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Thread: Electrical Question

  1. #1
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    Default Electrical Question

    Calling a pro...

    So right now I have a rather large UPS in one of my three racks and right now it's doing a very inefficient job powering one infrequently used HP 9000 system instead of the rest of my servers where I really need it due to frequent drop-outs. The UPS has an L14-30P (X, Y, neutral and ground) plug for connecting to the breaker panel but in the output side I only have an L6-30R (X, Y, ground) receptacle to connect a PDU. For the HP that's fine because I can flick it to 230v service and that's fine but the second rack has several 120v-only devices, so I have to use a PDU that has both 230v (x, y) and split-phase 120v (X or Y, neutral) but that then comes with an L14-30P, which won't connect to the UPS currently.
    Replacing the receptacle on the UPS is fairly trivial as it's just a regular box mount unit....



    ...however adding the Neutral raises questions.
    If I remember right, in Canada and most of residential North America, the Neutral wire from the wall, at the breaker panel and at the pole is just ground, however it is only allowed to connect directly to ground at the breaker panel and at the pole. Doing so anywhere else isn't allowed.
    My theory is that so long as my neutral to the breaker panel isn't broken, I can route the neutral from the UPS input to the output and in a power failure the UPS will handle the 120/230v and the neutral will still work on its way back to ground and everything will still operate. Is this correct?
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  2. #2
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    It depends on the UPS. The output may have a grounded conductor tap that you can use. What brand and model UPS?
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    Bughlt: Sckmud
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  3. #3
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    Hewlett Packard Power Trust A2998A. I got no docs for the thing. I talked about it on badcaps.net about a decade ago and all the support links are long broken.


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    Neutral isn't ground. Due to the way most residential power distribution is in our region, neutral is *usually* connected to ground at the service point, because your utility connects the neutral from their transformer to ground. This serves to keep all your service conductors less than 150V referenced to ground, for safety's sake.

    When your UPS is operating, ground is only ground. The UPS must provide a neutral, and yours doesn't. Circumventing the UPS with your neutral is a really bad idea. And most likely, you just won't have a neutral when the mains power is off and the UPS is powering its load.

    The best case scenario is that your 120V loads will not function at all. The worst case scenario is that some of them receive 250V. Actually the worst case is that you could conceivably raise a safety ground to 250V.

  5. #5
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    You might be able to get away with it. I've got a wall oven that's a 3-wire setup. 240V 40A with a 120V clock, permanently wired. Neutral is strapped to the oven cabinet. That was NEMA code prior to 1994. Modern installations use a 4-wire setup with the connection between ground and neutral broken.

    Most wall oven installation manuals will show how to wire for either 3 or 4 wire service. You could claim an exemption under grandfathering old equipment. I suspect that the local authorities wouldn't have a problem.

    Now, if it's 3-phase wiring, that's a different matter...




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    You might get away with using a common circuit breaker as a backup generator transfer switch, too, but I sure wouldn't do it. Too much chance for too much error.

    When the UPS is online, it's not much different than an isolation transformer. If your isolation transformer doesn't have a centre tap, the only way you might get safe split phase power is if all your loads are identical.

    IMAG0087.jpg

  7. #7
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    Looking over the spec sheet for the A2998A (HP museum has one), it appears that the output is 240V fixed. No mention is made of 120V capability. Get yourself a 240V-120V transformer for your 120VAC equipment and then ground everything. An autotransformer is not suitable for this application.

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    +1 for Chuck's isolating transformer. Be safe!

    Dave

  9. #9
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    I agree with the isolation transformer idea, as that allows you to be sure about the grounded conductor (that's a narrowly-defined NEC technical term that is distinct from the "grounding conductor"). The grounded conductor can be the neutral, but, in the absence of a neutral, one of the phase conductors can be the grounded conductor, as in corner-grounded three-phase delta systems.

    Looking at your photo, it doesn't look like there is a neutral on the output, and if it has a built in transformer like our Symmetra Power Arrays do connecting the neutral of the primary to the output won't actually work.

    The isolation transformer will generate the neutral properly, but do make sure that you properly bond to the ground at the transformer output. See the NEC for the subject of separately-derived systems. A discussion of this subject can be read at https://www.ecmweb.com/bonding-amp-g...g-transformers. The purpose is to keep from an unsafe condition during a ground fault.
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  10. #10
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    That's one of the reasons that I ruled out an autotransformer (probably easier to come by). If you used one of those, one side of the 240V would also be one side of the 120V output, which could be a very dangerous thing.

    Side story about safety:

    A couple of years ago, our pad-mounted distribution transformer blew a fuse. The lineman opened the box, and got out a 10 ft fiberglass pole and warned me to stand to one side. Using the pole, he unscrewed the fuse and inserted a new one. He said that occasionally a blown fuse was due to a transformer fault and it was not nice to be around for the fireworks.
    (Our transformer is fed by 12KV)
    .

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