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surge suppressor voltage limit

Discussion in 'Electrical Engineering' started by [email protected], Sep 16, 2008.

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  1. Guest

    In a previous thread a reference was made to an expert recommendation that
    surge suppressors have their clamping voltage doubled from 330V to 660V for
    120VAC suppressors.

    Originally, I thought that was a bad idea. But now I think maybe that will
    be useful. How I came to that conclusion was finding that suppressors were
    not readily available for 240VAC power systems. With such a change in the
    clamping voltage, I could use these more common suppressors as part of the
    changeover from 120V to 240V for my computers.
     
  2. TWayne

    TWayne Guest

    In a previous thread a reference was made to an expert recommendation
    The clamping voltage statement doesn't sound right. I'd go back and
    reread the information and pay particular attention to the knee voltage
    and the current rank just prior to it. I'm not saying it's a bad idea;
    just that it might have a lot of leakage on 240 that surpasses the
    component's temp ratings.

    You don't give enough info on how you plan to use these, but I do know
    that the UK style 230Vac 50Hz does have accomodating surge suppressors.
    Their grid is configured differently than here in the US where I am of
    course.

    HTH
     
  3. Guest

    |
    | |> In a previous thread a reference was made to an expert
    |> recommendation that
    |> surge suppressors have their clamping voltage doubled from 330V to
    |> 660V for
    |> 120VAC suppressors.
    |>
    |> Originally, I thought that was a bad idea. But now I think maybe
    |> that will
    |> be useful. How I came to that conclusion was finding that
    |> suppressors were
    |> not readily available for 240VAC power systems. With such a change
    |> in the
    |> clamping voltage, I could use these more common suppressors as part
    |> of the
    |> changeover from 120V to 240V for my computers.
    |>
    |> --
    |> |WARNING: Due to extreme spam, googlegroups.com is blocked. Due to
    |> ignorance |
    |> | by the abuse department, bellsouth.net is blocked. If you
    |> post to |
    |> | Usenet from these places, find another Usenet provider
    |> ASAP. |
    |> | Phil Howard KA9WGN (email for humans: first name in lower case at
    |> ipal.net) |
    |
    | You mean use a TVSS built for 120V and apply it at 240V? I worry it
    | would be bleeding catastrophic amounts of current with double nominal
    | voltage applied. Plus, a terminal intended for neutral (0V) would not
    | be insulated for the 120V applied if the 240V is +-120 V. If the 240V
    | is 240V and neutral, the 240 V terminal may only be good for 120 V

    If 330V clamping level works on 120VAC, then 660V clamping level should work on
    240VAC. The disadvantage would be that the L1-G and L2-G paths would be 660V
    as well, and those potentials are 120VAC each on a North American 240V system.

    I have disassembled power strips, including a nice TrippLite one that became
    unusable due to water damage. These things are wired symmetrically.


    | nominal. Also I doubt it is 'acceptable' to apply a device intended
    | for 120 V at 240 V.

    I find it not 'acceptable' that the manufacturers don't make decent 240V ones
    in the first place. If they did, I'd buy one.

    I did find one on the TrippLite web site for 240V. But it has NEMA 5-15 plug
    and outlets. There's something wrong with that concept. And it has a poor
    level of protection.

    The fact that there is not a genuine decent one on the market means I have no
    choice but to "construct" one in some way. Feel free to advise HOW to do that.
    But any suggestion to not do that will be ignored since it is not a valid path
    to the goal.
     
  4. Guest

    |> In a previous thread a reference was made to an expert recommendation
    |> that surge suppressors have their clamping voltage doubled from 330V
    |> to 660V for 120VAC suppressors.
    |>
    |> Originally, I thought that was a bad idea. But now I think maybe
    |> that will be useful. How I came to that conclusion was finding that
    |> suppressors were not readily available for 240VAC power systems.
    |> With such a change in the clamping voltage, I could use these more
    |> common suppressors as part of the changeover from 120V to 240V for my
    |> computers.
    |>
    |>> WARNING: Due to extreme spam, googlegroups.com is blocked. Due to
    |>> ignorance | by the abuse department, bellsouth.net is
    |>> blocked. If you post to | Usenet from these places, find
    |>> another Usenet provider ASAP. |
    |>> Phil Howard KA9WGN (email for humans: first name in lower case at
    |>> ipal.net) |
    |
    | The clamping voltage statement doesn't sound right. I'd go back and
    | reread the information and pay particular attention to the knee voltage
    | and the current rank just prior to it. I'm not saying it's a bad idea;
    | just that it might have a lot of leakage on 240 that surpasses the
    | component's temp ratings.

