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  1. #41
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    Quote Originally Posted by JST View Post
    ALL GFCIs have sensitivity problems. Q0 are better than Homeline, does not mean they are perfect.

    Issues with machinery and GFCIs are usually due to trying to use a type A consumer GFCI that is intended to handle household loads, in the shop, where all sorts of loads with higher leakage currents are present. The type A GFCI trips on just a few mA current, and is unsuited to machinery with large motors and so forth.



    "common mode" definition is actually a current (or voltage) common to all wires of a circuit..... differential is between wires, as per normal.

    So in these breakers, which have 3 wires (2 hots and a neutral), a current common to all would clearly trip the GFCI, since it would be not returning on any of the sensed wires.

    For a 230V load, voltages common to both hot wires (neutral is not used) would be "common mode".

    Yes, capacitances out on the various wires would be a cause for such a current. It would likely have a differential component since the capacitances are likely not all equal.

    Disconnecting the loads, which the OP seems resistant to doing, would determine this.
    I always work with common mode voltages in a 3 phase circuit and essentially that equals to neutral to ground voltage/current, but theoretically you are correct in that it is the summation of all phase voltage/current to ground. But common mode is a reference to ground and this is the googled definition (at least one that I quickly found):

    A common-mode voltage is one-half the vector sum of the voltages between each conductor of a balanced circuit and the local ground. Such signals can arise from radiating signals that couple equally to both lines, a driver circuit's offset, or a ground differential between the transmitting and the receiving locations.

  2. #42
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    Point being that if all the active wires that pass through the toroid have net current in them in the same direction through the toroid, then the toroid will have a magnetic field induced into it, and the detector winding will produce a voltage. If the current is in the detection range for the class of GFCI, you will get a detection and a trip.

    That is equivalent to saying there is current going out that comes back on some other path.

    So, if, for instance, there is a drop in voltage due to turning on the RPC, then if there is capacitance to ground out there somewhere, the drop in voltage can lead to a current pulse in the wires through the toroid, and the possibility of a trip exists, depending on how much current that is.

    I'd mention that contactors, relays, and switches all suffer from "contact bounce", so when they make contact, there can be a series of current change events in a short time. The bounces generate high frequency noise that can be simply transmitted out on the wires through an adjacent GFCI, and cause a trip even with a capacitance considerably smaller than what would trip a typical 120V GFCI at 60 Hz (and they tend to trip early).

    I spent several years doing EMI mitigation at a former employer, we did in-house pre-testing to assure a pass when we sent the products out for testing. So my idea of "common mode" may differ from a strict academic definition. But generally, a two or more wire circuit can have one or more differential signals, which are normally the desired ones, and a common mode signal traveling on all wires together, which is normally not the desired signal.

    In general, "ground", either local or earth, is the catch-all for current, so yes, current that goes out on wires and does not come back in on them tends to be returned via ground, although not necessarily on a specific ground wire (can be through the chassis, etc).

  3. #43
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    Quote Originally Posted by thermite View Post
    Glad yer here!

    Airey-fairy theoretical side of fornic-all-culation was gittin' too far distanced from the basic task of avoiding DIY rectumfrication fry-ups!

    15K not 1.5K on the bleeders, BTW

    Thanks!

    Yeah, he mentioned 1.5k originally, and then he corrected it to 15k. I just got lazy with the quoting and left that out.

    But I used 15k for my math (power ratings and RC time), but as I'm just a product of the public education system, I would double check any of *my* math.

    I'm just an old re-tooled electrician posing as an engineer. ;>

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    And it occurs to me, belatedly...

    That twisting the hot and neutral wires to the GFCIs together, and terminating them as close to each other as possible in the panel, just might cancel out enough noise to keep them from tripping.

    Worth a shot...

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    Quote Originally Posted by JST View Post
    I spent several years doing EMI mitigation at a former employer, we did in-house pre-testing to assure a pass when we sent the products out for testing. So my idea of "common mode" may differ from a strict academic definition.
    Digressing from the topic here, but my common mode experience is primarily with common mode voltage and subsequent current generated by VFDs. So for me, the definition is the instantaneous summation of all phase voltages (3 phase circuits)to ground. In a balanced 3 phase circuit, the voltage summation should be zero, but in a VFD due to the PWM waveform it is not. The common mode voltage waveform is totally predictable.

    In a motor, the common mode voltage is effectively the neutral point to ground. To measure the common mode voltage, we generate a virtual neutral point at the motor using resistors and measure to ground. In a motor, you also get common mode voltages when the motor is run across the line on imbalanced voltages. Much lower level than on a VFD, but at times they are harmful.

    So I believe that really don't utilize the textbook definition of common mode either..........

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    Quote Originally Posted by markz528 View Post
    So I believe that really don't utilize the textbook definition of common mode either..........
    S. Finley Breese M. didn' t so much "invent" telegraphy as to demonstrate a reasonably useful reach for it without getting anyone kilt off the back of atmospheric always-present "somewhere" electrical phenomena.

    Telegraphy & telephone early adopters had to learn about differential and common mode before the books could be written.

    I stll like it as an example for not going complacent nor arrogant.

    Do wotever you do, the "wild card" is that thunderstorm as just rolled-in not far enough away from your ass.. or the powerlines leading toward it.

    It can get comical!

    Some naif outta HQ, London is in NOVA dead-set you are to shed yer "not corporate standard" Arcnet & TCNS on Coax for his silly Ten-base T... and AS he is pontificating looks past me over my shoulder as an adjacent blue glass-clad building goes dark as night off a monster-shaped shadow as if in a mirror....

    "Oh, my God! What IS that!"

    "Standard-issue summer afternoon Northern Virginia air-mass thunderstorm!'

    Outside the office, running feet, the usual drill being relayed through the workspace:

    "Save your files, power-down, and unplug! Thunderstorm coming!"
    "Save your files, power-down, and unplug! Thunderstorm coming!"


    "Dont YOU have to shut off your computer?"

    "Nope. Those are your Ten-base-T people.

    "My staff are not involved in that daily cluster-fuck. We just keep on working all day all week. "

    "Why did you THINK I was running ARCnet & TCNS instead of Metcalf's folly?"
    Last edited by thermite; 07-02-2020 at 07:44 AM.

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    Quote Originally Posted by JST View Post
    Point being that if all the active wires that pass through the toroid have net current in them in the same direction through the toroid, then the toroid will have a magnetic field induced into it, and the detector winding will produce a voltage. If the current is in the detection range for the class of GFCI, you will get a detection and a trip.
    Or, if one lead through the torroid has a different current than the other one. All that is needed is for the sense winding to get a set amount of voltage induced and then it trips.
    Does not matter if one winding has no current and the other has over the min to make it trip, or if one winding has 15 amps and the other 15 minus delta.

    So far the results of the"take all the black wires off a 20 amp breaker and try it then" experiment are not forthcoming.


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