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    Welders

    Have a question for anyone who is up on welding. Have a 3 by 5 ft heavy build steel trailer. Have the welder, plasma cutter and wire feed all mounted on the trailer and all Miller. Just added a 6-3 with ground 25 foot feed cable and a sub panel. Does anyone know for sure if it is safe and allowable to run the clamp from all the units to one point and then use only 1 work clamp cable instead of having 3 separate cables. All the cases have the common (green)ground wire attached but not sure if there would be any internal circuit issues bu using one common cable??? As they say a ground is a ground the world around, but does the work side clamp have the common ground potential or is it a floating return to the unit and not common to all 3.
    Last edited by TonyBMI; 07-10-2015 at 10:09 PM.

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    You can link all of the green-wire chassis grounds but that's it.

    You work clamps are really "returns", not "grounds". There is a subtle difference between those. A return may be grounded in some situations, but not all. You may also run into some safety situations if you connect all the returns together - in particular, if multiple machines are powered on and one is set to DC- and another is set to DC+ you'd be creating a potential loop between the electrode leads.
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    Quote Originally Posted by JimR View Post
    if multiple machines are powered on and one is set to DC- and another is set to DC+ you'd be creating a potential loop between the electrode leads.
    Yet there's numerous different welders in use on construction projects simultaneously every day, and they have no problems.

    I think the only time you'd even have a possibility of a problem is if the machines were out of phase from each other, and operated at the same time. If their polarity was the same (EN and EN for instance), and their input leads were out of phase (one hot leg switched from the other so the transformer poles were reversed at the power inlets on the welders) then you'd have a 0 voltage condition across the two connections and you wouldn't get any spark, but this too would only happen in AC, as DC is rectified and isolated from the input power. The AC voltage is also run through the main transformer (or generated through the PWM circuitry in an inverter), so the input phasing may be irrelevant all together.

    I'm not an EE, so this is just my observations from real world applications. I have two arc welders powered on and commonly bonded across my fabrication bench, and have for about 8 years now. One mig and one tig, but they're not used at the same time.
    Last edited by Jim Timber; 07-11-2015 at 10:59 PM.
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    Quote Originally Posted by Jim Timber View Post
    Yet there's numerous different welders in use on construction projects simultaneously every day, and they have no problems.
    This is true but they also have equipment that you wouldn't typically see most homeowner's/hobbiest's using and they follow ANSI Z49.1 which is a safety standard laid out specifically for that sort of situation. The short of it is that they aren't just tying all of their returns together. They tie all of their returns to ground (often by using a building frame as the ground.) The OP could follow the same ANSI standard with his setup but he'd need to make sure he has a proper ground anywhere he sets that trailer up. That might mean having to drive his own ground rods.

    I think the only time you'd even have a possibility of a problem is if the machines were out of phase from each other, and operated at the same time. If their polarity was the same (EN and EN for instance), and their input leads were out of phase (one hot leg switched from the other so the transformer poles were reversed at the power inlets on the welders) then you'd have a 0 voltage condition across the two connections and you wouldn't get any spark, but this too would only happen in AC, as DC is rectified and isolated from the input power. The AC voltage is also run through the main transformer (or generated through the PWM circuitry in an inverter), so the input phasing may be irrelevant all together.
    Reversing an input lead won't put you out of phase. Phase and polarity aren't the same thing. I'm assuming here that he's working in his garage/shop and has standard residential power which is all single phase. It isn't possible to be out-of-phase if your power source is single phase unless you add an inductive or capacitive circuit to specifically push a line out of phase. He *could* however, get an out-of-phase situation of he has 3-phase power and that is wired incorrectly.

    I'm not an EE, so this is just my observations from real world applications. I have two arc welders powered on and commonly bonded across my fabrication bench, and have for about 8 years now. One mig and one tig, but they're not used at the same time.
    It may work fine for years (or decades) - especially if one or the other has a foot switch or trigger that needs to be activated to energize the work lead. The only time someone is going to care is that one time when it doesn't work fine.