    If you designed and constructed a real 240V protector, would you not make
    it have twice the clamping level?


    | You don't give enough info on how you plan to use these, but I do know
    | that the UK style 230Vac 50Hz does have accomodating surge suppressors.
    | Their grid is configured differently than here in the US where I am of
    | course.

    I would use it here in the US on the US style 240V system which is 120V to
    ground on each of two hot wires in opposing polarity (180 degrees in the
    case of genuine 240V single phase).

    I have considered using a German power strip, and just make my computer
    wiring all based on the Schuko. The Schuko is unpolarized and symmetric,
    even though the electrical system it is based on is 230V L-N. But they
    require devices to be safe when plugged in either way. So these devices
    should be able to handle 230V relative to ground on either conductor, as
    we as between conductors.

    The change of clamping voltage from 330V to 660V would means I might be able
    to find devices with protective components that could handle 240V L-L *AND*
    be able to support replacing NEMA 5-15R outlets with NEMA 6-15R outlets. I
    would prefer to use the NEMA 6-15's over the Schuko.

    Alternatively to the Schuko, which is more common, I might use the Italian
    plug/outlet design.

    Most of my "wall wart" power supplies are rated for 100-240 volts. But they
    all have NEMA 1-15P on them. So I'll still need to either put 240V on a
    NEMA 5-15R or just run these on 120V.
     
  5. Guest

    On Tue, 16 Sep 2008 19:03:19 GMT wrote:
    | MOVs used in surge suppression circuit for 240VAC are normally selected
    | based on 240 +10% and then + 10% again. This allow for a 10% increase in the
    | nominal supply voltage to 264 VAC and for the 10% manufacturing tolerance on
    | the AC rating of the MOV. Therefore for 240VAC you would normally select a
    | MOV with a nominal AC rating of at least 290 VAC. The standard rating for an
    | MOV is either 300 VAC or 320 VAC depending on the manufacturer. These
    | normally clamp the surge at about 800V.
    |
    | In your posting you talk about 660 V, I would guess that this is the
    | clamping voltage, not the AC rating. If you use a surge suppressor rated at
    | 120VAC these normally use a MOV rated at 150 VAC. These will fail very
    | quickly if are used at 240VAC, due the high leakage current.
    |
    | Best regards
    |
    | BillB

    Don't forget, Bill, that I am talking about 120VAC protectors based on DOUBLING
    their clamping level ... NOT the current crop of 120VAC protectors that perhaps
    still have the low clamping level.

    I've read in multiple places that the common clamping voltage for 120VAC is
    330V and for 240VAC is 660V. Fewer of the readings mentioned the 240VAC and
    660V clamping level. But the ratio makes sense.

    The PEAK voltage on 120VAC is 170V. The PEAK voltage on 240VAC is 340V. So
    the 330V level for protecting 120VAC, and the 660V level for protecting 240VAC,
    is well above the peak operating voltage.

    If you DOUBLE the clamping level AND double the AC voltage, what do you think
    happens?
     
  6. Guest

    | On Sep 15, 11:44 pm, wrote:
    |> In a previous thread a reference was made to an expert recommendation that
    |> surge suppressors have their clamping voltage doubled from 330V to 660V for
    |> 120VAC suppressors.
    |
    | Those voltages are ball park numbers. Actual voltage curve is not a
    | step function or 'knee voltage' as so many want to believe. A
    | protector rated at 330 volts will conduct current even at 130 V RMS.
    | How much current determines how quickly an MOV degrades. Yes,
    | degrades because MOVs inside protectors must fail only by degrading; a
    | change in its threshold voltage. A 10% change means the MOV has
    | failed.
    |
    | Typically, MOVs for 330 volts will also conduct at 180 volts AND
    | will increase voltage to greater than 800 volts as a surge current
    | increases. So what is its voltage? That would be the MOV inside a 330
    | volt protector. 330 or 660 volt numbers are ballpark - arbitrary.
    | See MOV datasheets for the V-I charts.
    |
    | An MOV used in 120 volt operation must conduct less than acceptable
    | current at 185 volts. Idea is to select an MOV that conducts under a
    | minimum acceptable current constantly. How do we measure MOV
    | threshold voltage? One standard is to push a constant 1 milliamp
    | through the MOV and read its voltage. If that voltage changes by 10%,
    | then the MOV has degraded - failed. IOW that 1 milliamp voltage
    | should be sufficiently above 185 for 120 VAC operation and above 370
    | for 240 VAC operation. These would also be 330 volt and 660 volt let-
    | through voltages.

    So you think maybe I should fall back to the old plan of using Schuko power
    strips protectors?
     
