Surge protector when no Earth connected.

Discussion in 'General Electronics Chat' started by alphacat, Nov 24, 2009.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    My product is main-operated and comprises a plug which is entered into the mains (into the wall outlet).

    The surge protector in my product consists of two MOVs -
    one is connected between the Live wire and the Earth wire,
    the second is connected between the Neutral wire and the Earth wire.

    Some houses (in some countries) dont have an Earth connection in there wall outlet (meaning, the wall outlet has only two holes - Live and Neutral).

    How can a surge protector protect an appliance if it is connected into a wall outlet with no Earth connection?

    Thank you.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Just connect MOV between HOT & NEUTRAL.
     
  3. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    I see, thank you.

    What are the advantages of connecting 2 MOVs - Live-Earth, Neutral-Earth - over connecting a single MOV - Live-Neutral?
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    There are other forms of surge protector which may be more appropriate in any case.

    The MOV is a 'one shot' protector - it dies is doing its duty and has to be replaced. Further it has an operating or striking voltage below which it is not operational.

    You can incorporate surge filters made from L, T, ∏ or ladder sections with inductors in series and capacitors in parallel.

    Ready made units for inclusion at the mains inlet to equipment and even mains inlet modules including these are available.
     
  5. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Thank you for this.

    Currently, since the PCB is already designed for MOVs, and its no problem of adding up another MOV between the Live and Neutral wires, then i'd rather consider the option using the third Live-Neutral MOV.

    But i dont understand why is it better to connect two MOVs from Live/Neutral to Earth, instead of just one MOV between Live and Neutral?
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You say that you are designing for volume production?

    If you do not fully understand protection circuits and components and what they do and why you should get expert help.

    It is irresponsible folly to do otherwise.

    Electricity supply circuits which have only 2 wires are often not differentiated at the device - eg bayonet fitting light bulbs.
    Further you must ask yourself what is the purpose of the protection?
    An MOV is meant to divert short term excessively high voltage spikes to somewhere safe.
    As such it's principle use is in lightning protection.
    How does diverting an excessively high voltage between line and neutral to the neutral, make anything safe?
     
  7. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    In the case of Live to Neutral MOV, You dont divert high voltage from the Live to Neutral, but you divert high current from Live to Neutral.

    So whats the problem here?

    The purpose of a surge protector is to protect the product from high voltage surges which might cause to high current surges and therefore damage the appliance.
     
  8. studiot

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    Last edited: Nov 24, 2009
  9. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Wow, thank you very much for this great article!

    When the MOV senses a surge, its equivalent resistance gets very low and therefore most of the current (a very large one) passes through it and not through the load.

    So if a large surge occurs in the Live wire, the Live-to-Neutral MOV will shunt all the current straight to the Neutral and not throught the appliance itself.

    Isnt it what MOV supposed to do?
    To shunt current away from the appliance?
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes indeed a large fault current can flow when an MOV starts clamping.

    However it clamps on voltage not current - and will be unaffected by a large current, due to a fault or not, without overvoltage.

    Any fault current should therefore operate additional or alternative protection, which is normally a 'disconnection device'. In fact this is one of the fundamentals of protection.

    Also a MOV is 'fail unsafe' in that over time lots of small jolts (overvoltages) will cause it to fail open circuit. There will then be no surge protection and no indication that the MOV has failed.
    For serious protection duties MOVs are often mounted in parallel banks for this reason. The idea being they won't all fail at once.
     
  11. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Yes, I agree with everything you said.

    Though, i cant imagine a situation where there will be a too large current flowing through the appliance while the voltage remains in its nominal value.
    Ohms low: V = I * RL ===> if the current increases, the voltage increases as well.

    My main question is actually why connecting 2 MOVs from Live/Neutral to Earth, instead of just one MOV from Live to Neutral?
    What are the advantages/disadvatages of this?
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    too large = fault

    Fault currents are also deliberately made large to definitely trip the disconnection device.

    It is not a question of diverting fault currents, they do not exist in normal operation. They are deliberately created to trip the cutout. They may be created by a varistor going low resistance, then the fault current will pass the varistor as you suggest.
    There are also other ways a fault current can arise that does not pass (or activate) the varistor.

    I don't follow your question about voltage rising. Mains supply is controlled to remain within pretty close limits.

    All appliances connected to the mains should be able to operate normally at or above these limits, preferably well above with some spare in hand. This is a sensible legal requirement in most countries.

    However nothing is perfect, so designers need to understand what they are protecting against.

    You should also be enough of an engineer to recognise the 'joule rating' of a surge protector as marketing moonshine. The mains can supply many thousands of times this on demand - a surge is not 'so many joules' , but so many volts too high. The current is determined by the load resistance, nothing else. Hence the power is determined by the voltage and the load resistance.
     
  13. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    I see,
    Thank you very much.

    About the configuration of the MOVs connections.
    Could you please tell me the advantages and disadvantages when comparing the two following configurations:
    1. 2 MOVs from Live/Neutral to Earth.
    2. 1 MOV from Live to Neutral.
     
