Transformer with no ground?

Discussion in 'General Electronics Chat' started by Rogers1438, Jan 5, 2010.

  1. Rogers1438

    Thread Starter New Member

    Jan 4, 2010
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    Hi, I'm in the military and currently stationed in korea. My landlord gave me three transformers (2 kVa each) so i could use my American electronics at 120 V (korea is 220). Problem is though, i noticed that while it will accept an american plug with a grounding prong, the transformer itself has no grounding plug when it connects to the korean outlet.

    I'm pretty sure something is wrong with this. Am i missing something? Do transformers not need to be grounded? Am i putting myself in danger by using them? The brief instruction manuals, translated into english, don't mention anything regarding this.

    Appreciate the help!

    Rob
     
  2. steinar96

    Active Member

    Apr 18, 2009
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    Transformers themselves are not and do not have to be grounded. Grounding is mostly used for increased safety when it comes to appliances utilizing the mains electricity.
    You need to be aware though that since by using these transformers you forfit ground protection. You might wanna be alert when using large home appliances using these transformers. If the appliances themselves malfunction you might risk getting electrocuted if you form a conduction straight to earth.
     
  3. thyristor

    Active Member

    Dec 27, 2009
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    That's not correct. The output voltage at 220v will be floating having travelled through the transformer. There is no return circuit to earth and therefore nothing can flow through you to earth and back to the transformer to cause a shock.

    One may therefore safely touch EITHER output lead and not receive a shock as there is no earth reference. However touching BOTH output leads simultaneously will give one a shock.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    I agree with thyristor. This setup is actually safer. A medical grade isolation transformer is often used in medical system to improve safety.
     
  5. MaxSmoke

    Active Member

    Oct 29, 2009
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    These transformers you are using, are they auto-transformers?

    If they are auto-transformers then there will be no isolation from raw mains. Such a set up with no earth and no galvanic isolation is potentially dangerous in the event of an equipment fault.

    While it is more likely they are isolation transformers and therefore should provide galvanic isolation, it is a good idea to check they are not auto-transformers!
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    You can check this by looking for markings for double insulation. It should be labelled "Class II", "double insulated" or bear the double insulation symbol (a square inside another square).
     
  7. Rogers1438

    Thread Starter New Member

    Jan 4, 2010
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    thanks for all the comments guys. I haven't noticed anything on the device itself that would indicate its an auto transformer, but then again 90% of it is in korean so i could be missing something. The manual thats translated into english also seems to omit a large portion of whats in the original instructions. I'll bring the manual to work and see if any of the Koreans that we work with can lend a hand.

    Rob
     
  8. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Having both sides of a control transformer secondary circuit floating sounds ok in theory, but practically speaking, its downright dangerous. Lets say the neutral (un-switched) leg of your circuit is floating and something on the switched side gets grounded. Now the neutral becomes HOT! Thats a big NO NO in control circuit design! ALWAYS ground the neutral side of your control transformer, and fuse appropriately the switched leg!

    Cheers, DPW [ Everyone's knowledge is in-complete...Albert Einstein]
     
  9. thyristor

    Active Member

    Dec 27, 2009
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    But if the transformer output is floating, there is no neutral and hot, just two outputs, from which you will only feel a shock if you happen to touch both of them at one time.

    If EITHER of the two outputs becomes inadvertently grounded, of course the other will then be hot and will indeed give you a shock if touched.

    If you're worried about switching in a floating system use a double pole switch.
     
  10. Duane P Wetick

    Active Member

    Apr 23, 2009
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    That is still no protection if something becomes in-advertently grounded. Look at the incoming power into your house and you will see that the neutral is always at ground potential and is NEVER switched!

    Regards, DPW [ Everyone's knowledge is in-complete...Albert Einstein]
     
  11. thyristor

    Active Member

    Dec 27, 2009
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    Of course..... but if there is a real danger of inadvertent grounding then don't use a floating system.

    The advantage of a floating system (isolation transformer) is where the danger exists of inadvertently touching one terminal. That's why they're used in (damp) bathrooms as electric shaver sockets
     
    Last edited: Jan 6, 2010
  12. Duane P Wetick

    Active Member

    Apr 23, 2009
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    If you must have an isolated line voltage power system, then a ground fault circuit must be added to the system that will effectively open the circuit should a fault to ground occur. Thats the only protection against electrical shock. Un-grounded power systems are an accident waiting to happen, nothing less.

    Regards, DPW [ Everyone's knowledge is in-complete...Albert Einstein]
     
  13. thyristor

    Active Member

    Dec 27, 2009
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    That would require bonding one output lead of the transformer to earth (forming a neutral) in order for the RCD to trip. Without this connection, the RCD will not trip in the event of an earth fault. Therefore, the system would no longer be a floating one anyway.
     
  14. Duane P Wetick

    Active Member

    Apr 23, 2009
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    There are lots of instances where people have been seriously hurt when their isolated AC power systems suddenly became not so isolated (boats) for a variety of reasons.
    Better to have some ground fault protection than none at all. Its your life, afterall...

    Regards, DPW [ A little bit of knowledge can be a dangerous thing.]
     
  15. pkennedy

    Active Member

    Feb 27, 2009
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    I agree with Duane. One leg of the transformer must be grounded or you will be at risk. It is common practice to always ground one leg this grounded conductor will be your nuetral wire. That way if the other conductor (ungrounded conductor) is shorted it will trip the breaker or blow a fuse. I would highly recommend that you do not use this without being grounded.
     
  16. thyristor

    Active Member

    Dec 27, 2009
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    I agree. Boats should NEVER use a floating system (excusing the pun). The hull could go live which you, on board, won't feel because you will also be floating at hull potential, but a nearby swimmer, or someone stepping on board, may be electrocuted.

    There are a number of reported instances of this causing deaths in the US press.

    In the UK (unlike the US) it is illegal to install any electrical socket in a bathroom other than a (non-earthed) transformer-isolated output designed to power only an electric shaver or the like. Even the light switch must not be installed within the bathroom unless it is a pull-cord with a plastic-housed insulating air gap in the cord. A conventional switch must be installed outside the bathroom door.
     
    Last edited: Jan 7, 2010
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