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    Quote Originally Posted by JimR View Post

    Reversing an input lead won't put you out of phase. Phase and polarity aren't the same thing. I'm assuming here that he's working in his garage/shop and has standard residential power which is all single phase. It isn't possible to be out-of-phase if your power source is single phase unless you add an inductive or capacitive circuit to specifically push a line out of phase. He *could* however, get an out-of-phase situation of he has 3-phase power and that is wired incorrectly.
    Power derived in residential 240vac is center tapped off a single transformer at the pole that ensures both legs are sinusoidal and in time with one another. If you have two loads off that 240v and one is connected out of phase from the other, and both outputs are presented to one another (as in an arc or short), you'll negate the current flow as the two sides will neutralize each other out.

    Where this gets muddied is the main transformer in the welding machines, since they use a 10:1 step down to bring the line voltage from 240 to 24ish (sometimes lower yet) and increase the current capacity. While you might think this de-couples the input from the output, the behavior of AC transformers doesn't work that way. Once rectified on the output to get DC, there's no coupling at all with the main leads though, so this only becomes an issue in AC welding, which TIG and SMAW (stick) can use simultaneously - and that's not a likely scenario to encounter. I can't think of an instance where someone would be tigging in AC and running a buzzbox on the same work as you can't weld aluminum in AC with SMAW.
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    Welders

    What would be the difference between what the OP is wanting to do and multiple welders using the same steelwork for their common return? It's common practice in the building trades. In essence they are using the building iron as a long big wire. Multiple welders can do this. I see no issue with this as long as the ground wire is of sufficient size. Heck, I used to weld on the same piece of iron as two other guys at the same time with different machines.

    I used to think multiple circuits couldn't use the same wire. I found out this isn't the case. In the Mitsubishi mini-split heat pump I installed, the outdoor unit feeds the indoor unit power and control circuits. But it only does it with 3 wires.

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    You can see L1 & L2 jump directly over to S1 & S2. That's power supply, 240VAC. But put a multimeter between S2 & S3 and you get a 30VDC control signal circuit. They both share the S2 wire. The ground isn't used, can't be to be code compliant. I found this out troubleshooting a small issue....

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    I had a pinched wire between the inside and outside units that had 240VAC bleeding over to S3. That little resistor didn't care for that.
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    Quote Originally Posted by Jim Timber View Post
    Power derived in residential 240vac is center tapped off a single transformer at the pole that ensures both legs are sinusoidal and in time with one another.
    Yes, that's true. And that gives you a single 240Vac single phase line. It isn't two phases. The two legs are exactly 180° out of sync with each other which creates two opposing voltage peaks happening at exactly the same time. As a result you have a +120V peak happening at the exact same time as a -120v peak. You get your 240vac by connecting between those two peaks or 120vac by connecting to one of those peaks and a return.

    To be a different phases the peaks would have to occur at different times i.e. 208v 3-phase has 3 120v peaks that occur 120° out of sync with each other.


    If you have two loads off that 240v and one is connected out of phase from the other...
    How are you going to connect something out of phase? What are you going to do to your line to cause those voltage peaks to occur at different times?

    ...and both outputs are presented to one another (as in an arc or short), you'll negate the current flow as the two sides will neutralize each other out.
    You'll have a short but that short has nothing to do with phases. The short will cause an over-current situation and (if everything is wired to code) your breaker will trip. If current flow was neutralized, you'd have zero current flow and the breaker would never trip.
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    Quote Originally Posted by dieselshadow View Post
    What would be the difference between what the OP is wanting to do and multiple welders using the same steelwork for their common return? It's common practice in the building trades. In essence they are using the building iron as a long big wire. Multiple welders can do this. I see no issue with this as long as the ground wire is of sufficient size.
    The common practice in the building trades is to ground the steel frame of the building IAW the ANSI standard I mentioned above. Then by connecting all of the returns to the building frame, they tie all the returns to a common earth ground. And as I said earlier, he could do that as well. But he's got all of his equipment on a trailer so if he hauls that trailer to another location, he has to make sure he has an actual earth ground to connect to.
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    Quote Originally Posted by JimR View Post


    How are you going to connect something out of phase? What are you going to do to your line to cause those voltage peaks to occur at different times?



