What rules for ground wire attacment in a control box?
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  1. #1
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    Default What rules for ground wire attacment in a control box?

    You have all seen the grey control box with hinged door. Open door and a stud exists on the door.
    The bottom of the box has a white colored plate which attach to the bottom of the box with a screw in each corner.
    The screw in each corner mates with a welded stud in the bottom of the box.

    A 120v line into the box will connect to a circuit board mounted with aluminiun stand-offs on the white plate.
    A second circuit board will be mounted on the door. This board will be on aluminium stand-offs in each corner,
    the standoffs mounted to a aluminium plate, with the plate held by the door ground stud.

    The first circuit board is a power supply to the cpu board on the door. So there are four wires going from the first
    board to the board on the door (24v, 5V, 3.3v, power supply ground, not frame ground).

    So the questions:
    1. Can the incoming power (2 wire 120v) with ground (green wire) be connected at the circuit board 1 (power supply)?

    2. Or does the green wire always go to a stud? And is the same stud used to connect a wire braid to the door?

    3. Is it normal to connect the circuit board ground to frame ground via one of the weld box studs?
    This is done by a circuit board trace routing directly to a board mounting hole.Or should a separate
    wire go from the circuit board to a single stud for all ground connections?

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    My opinion is #2 Yes and #3 No. I think it is best to keep the AC safety ground (the green wire in the power cord) and signal ground separate. All signal ground connections should go to the same point and that point should be isolated from the green wire ground.

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    Per NEC all grounds must go to the same buss, no room to free lance...Phil

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    Thanks. I'm making those circuit boards and don't want to do it twice. Using this stuff.

    Toner Reactive Foil - Green

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    Your ground cannot go through a PCB, because PCBs aren't as robust as a cable - it's too easy for a solder joint to fail (solder joints are probably not allowed for grounds anyway), the cross-sectional area of the copper PCB trace is probably much smaller, and it's much easier for it to be inadvertently scratched or cut.

    A few options:
    • A ground busbar bolted to something, with tails to everything needing to be grounded.
    • A ground stud, with a lug for everything needing to be grounded. Avoid having too many lugs on this stud.
    • Redesign to minimize number of ground points needed, by e.g. bolting equipment needing grounding to the already-grounded chassis, or going for double-insulation and plastic construction.

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    Welcome to the insane world of ground loops.
    At first it seems simple.

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    You can run whatever voltage you want to door-mounted controls. 120VAC is a common control voltage to see with 30mm (bulletin 800T, etc.) pushbuttons, selector switches, indicator lights, etc.

    Every normally non-current carrying exposed metal part likely to become energized in the event of a ground fault must be bonded back to the source. Cabinet hinges do not count as an effective bond.

    If the board provides a separate ground terminal, it must be connected to the equipment grounding conductor supplying the equipment via a suitably sized equipment bonding jumper. If the board does not provide a separate ground terminal then the licensed engineers who signed on the dotted line deemed it unnecessary.

    Note that an equipment grounding conductor terminal and a 0V circuit conductor terminal are not the same. Sort of like connecting neutral and ground together - very bad juju.

    Fun unrelated fact - touching a screwdriver between the neutral bus and ground in a 400A switchboard downstream of a 1MVA unit-sub makes tiny little sparks and some cool audible buzzing. Tangible manifestation of objectionable current. Don't try it at home, obviously... this gear was locked out upstream.

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    You should not have too many safety ground connections, small buss bar or stud, both work.

    Power cord or source cabling has first stop at safety ground.

    Never go across a circuit board, all connections from circuit board to safety ground need to be single point.

    Avoid using mounting screws to make any connection.

    Ground cable to attach any moving part to ground.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    I had a teacher say to the class how he liked to have a small carpet underneath his footsies in the morning when sitting on the reading chair in the morning.

    I discovered another way to warm up the footsies. Go out to the lathe with bare feet and turn on the power. Touch the lathe and feel the tingling in my feet from excess electrons making attachments. Even better when not taking any anti-oxidants.

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    Equipment grounding conductor integrity cannot be such that it is contingent on the presence of a device either. This means no daisy-chaining through device terminals. Pigtail everything. E.g. when the next guy comes along to replace a worn out receptacle, the grounding system integrity must not be interrupted by the removal of said receptacle. Same for neutral conductors where part of a multiwire branch circuit. 'Neutral shocks' are a very unpleasant surprise.

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    In the past I did a lot of wiring of new machine tools with the hoffman box like you are describing. We used a ground bar and scraped the paint off from where the ground bar would screw on, attach with 2 screws to keep it from rotating. This is mounted to the white sub panel, and not a sub panel mounting bolt or screw.

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