  14. studiot

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    Nov 9, 2007
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    Protection is not an academic exercise, listing advantages and disadvantages for some professor.

    You should be designing against some specific event, not just a nebulous 'rise in voltage' whatever that means.

    So are you designing against mains voltage spiking to say 300 volts on a 240 system?
    or are you designing against lightning strike?
    or what?

    The answer may well be different in each case.
     
  15. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    Thank you.
    The protection is againts lightning strike.
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
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    When you have two wires in close proximity, eg an overhead line and neutral supply, the lightning will affect both wires.

    Thus in order to provide protection you have to provide a surge arrestor path to an earth from each wire individually. Strapping an arrestor across the wires will be ineffective.

    If no earth exists it is necessary to provide one.
     
  17. creakndale

    Active Member

    Mar 13, 2009
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    Back in the late 70's, my Dad was doing some tree trimming when he miscalculated. A large heavy branch fell directly on the power lines coming into the house. That catastrophic failure took out the Line/Earth and the Neutral/Earth MOV's in his computer surge protector.

    I suppose a lightning strike near power lines could impress a high enough voltage on the Neutral line to trigger a MOV's threshold.

    In theory, the Neutral and Earth are tied together at the Breaker/Fuse box. But who knows how apartments and older homes handle this connection if it even exists. Plus, all electrical wire and especially all their connection points have resistance associated with them. A current surge multiplied by this resistance could cause a voltage spike even on a Neutral wire such as a short from Line to Neutral or a lightning strike.

    IF you have an Earth connection, it's hopefully at the safest/lowest voltage potential. So a surge protector with 3 MOV's placed:
    Line to Earth,
    Neutral to Earth, and
    Line to Neutral, protects the load from a wide range of potential voltage spikes.

    creakndale
     
  18. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The original question ( and thread title ) was about what if there is no earth

    But it was an interesting experience you relate, thanks.
     
  19. westom

    Member

    Nov 25, 2009
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    5

    Numerous factors are not in your posts. Let's start with basics. Lightning struck a church steeple. Why? Because the electricity called lightning makes a connection from the cloud to earth borne charges. The better (more conductive) path to earth was through a wooden church steeple.

    Yes, wood is even a conductor. But not a very good one. A typical 20,000 amp lightning current creates voltage in that wood. Current times a voltage means destructive energy dissipates in that wood.

    How did Franklin stop church damage? The connected a lightning rod to earth. Nothing stops surges. Franklin diverted that current on a conductive wire - near zero voltage because wire is more conductive. Current times near zero voltage means near zero energy - no damage.

    That is also what protector components do. Either you connect that electricity to earth before it enters the building. Or it hunts for earth destructively via appliances. Surges are earthed before entering the building. That is how it is done in every facility that suffers no damage. And why the single point earth ground is so essential.

    MOVs connected between wires do what? That surge comes down one or all AC electric wires seeking earth. MOVs simply connect that surge from one wire to all wires - surge still seeking earth ground. What path will the surge take? Wire impedance means a better path is destructively through some adjacent appliance - or why one appliance may be damaged while other nearby appliances are not.

    Wire impedance is why the 'whole house' protector connects as short as possible to earth ground AND has a greater separation between appliance and protector. Typically, that protector must connect from every incoming wire in every cable to earth ground 'less than 10 feet'. Wire must also have no sharp bends, no splices, not inside metallic conduit, etc. Other factors that would also increase wire impedance.

    A 'whole house' protector is typically installed in the breaker box. But if that earth wire goes up over the foundation and down to earth, then it is too long - too much impedance. Energy will seek earth ground destructively inside the building. That earth ground wire must go through the foundation and connect short to the single point earth ground. Earthing that must also be used by all other utility wires - telephone, cable TV, satellite dish, etc.

    MOVs do not provide protection. MOVs are connecting devices that are effective ONLY if something exists to connect to. Earth ground is what energy must be harmlessly dissipated.

    Cable TV and satellite dish need no MOVs - no protectors. These wires connect directly to earth. Therefore no protector is required to connect to earth. That is the point. Protectors are only connecting devices - no protection. Protection is what absorbs that energy. Your concern is the energy absorbing device. MOVs at a few hundred joules will never absorb typically destructive surges that are hundreds of thousand of joules. MOVs located too close to the appliance must either absorb that massive energy - or are not effective.

    Your protector circuit is only as effective as the only device always required in every protection system - earth ground. Wire impedance is why effective MOVs are connected 'less than 10 feet' to earth. Wire impedance is also why safety ground is not earth ground. A protector is only as effective as its earth ground.
     
  20. lmartinez

    Active Member

    Mar 8, 2009
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    6
    MOV (metal oxide Varistor)The purpose of such device is to direct the energy into the earth. Hence, it is imperative to provide a path for such energy to be dissipated into the surface of the earth......Caution!!!!!!
     
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