Can a reversed polarity outlet damage a power supply?

Discussion in 'Highly Technical' started by totalcommand, Aug 1, 2007.

  1. totalcommand

    totalcommand Platinum Member

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    I recently destroyed the power supply on my computer.

    There was a 2 prong outlet that I used a 2 prong to 3 prong adapter on (this usually works by using the screw of the socket as ground). However, the outlet turned out to have the live wire and neutral wire swapped.

    This 3 prong adapter allowed me to plug in my surge protector, which the computer power supply was connected to, into the 2 prong outlet. After a minute of being plugged in, and without me even turning the computer on, sparks started spewing from the power supply, rendering it toasted.

    An electrician came in and tod me that the swapped live/neutral wires resulted in the damage to my power supply. However, other people I have asked have said that reversed live/neutral with AC current cannot possibly damage the power supply.

    I trust your wisdom very much, so before I make a claim to cover the damages to my landlord, I wanted to ask you this: Who is right? Can a reversed polarity (swapped live/neutral wires) in an outlet damage a power supply?
     
  2. f95toli

    f95toli Golden Member

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    Reversed polarity can (almost) never damage an AC system. In many countries you can turn the connector whichever way you want so most people do not keep track of live/neutral. Your PSU is designed to work all over the world so that was definitly not the problem.
    However, are you sure that the surge protector is not the problem? It could be that it needs a proper earth connection in order to work properly; but it is unlikely that a bad connectioj to earth would damage anything.

     
  3. HVAC

    HVAC Member

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    This electrician's causality gene has mutated and rendered them unable to determine proper chicken vs. egg outcomes.

    This is a design or component failure on the part of the power supply. If the power supply required a certain configuration of line-neutral, it would only be an issue with respect to ground. Even then the designer was intentionally dense if they didn't consider that people use such things as isolation transformers and the like on occasion.

    Now if it was a swapped live/ground ..... smoke...... fire ......ashes.......boom. (Or just plain not work.)

     
  4. Paperdoc

    Paperdoc Golden Member

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    Seems to me unlikley that the reversed polarity would cause this, but I can't be sure. Depends a lot on the details of your power supply's design.

    Your story, however, MAY illustraste a good reason that those 2- to 3-prong adapters are a bad idea. As you say, "this usually works ...". But "works" simply lets you physically plug one style of plug into another style. The flaw is the assumption that, although the outlet has no third hole for a ground connection, the metal box in the wall still has a proper electrical ground connection back through the supply panel. IF it does, the screw that fastens on the cover plate is a ground. BUT in most cases with 2-prong outlets, it is there because the supply cable to the box does NOT have any ground lead in it, and the screw is useless electrically. You have no ground, and you don't know there is a problem! If that's the case, the surge protector cannot do its job. I don't know whether the surge protector itself, in this no-ground case, could actually cause your problem.
     
  5. totalcommand

    totalcommand Platinum Member

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    Interesting. This is a high quality power supply from Enermax. All other components that were plugged into the surge protector ran fine, it's just the power supply that sparked out. I had tested the computer less than 12 hours before in a properly configured three prong outlet on a different circuit across the room, and it ran perfectly fine. Only when I moved it to the configuration described above did it fail.

    Does anybody want to do an experiment ;-)

    Take the line going into your desktop computer, and reverse live and neutral. If you can still post on AT, let me know ;-)

    edit: does anyone have knowledge of power supply circuitry here?
     
  6. Peter

    Peter Elite Member

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    It might have been some ugly potential buildup due to lack of earth, but much more likely, it's simply been 110V equipment plugged into a 230V net. Or vice versa.
     
  7. JohnCU

    JohnCU Banned

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    The polarity is constantly changing anyway, it wouldn't matter.
     
  8. f95toli

    f95toli Golden Member

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    As I pointed out above, I used to do this all the time. Most countries in Europe (including Sweden where I am from) use plugs that you can insert "upside down" if you want. However, I must say I prefer the system here in the UK where you can only connect it one way and there is a fuse in the plug.
    Also, there is no "world standard" for how the wires are connected IN the equipment. This is e.g. a constant worry for some hifi-enthusiast who want to connect their amps "the right way" (not as silly as it sounds actually, the "right way" minimizes the parasitic capacitance in the transformer, you can measure the difference) since different manufacturers use different standards for where live and neutral are positioned.







     
  9. Modelworks

    Modelworks Lifer

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    Your just unlucky :(
    The power supply doesn't really care about the ac "polarity"

    The only other thing I can think of would be that the supply was made so the case of the supply was aligned with the neutral.
    But that wouldn't matter either unless some part of your pc was plugged into another outlet seperate from everything else that would have put the "hot" side onto your pc's casing.

    The above would be a one in a million chance though.
    And wiring a supply or anything in the usa like that has been against code for years now.
    Ever since the ground prong was added.
     
  10. Mark R

    Mark R Diamond Member

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    One possibility is that the socket was incorrectly wired with 240 V. Plug 120 V equipment in - boom! I can imagine that a switched off PSU would tolerate double voltage for a few seconds before exploding.

    To get 120 V, the socket is connected to a live and neutral wire. To get 240 V, the socket is connected to two opposite lives.

    I find it very hard to believe that simply swapping the places of a live and neutral wire would do anything.
     
  11. Peter

    Peter Elite Member

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    What? 240V has a live and a neutral just like 120V. The swing on Live is twice as much, that's all.

