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Mixing Ground Connections For 120V AC and 24V DC

  1. Sep 21, 2016 #1
    I have a questions about AC and DC components being grounded to the same spot.

    I have a power supply box that takes in 120V AC and brings that into a transformer that converts it to 24V DC. On the way into the box the ground wire from the 120V AC is brought into a grounding terminal block and from there it goes to the metal base plate on the enclosure and into one of the inputs on the 120V AC to 24V DC transformer.

    Outside of the box I have a 24V DC pump motor. This motor has 3 wires coming out of it 2 for the 24V DC power and a green ground wire. These 3 wires are brought into this power supply box thru a cable gland. The 2 power wires connect to the output of the transformer. My question has to do with the ground wire for the pump.

    Can I connect this wire to the same terminal block as the 120V AC ground wires. The terminal blocks that the ground wires go into have a metal to metal contact with the base plate of the power supply so everything is grounded with the 120V AC ground wire. That being said if I cannot mix these ground wires then I do not know what to do with the 24V DC pump ground.

    Could I install a diode in line with the pump ground so current could flow from the pump into the ground in my enclosure but not from the enclosure into the pump? Is this even needed? Any help is appreciated. Thanks.
     
  2. jcsd
  3. Sep 21, 2016 #2
    Hi!
    How do you connect your apparatus to the 120AC line?
     
  4. Sep 21, 2016 #3
    There is a 3 prong plug that plugs into a wall outlet.

    The cord then comes into my enclosure thru a cable gland. From there the live and neutral wires go to the input of my transformer. The ground wire goes to a terminal block. Then the ground wire also goes to the input of the transformer.
     
  5. Sep 21, 2016 #4

    berkeman

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    Yes, connecting all of your device grounds to Earth ground and the metal case should be fine.
    But you have a fuse and a switch in the Hot lead before the transformer, correct?
     
  6. Sep 21, 2016 #5
    Yeah I should have said that but I was trying to cut down on typing. The hot lead for the 120 goes thru a switch then a fuse then to a terminal block then to the transformer.

    So I can connect the grounds together. Should I even bother with the diode inline with the 24V DC pump ground?
     
  7. Sep 21, 2016 #6

    berkeman

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    Just connect the grounds together. There are times you might want a floating output ground, but your device should work fine with all the grounds tied together.
    To meet safety standards, I believe that the fuse must be the first thing that the Hot lead connects to in your device. Route the Hot lead through the fuse first, then to the switch. :smile:
     
  8. Sep 21, 2016 #7
    Sounds good. Thank you for the reply.
     
  9. Sep 21, 2016 #8

    berkeman

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    Your welcome. :smile:

    One example of a time when you would want to float the output "ground" is for DC power supplies. You might have a 5V and 0V output from your transformer, rectifier and voltage regulator, and you might have a separate Earth ground connection at the outputs. That way you can "stack" your DC ouput voltage on top of other voltage sources, or power "floating" circuits that do not have an Earth ground reference.
     
  10. Sep 21, 2016 #9
    I am still kind of new to all of this. Could you explain a little more. What do you mean by stack? Are you saying this only applies if there is more than 1 voltage source?

    The 24V transformer I am using does not have a ground connection on the output side only the input side (120V). Would this apply to me then. I included a link to the data sheet of the transformer if it helps. I have part number TSP-090-124. Thanks.

    http://www.tracopower.com/products/tsp.pdf
     
  11. Sep 21, 2016 #10

    berkeman

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    Say you have a triple output DC power supply, providing 5V and +/-12V. Generally there will be a separate "-" terminal for the 5V rail and for the +/-12V rails. Since they are floating, you can use the +/-12V rails to make a +24V supply, and connect the -12V output to Earth ground if you want it Earth ground referenced. And you could make a +29V supply out of the 3 outputs by staking them appropriately. You could choose to reference the stack to Earth ground by making an appropriate tie between the Earth ground connector and one of the PS outputs.

    You also sometimes do not connect your device's ground to Earth ground if you know that the Earth ground connection is noisy, and your device needs a quiet ground for your application (like in A/D converter applications). The Earth ground can be noisy when you have a large powerful motor running nearby, for example.
    That looks like more than a transformer, if it is giving you a DC output. Most likely it is a transformer followed by a bridge rectifier and a voltage regulator. The transformer itself does not have an Earth ground connection (it has no DC connections). The input Earth ground connection most likely just goes to the metal case for safety reasons. You can choose to Earth ground the output "-" terminal, or leave it floating for the applications I mentioned above.
     
  12. Sep 21, 2016 #11
    Thanks for the explanation. Just to make sure we are on the same page. If I have a DC power supply that has a +12V terminal and a -12V terminal I could get either 12V or 24V depending how I connect things and if they are left floating.

    Then back to my situation if I leave the DC outputs on my power supply floating and then just ground my pump motor to the same place as the ground wire from the cord that has the 120V AC coming in I should be ok. Thanks again for all the help.
     
  13. Sep 21, 2016 #12

    berkeman

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    Correct. One way to check that the "-" output terminal is floating is just to use your DVM to check for continuity between the "-" output and Earth ground. Generally there will be no DC continuity (but a fair amount of capacitance).
    Yeah, most likely the "ground" connection on the motor is just to ground the metal case of the motor. You should be able to float the DC input power to the motor windings. In fact, this will help to keep your motor from injecting a lot of noise into the Earth ground connection. Again, you can beep from the motor ground connection to the motor "-" DC input to see if there is a DC connection inside the motor or not.
     
  14. Sep 21, 2016 #13

    berkeman

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    Quiz Question -- Why is there generally a fair amount of capacitance from the + and - outputs of this kind of power supply to Earth ground? :smile:
     
  15. Sep 22, 2016 #14
    FYI, airplanes have numerous voltages, but all share the airframe as the ground... 115VAC 400Hz and 60Hz, 28VAC 400Hz, and 28VDC. The 28VDC is created from Transformer Rectifier Units (TRUs) fed from the 3-phase 115VAC. Neutral and ground are both connected to the airframe relatively close to equipment, while the power wires typically run anywhere from 10' to >200' to circuit breakers.
     
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