Grounding of a single phase dry transformer used in reverse
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    Default Grounding of a single phase dry transformer used in reverse

    I have a 240x480 primary 120x240 secondary single phase transformer that I need to configure to run a 13amp 460v single phase electroarc tap burner on 240 service (to be used very infrequently, primarily to make sure it works properly). Transformer schematic below. To use this as a step up transformer I have my 240 supply running to X1 and X4. x2 and x3 are jumpered together, and tested momentarily I get the proper 460 out from h1 and h2 with taps 5&6 connected. I didn't attach the supply ground for the test. There is a jumper running from x2x3 to the cabinet left over from the previous, conventional step down installation equipment side. My question is should this jumper be removed or left in place? My supply ground will then go to the cabinet together with the ground from the electroarc. If there is supposed to be a jumper from the "center" to ground on the equipment side should there then be a jumper from the 5/6 to ground?
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    Ground 5&6 on the equipment side. If an undesired electrical path was made from h1 or h2 to ground this would allow your fuses/circuit breakers to clear the fault. Of course this means you should provide fuses/circuit breakers on the output of the transformer. This is now what is known as a separately derived power source per NEC.

    The ground on what is now the center of the input side of the transformer should be removed. If not removed, any lack of symmetry in input voltage, or turns ratio of the transformer would result in ground current of potentially very significant quantity.

    Do bond the case of the transformer to ground.

    Bill

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    Thanks, that makes sense to me. Just out of curiosity what amperage draw might I expect on my supply side when there is no secondary load on the transformer?

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    Before connecting that 5/6 jumper to ground, for the heck of it I put a voltmeter between that conductor and the ground coming from my supply and got 9.6v. Is this a problem as they will all be conected?

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    Quote Originally Posted by Bill Simmons View Post
    Ground 5&6 on the equipment side. If an undesired electrical path was made from h1 or h2 to ground this would allow your fuses/circuit breakers to clear the fault. Of course this means you should provide fuses/circuit breakers on the output of the transformer. This is now what is known as a separately derived power source per NEC.

    The ground on what is now the center of the input side of the transformer should be removed. If not removed, any lack of symmetry in input voltage, or turns ratio of the transformer would result in ground current of potentially very significant quantity.

    Do bond the case of the transformer to ground.

    Bill
    Do NOT ground 5 and 6!!!! You CONNECT 5 to 6, but NOT to ground!

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    I know that the schematic shows to connect tap 5 to 6 to get my intended voltage, but keeping in mind that I'm using this transformer as a step up rather than step down as originally intended, the separately derived system of the original step down configuration was grounded at the center, so now I'm confused...does not the 5/6 (center) need to be grounded as IT is now the separately derived system? (this old photo is before changing any connections)
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    Quote Originally Posted by tomjelly View Post
    I know that the schematic shows to connect tap 5 to 6 to get my intended voltage, but keeping in mind that I'm using this transformer as a step up rather than step down as originally intended . The separately derived system of the original step down configuration was grounded at the center, so now I'm confused...does not the 5/6 (center) need to be grounded as IT is now the separately derived system? (this old photo is before changing any connections)
    "More information needed". The EGS/Hevi-Duty we can see, the load, not so far.

    Your description indicates that the original Centre-Tap provided a form of local Neutral. Was it needed and utilized? Was it tied to a split-phase supply service Neutral?

    What to look for? Another transformer, even a small one.

    And some answers:

    Does the original system need 120 V off one leg to Neutral of a 240 V supply to run controls, relays, contactors, indicator lighting? Is there a need of - for example - 24 VAC for any of those things, anywhere?

    If either/both are so, more planning is needed.

    IF not so, then "floating" single-phase off the transformer secondary needs no local neutral.

    Only the proper Protective Earthing AKA "ground".

    Which does NOT involve 5+6 nor any other transformer winding tap. Only the case. Transformer case AND load cabinet/frame. "Each".