     
  12. JohnCU

    JohnCU Banned

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    I was under the impression that one wire was 120V and the other -120V for 240.
     
  13. f95toli

    f95toli Golden Member

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    I think it can be done both ways, at least here in Europe.
    99% of all installations use live/neutral. However, in some places (e.g. in the students labs at the university where I used to work) +-115V is used instead, from what I understand this is just a safety feature , lower voltage should mean less risk (not that I think there is a huge differene between a 115V and a 230V shick).
     
  14. Peter

    Peter Elite Member

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    Live and Neutral all across western and central Europe. Silly special cases (see above) notwithstanding :)
     
  15. DrPizza

    DrPizza Administrator Elite Member Goat Whisperer
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    It's not that way in the US. 240 V is two 120V 180 degrees out of phase. Since the OP is in North Carolina, having 240 V on a 120 V outlet would be considered a major major screwup. A typical 10 year old being exposed to a circuit breaker box for the first time and told to connect a new circuit to that box would probably be smart enough to figure it out without resulting in a 240 volt circuit. (Make sure the power is off to the box first.) ;)

    Personally, I'd have a hard time believing that having a reversed circuit - hot&neutral reversed would ever make a difference to how well something runs. The only difference should be one of safety.
     
  16. jjzelinski

    jjzelinski Diamond Member

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    Actually it's 120 degrees :)
     
  17. Mark R

    Mark R Diamond Member

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    Nope. 180 degrees. In the US, the normal domestic supply is 1 phase center tapped. So you get 2 phase connections 180 degrees out of phase.

    3 phase is the standard 'higher capacity' supply in most other regions of the world. E.g. in Europe, if your power demands exceed a normal 230 V single phase supply, you'll get 3 phase 400 V (120 degrees apart).

    In the US, 3 phase tends to be reserved for industrial use.
     
  18. silverpig

    silverpig Lifer

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    You could buy the cheapest POS powersupply from your local store to see what happens when you do it manually.
     
  19. bruceb

    bruceb Diamond Member

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    240V does not use the Neutral as the Load is connected to Both Hot wires
    The only time you use the Neutral on a 220V-240V AC circuit is if you want
    to say run a Fan Motor or some other device on 120V without running in another circuit.

    As to the OP's original question, the only way the Outlet Polarity might damage anything
    is if the power supply or device connected to the outlet had one side of it's internal wiring
    connected to the Case Ground ... (aka a Hot Chassis) .. in that case you have the potential
    of a severe shock hazard to the end user. That condition has been remedied in the last 10-20
    years through the use of Polarized 2 prong outlets, where the WIDE SLOT is the Neutral Wire.
    And of course 3 prong grounded outlets can only be plugged in one way, but it is possible for
    some idiot to miswire the connections at the outlet.
     
  20. NeoPTLD

    NeoPTLD Platinum Member

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    Only for houses.

    Not true. 120/240 isn't used in big buildings. Dorms, offices and big apartments get 120v from 208/120 3ph wye. A residential unit within a large building simply have two of the three phases pulled in. First floor may get phase 1 & 2, second floor 2&3, third floor 1&3 and so forth as to balance the phase.

    This setup gives 3 phase access to mechanical equipment, such as commons area HVAC, elevators and water pumps.

     
  21. Mark R

    Mark R Diamond Member

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    Thanks for the clarification.

    So what do apartment residents do for their 240V applicances? Are they SOL?
     
  22. bruceb

    bruceb Diamond Member

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    They are not sol as you say. All modern buildings, apartments and homes built or upgraded
    since say the late 1960's era have the ability for 220V appliances to be installed. The most
    common need for that voltage in a home is for Electric Ranges, Heavy Duty window AC units,
    Central Air Conditioning and if you like a workshop, your Lathe, Drill Press and Air Compressor
    would run on 220V .. Most commercial buildings, get power at 4800V 3 Phase from the power
    company. It is then run thru a Huge Transformer to bring it down to 480V ... after that sub transformers
    take it down to 220V or 110V for lighting or regular outlet type loads. Most of the larger motors in a
    commercial building, like for elevators will run on a 3 Phase circuit of 480V .. one of the reasons
    they do this, is it is much easier to get power into the building from the Pole by using the higher voltage.
    You need a much smaller wire to carry the needed amount of power.
     
  23. NeoPTLD

    NeoPTLD Platinum Member

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    Motors- almost always rated for both 208/230v. Motors rated for just 230v might not be able to handle the higher current draw at 208v and might have to be derated.

    http://www.lodgemart.com/ama-ae123a35ma.html
    Even window ACs are rated 208/230 here.

    Heaters -
    You're sort of SOL. They'll operate at 75% power. Water heater will have to be fitted with elements rated at 208v, or get 240v elements that are 33% higher wattage output than desired.
     
  24. Analog

    Analog Lifer

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    Reversing the polarity can be cause for concern. It depends on how the power supply was wired. For example, the neutral and ground are at the same potential, so a surge arrestor metal oxide varistor could be from hot to ground. If the neutral and hot are switched, then the MOV would never see the surge. So if, unfortunately, a line surge was there at the time you plugged in, you would have no protection. Also, the supply may have devices that are polarized in the same manner that wouldn't like reversed hot/neutral with respect to ground. It is too difficult to speculate here without a schematic and understanding what blew in your supply. Perhaps you could supply this information.