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Thread: Electrical Question

  1. #11

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    In a house, nuetral is a current carrying path. When the UPS takes over, there will be no current carrying path. The ground of the fuse box is a current carrying path, ground isn't that path. When the UPS kicks in, just connecting the neutral wire to ground does not complete the circuit to the UPS output, that has been disconnected from all input leads. All are right, you need to use a isolation transformer.
    Dwight

  2. #12
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    "Ground" is the lead, that when firmly grasped while standing barefoot in a mud puddle isn't going to electrocute you.

  3. #13
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    And neutral is really only neutral because it's referenced to ground. (But it is not ever "ground".)

    Neutral is most notable for being able to serve two circuits (on opposite "phases") with a single conductor. This is being phased out.

    I wish we'd come up with a new word for safety ground and start calling earth ground "earth". The word ground makes everything far too confusing.

  4. #14
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    Yeah, I'd go with Chuck's suggestion of an isolation transformer. You can get smallish ones sorta cheap, it's common to need one to go from one phase of 240V or 480V delta to 120V or 120-0-120 split phase (center tapped secondary). They're often reconnectable/multitap or carry multiple ratings, so you might end up with e.g. a 480V/240V primary and center tapped 240V/120V secondary, which would do the same job. Shipping will undoubtedly be the big cost.

    If it were me, and the big HP UPS is efficient enough to make running it practical (old ones often aren't worth running), I'd probably just get a 240V PDU and make sure I had 240V-capable supplies in the servers in that rack. There are PDUs that use a different IEC plug on the pigtail, which makes getting 120V-only equipment mixed up harder to do.

  5. #15

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    Many power supplies are designed to run from 120 or 240. They often have a jumper inside for that purpose. You have to realize that they sell units to other countries as well. These jumpers are often not brought out to switches as most don't like being connected to the wrong voltage. Before going to the trouble of using transformers on units, you might take a look under the covers. It might void warranties as well.
    Dwight

  6. #16
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    The NEC differentiates thusly:
    Grounded conductor: the current-carrying conductor of a circuit that is bonded to the grounding conductor at the service disconnecting means or by the system bonding jumper of a separately-derived system ( this might or might not be a neutral);
    Grounding conductor: the non-current-carrying conductor of a circuit that is bonded to the grounding electrode(s).

    Bonded is a technical term in this context. This wheel has already been invented and these terms are the terms used by electricians anywhere the National Electrical Code is used by the authority having jurisdiction. No need to reinvent this wheel.
    --
    Bughlt: Sckmud
    Shut her down Scotty, she's sucking mud again!

  7. #17
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    When dealing with electrical (not electronic) systems, "ground" to me means just that--something that is normalized to earth potential (if that makes sense). "Neutral" is a current-carrying conductor that may have a relationship to ground.

    In electronics equipment, the term "ground" need only relate to an electrical common path. There's no particular requirement for safety, nor connection to a case, although both may be observed.

    There are at least three schematic symbols:



    which are variously abused. I agree that the terminology is confusing at best.

  8. #18
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    I read this thread and I think to myself someone is gonna fry themselves or a piece of equipment or make very large capacitors go boom. Please don’t f around with 120 v. AC is just not safe. AC kills people.

  9. #19
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    So does DC. Use reasonable care and common sense and you'll be fine.

  10. #20
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    240V (what we're dealing with here, not 120V) usually won't kill you. But it sure will knock some sense into you if you're irresponsible.

    765kV is perfectly safe if you are responsible.

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