    You'll have a short but that short has nothing to do with phases. The short will cause an over-current situation and (if everything is wired to code) your breaker will trip. If current flow was neutralized, you'd have zero current flow and the breaker would never trip.
    If you hook the leads on your transformer backwards at the plug from one machine to the other, you'll have a zero-potential cancelled sine wave because the leads will be out of phase from one another. It's not a problem in this scenario, because the welder's aren't being used to create AC simultaneously. If they were, you could have a problem, as I mentioned before.

    How you'd achieve this: Red to Red on one machine, and Black to Red/Red to Black on the other. Very simple and not an issue in the slightest unless you're trying to use both legs at the same time. It won't necessarily blow a breaker either, as it's not a short to neutral or ground. You'll just have no potential across that connection (voltage of 0).

    You're thinking too hard about 3 phase issues here. I'm not even getting into phase separation of 3 phase legs and the fun that can cause. Phase of the sides of the sine wave in 240v single phase work just like 3 phase, but they're 180 out instead of 120.
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    Quote Originally Posted by Jim Timber View Post
    If you hook the leads on your transformer backwards at the plug from one machine to the other, you'll have a zero-potential cancelled sine wave because the leads will be out of phase from one another.
    No, the leads will not be out-of-phase. They will be reversed polarity. Not the same thing.

    How you'd achieve this: Red to Red on one machine, and Black to Red/Red to Black on the other. Very simple and not an issue in the slightest unless you're trying to use both legs at the same time. It won't necessarily blow a breaker either, as it's not a short to neutral or ground. You'll just have no potential across that connection (voltage of 0).
    Again, what you are describing is reversed polarity and it will not result in zero volts. You'd be connecting +120 volts to -120 volts with zero resistance. According to Ohm's Law that will give you infinite current flow. Your breaker has no connection to ground or neutral. It doesn't care where the short is to. It only measures current flow and if you exceed the breaker's rated current flow, it will trip. This is no different than connecting a battery's positive terminal to the negative terminal. If you do that with nothing but a circuit breaker between them, the breaker will trip even though there is no ground or neutral involved. To get to zero volts you'd would have to shift one of your two sine waves another 180° so that they are both at 0°. Reversing polarity won't do that for you. This is exactly why the National Electric Code now requires that electrical outlets in your home be polarized - so that you can't connect devices with reversed polarity. If reversing polarity resulted in zero volts, it wouldn't be a danger to anyone and they wouldn't care.

    You're thinking too hard about 3 phase issues here. I'm not even getting into phase separation of 3 phase legs and the fun that can cause. Phase of the sides of the sine wave in 240v single phase work just like 3 phase, but they're 180 out instead of 120.
    I'm focusing on phases because you keep saying you can connect a device "out-of-phase" but you keep giving examples of connecting devices with reversed polarity. They are NOT the same thing. An electrical "phase" has nothing to do with the "side" of a sine wave. Nor does it have anything to do with the number of sine waves. That difference of 180° vs 120° makes a world of difference.

    In electrical terms, a phase is the number of times you see the maximum apparent output voltage in an AC circuit during 1/60th of a second. (for places that use 60Hz power). That's it. Nothing more, nothing less.

    In a single-phase system the sine waves are aligned 180° from each other. That means you will see the max voltage ONE time during 1/60th of a second. In a 2-phase system (which aren't used any more) the sine waves are aligned 90° out of sync so you get max voltage twice during 1/60th of a second. In a 3-phase system the sine waves are aligned 120° out of sync so you see max voltage 3 times during 1/60th of a second, etc, etc...

    If you have 1,000 sine waves that are all aligned at 180° so the max voltage only occurs once during 1/60th of a second, you have single phase power. And if you only have one phase it is not possible to connect two devices to it and be out-of-phase.
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