the existing outlet. Did it
    have a three-prong pigtail or four? For proper grounding, it should have
    been a four-prong one.

    Either that, or you're not measuring the current in the
    equipment-grounding-conductor correctly.

    daestrom
     
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