    "It's PM"

    Share the make, model, and revision level / age of your load device? Some other Pilgrim may have one, have the schematics for it, or even have already done what you are now setting out to do.

    I am not he, but PM is a seriously big tent. Damned seldom NO ONE has "seen this movie before".

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    Quote Originally Posted by tomjelly View Post
    I know that the schematic shows to connect tap 5 to 6 to get my intended voltage, but keeping in mind that I'm using this transformer as a step up rather than step down as originally intended, the separately derived system of the original step down configuration was grounded at the center, so now I'm confused...does not the 5/6 (center) need to be grounded as IT is now the separately derived system? (this old photo is before changing any connections)
    You are using this to make 480 single phase, created from 240V source, correct?

    There is no single phase 480V service with a grounded center tap, that only occurs for 240V, because that is 120-0-120V. (there is 277/480, but that is "star grounded")

    You can ground one end. Grounding 5,6 would not make any sparks in and of itself, it would just be confusing to someone later. But corner grounded 480 is a "thing" (even if not very usual), so the end grounding is not so unusual, it just makes an equivalent to what two legs of corner grounded 480 would have.

    Now, if there is a NEED for the center tap in a machine, that is a different matter, what is in a machine may not be what you find at an outlet or branch circuit.

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    Referring back to the original post, this transformer is going to be used solely for this single 460v load only, no need for any other output. in its original application there WAS a lead running from the x3x2 jumper to the transformer cabinet. Neither that jumper nor the line from it to the cabinet is now connected. So at this point I will use no jumpers from any transformer terminals to the transformer chassis cabinet, and I WILL collect the equipment ground and my 240 supply ground and bolt them to together to the transformer cabinet. Is that the correct configuration?

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    Quote Originally Posted by tomjelly View Post
    Referring back to the original post, this transformer is going to be used solely for this single 460v load only, no need for any other output. in its original application there WAS a lead running from the x3x2 jumper to the transformer cabinet. Neither that jumper nor the line from it to the cabinet is now connected. So at this point I will use no jumpers from any transformer terminals to the transformer chassis cabinet, and I WILL collect the equipment ground and my 240 supply ground and bolt them to together to the transformer cabinet. Is that the correct configuration?
    It surely SEEMS so... the usual disclaimer that "we" don't have any sight of what possible complications the load might introduce...

    That's not to suggest there is any problem.


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    You need to connect one side of the 480V coil to ground, H1 or H2. As JST has noted.

    Any fault current on the 480V side, needs a way to get back to that 480V output coil.

    You should consider replacing those corroded lugs, and cleaning the transformer bus tabs.

    25KVA @ 240V = 104A, your initial magnetizing surge will be way more than that to get it started,

    So your supply protection should be large enough to not trip on magnetizing the core.

    SAF Ω

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    Well you have some definite maybes now. My ground 5/6 had 1 definite no vote, a won't hurt anything but could be confusing vote and some not needed votes.

    And you have votes for corner grounding as it can be found in some industrial environments.

    As SAF has also stated, I believe there should be a fault clearing path. And grounding either the 5/6 or h1 or h2 provide such path.

    The reason I went with the 5/6 choice even though it admittedly provides a useless neutral point is that H1 or h2 would then have only 240 volts to ground should one get there finger in the works.

    On the plus side, I think every one agreed about the input connections.

    Of course my advice could be suspect as I don't seem to have a need to understand electrical power as we don't seem to have power where I live. I am without electric service for the third time in a week courtesy of our fine utility. I thought I was going to get my hands slapped yesterday for one of my electrical doings. The power company line inspector found a problem on one of the poles on my property and suddenly there were six power company trucks on the property. I was back feeding two services with out transfer switches at the time and expected a stern lecture and not one linemen said a thing.


    Bill

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    Quote Originally Posted by Bill Simmons View Post
    As SAF has also stated, I believe there should be a fault clearing path. And grounding either the 5/6 or h1 or h2 provide such path.