  7. bud--

    bud-- Guest


    What are the problems?
     
  8. Guest

    On Thu, 18 Sep 2008 18:40:50 GMT wrote:

    | Please explain what you mean by doubling their clamping voltage?

    Using a 660V level instead of ao 330V level.


    | All the surge suppression devices we have tested are suitable for use at
    | 240VAC and use 300V MOVs with rated clamping voltages of around 770V. These
    | have all had clamping voltages of around 800V when subjected to 8/20us
    | pulses.

    I've not tested these in labs. I'm just going on things I have read in the
    past. In particular someone did post here that an "expert" recommended that
    the levels be double.


    | I am talking of testing surge suppression devices not just an MOV by its
    | self.
    |
    | Similar sized MOV rated at 150VAC have rated clamping voltages of between
    | 360 and 400V.
    |
    | See Little Fuse website for details
    | http://www.littelfuse.com/searchresults.html?NN=0:Technology:156

    I'll bookmark that to check it out.
     
  9. Guest

    | writes:
    |
    |>I would use it here in the US on the US style 240V system which is 120V to
    |>ground on each of two hot wires in opposing polarity (180 degrees in the
    |>case of genuine 240V single phase).
    |
    |>I have considered using a German power strip, and just make my computer
    |>wiring all based on the Schuko. The Schuko is unpolarized and symmetric,
    |>even though the electrical system it is based on is 230V L-N. But they
    |>require devices to be safe when plugged in either way. So these devices
    |>should be able to handle 230V relative to ground on either conductor, as
    |>we as between conductors.
    |
    | I have seen (older) power strips that use 3 or so ordinary duplex 5-15R
    | recepticles in a row. If you want to make your own, try to find these and
    | replace the outlets with 6-15R's.

    That was the idea ... except in for some wall warts I'll still need 5-15R.
     
  10. Guest

    | On Sep 17, 8:35 pm, wrote:
    |> So you think maybe I should fall back to the old plan of using Schuko power
    |> strips protectors?
    |
    | I don't know what is inside that Schuko strip to suggest it is
    | safe. Question is whether you are doing this to learn, or doing it to
    | make a protector. Using (and modifying) a Schuko for North American
    | power is not acceptable for normal operation and may even violate your
    | fire insurance. But it might be educational if used as an experiment.

    Open one up and see. It's basically the same thing as used for 120V in USA,
    except it obviously has to be able to handle the 230V (+/- 10V) standard in
    Europe AND be able to handle being plugged in either way.


    |> The PEAK voltage on 120VAC is 170V.
    | No. Same standards that also define what surges an appliance must
    | withstand without damage also define how much voltage it must see
    | constantly. That standard then defines the maximum constant voltage a
    | protector must remain 'inert' for. For 120 VAC operation, that
    | protector must remain 'inert' at voltages above 185 - not 170.

    I was not saying the operative level was 170, only that the sinusoidal peak
    of a nominal 120VAC is 170V. Add 10% or more as needed.


    | What happens when AC mains goes to a completely normal 125 volts?
    | All part of a protector design.

    Of course. So the real level where the protector acts would be higher than
    170V, such as the 330V figure I have read in a number of places.
     
  11. bud--

    bud-- Guest

    Is the standard is too tough?

    At the risk of asking a dumb question, would the suppressors likely pass
    the US-UL tests?
    Are you the entity formerly known as ? (His posts were real
    interesting.)
    Still at TUV?
    In exile?
     
  12. Guest

    On Fri, 19 Sep 2008 23:18:47 GMT wrote:
    | I now know what you are speaking of regarding the situation in the USA. Your
    | 240 V supply is two hot wires with 120 V to ground and 240 V live to live.
    | In Europe we have one live, a neutral and ground. The live has 230 V to
    | Neutral and to Ground, Neutral has 0 V to ground.
    |
    | For the US system I would use 3 MOVs 2 with 150V AC ratings connected live
    | to ground and one with a 300 VAC rating, connected live to live. In Europe I
    | would use 3 MOVs all rated 300 VAC, one connected live to ground, one live
    | to neutral and one neutral to ground.
    | With the US system the MOVs will provide a clamping voltage of about 400 V
    | live to ground and 800 V live to live. While the European version will give
    | 800V live to ground, live to neutral and neutral to ground.
    |
    | The situation is even more complex in Europe as we have to limit the leakage
    | to ground, normally by the use of a Gas Discharge Tube, GDT. The GDT is
    | installed between live , the live to ground MOV and ground and neutral, its
    | MOV and ground. The GDT has negotiable leakage at line voltage, but during a
    | surge it will conduct, but it responds slowly increasing the peak clamping
    | voltage due to its slow response.
    |
    | So if you use a European surge suppression unit in the US it will work fine,
    | but will not give the same protection live to ground that a unit designed
    | for your supply would, but should be electrically safe.