    The reason I went with the 5/6 choice even though it admittedly provides a useless neutral point is that H1 or h2 would then have only 240 volts to ground should one get there finger in the works.
    Sympathetic. Had the luxury of no need of 4XX. Didn't want high-leg Delta nor even 600 V "class". I simply shed anything as needed over 300 V.

    And then... invested in a different EGS/Hevi-Duty.

    27 KVA, nominal 230 Delta => 230 Wye.

    Re-derived meself a "local" neutral that neither of Phase-Perfect nor RPC had, but the diesel gen set DID have. MEP-803A's 3-Phase option is 208 Wye, but adjustable to 220 WYE.

    And now am a 100 V and a bit better-off, yet.

    Around 140 VAC or a tad less, any-leg-to-Earth shock hazard. Near-as-dammit the same as the residential 245 VAC split-phase service.

    2CW

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    Quote Originally Posted by thermite View Post
    Sympathetic. Had the luxury of no need of 4XX. Didn't want high-leg Delta nor even 600 V "class". I simply shed anything as needed over 300 V.

    And then... invested in a different EGS/Hevi-Duty.

    27 KVA, nominal 230 Delta => 230 Wye.

    Re-derived meself a "local" neutral that neither of Phase-Perfect nor RPC had, but the diesel gen set DID have. MEP-803A's 3-Phase option is 208 Wye, but adjustable to 220 WYE.

    And now am a 100 V and a bit better-off, yet.

    Around 140 VAC or a tad less, any-leg-to-Earth shock hazard. Near-as-dammit the same as the residential 245 VAC split-phase service.

    2CW
    That will still get your attention!

    Tom

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    Quote Originally Posted by TDegenhart View Post
    That will still get your attention!

    Tom
    It can even kill yah, yes.

    But it don't mess-up the corpse as bad as the harder stuff does, so funerals can be a tad cheaper.

    The rectumfried Dinosaur Current is wot makes me wariest of all, though...

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    INSIDE a machine, all bets are off.... there is no absolute functional need to ground anything, although it is still a good idea, and an ungrounded winding SHOULD be only with a double-insulated transformer. Those are not common with large units.

    There is no HARM in grounding the secondary and load, so I'd go ahead and do it.

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    Here's my opinion, for what little it's worth:

    240v line to the secondary (240v) winding, no aspect of it connected ANYWHERE, as it's a balanced load.

    480v of the primary (480v) winding goes to your load, with no aspect of that winding grounded ANYWHERE.


    Chassis of the transformer grounded. Chassis of all machines powered by it grounded.

    Why?

    The transformer is an isolation stage. Sensible design isn't there to provide operational return paths for the sake of power, it's having a NO PHYSICAL POTENTIAL subjected to the operator in the event of a failure.

    Let's say the transformer 240v side shorts to core, which is grounded... that makes fault current flow through the transformer chassis to ground... but only for as long as it can continue running without tripping your breaker.

    Let's say the 480 side shorts to ground... it will have basically no impact other than eventually tripping the machine's 480v breaker. Machine potential to ground, however, will be extremely low.

    IMO, if there's a greater concern for transformer failure and fault currents, a sensing circuit that uses GF sensing to knock down the contactor that powers the big transformer.

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    Thats exactly what I ended up doing, with a 60a fused disconnect on the supply side and a 30a breaker on the machine side. The tap burner runs fine in this configuration.

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    Th point of secondary side grounding has almost zero to do with fault currents from shorts to ground on the secondary.... An ungrounded secondary has NO fault current with a single point ground, which is one reason that configuration is used. (generally there is some means to indicate that a ground has occurred)

    The real point of it is to cause fault current from a primary-to-secondary short to go to ground and open the primary side protection. Otherwise the secondary would be live with primary side voltage. This is a problem when you have a high primary voltage, and a lower secondary side voltage, since the secondary insulation is not rated for that. In this case, it is less of an issue because the secondary is the higher voltage. But the point remains..


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