    However, I believe it would get at least as good a protection level as the
    proposed doubling of the clamp voltage. The proposed 120V protection design
    would have 800 V on all pairs, L-G, L-N, and N-G. But maybe the European
    devices would be better with regard to leakage.
     
  13. Guest

    On Sat, 20 Sep 2008 07:55:42 GMT wrote:
    | Hi Phil
    |
    | The so called European devices are the same as the ones as the US version,
    | made in the same plant in China, tested to the same standard as a component.
    |
    | Yes I agree you would get less leakage to ground using a higher voltage MOV,
    | but you would also get 800V live to ground clamping voltage instead of 400V
    | live to ground when using the 150V MOV. The reason you fit a surge
    | suppression device is to limit the value of any surges on the line to the
    | lowest value economically possible to get the best protection for your
    | money. If reducing the leakage is the prime requirement you should
    | investigate other solutions to the problem eg GDTs.

    Is there any reason the 800V level of clamping would be bad? This is for a
    240VAC circuit.

    If course there is the argument I _can_ use a lower clamping voltage between
    either of the two live wires, and the grounding wire (there being no neutral
    in this case). My counter to that argument is that there is a suggestion being
    made to double the clamping level for 120VAC devices. If that is valid science
    then how would it apply to the North American 240V configuration?


    | What we have been discussing above is only half of the requirement, in
    | addition you need to consider thermal protection for the MOVs, plus short
    | circuit protection. An MOV without the protection can produce quite a good
    | explosion when it fails, which all will do eventually if subjected to repeat
    | surges. If a MOV fails you also need to know it has failed, in other words
    | you need some form of indication. The simplest is that when you plug the
    | device in all the circuits are dead, but this is not always possible the
    | achieve easily. Otherwise you will continue using it, but not have the
    | protection you think you have.

    Presumably some circuit within the device will test if the MOV is completely
    open, or sufficiently open to not offer the protection. Some implication in
    an earlier post is that the leakage itself might be a means to do this aspect
    of the test.

    For cases of the MOV being fused closed, being behind a fuse or circuit breaker
    would be expected to achieve that test.


    | Look inside any well know surge strip and you will find all these feature.
    | Also have at look at
    | http://www.littelfuse.com/data/en/Application_Notes/Littelfuse_app-note_an9767.pdf
    | and http://www.littelfuse.com/data/en/Application_Notes/EC638.pdf
    | and http://www.littelfuse.com/data/en/Application_Notes/EC640.pdf

    In what percentage of power strip type point of use protectors do you see
    these features?
     
  14. Guest

    On Sat, 20 Sep 2008 21:12:19 GMT wrote:

    | I cannot see any valid reason for doubling the live to ground clamping
    | voltage, it the same as saying why bother to fit surge suppression in the
    | first place. The lower the clamping voltage the less likely it is to damage
    | the connected equipment.

    I do agree with this concept. But my point is something else.

    If *they* go ahead and change the surge protector designs so that the clamping
    voltage is doubled for the 120V protectors, then maybe that creates a device
    that could be used on twice the voltage.

    If the clamping voltage in the USA is doubled, but this does not happen in
    Europe, then the devices in both would end up being about the same, right?

    I'm looking for devices to protect against surges at point of use for 240V
    as wired in the USA. I could use a protective device from Europe, but it
    would not be optimal. But my big point is, if they double the clamping
    voltage of protective devices from USA, they would basically be the same
    and this gives me another choice to find a device that should work be will
    be less than optimal.

    The optimal design would be one that considers the voltage to be 240V L1-L2,
    120V L1-G, and 120V L2-G. The less that optimal designs have one advantage
    over the optimal designs: they are available.

    So the remaining question is, if they do double the clamping voltage on the
    the models for the 120V market, are they usable on 240V? Would there be any
    difference between these "doubled for 120V" compared to the "normal for 240V"
    models in UK (not considering the NEMA vs. BS outlets and plugs, fusing
    requirements, etc).


    | One perceived advantage of using higher voltage MOVs is the transient energy
    | rating, for example a 20mm 150 VAC MOV typically has a rating of 120J, while
    | a 20mm, 300V MOV has a rating of 250J. Therefore the higher voltage device
    | can withstand a larger surge energy level before failure, but against this
    | you will have the higher clamping voltage that is more likely to damage the
    | connected equipment. Energy ratings are used as a marketing tool by some
    | manufactures. Finally it is the connected equipment that we are trying to
    | protect not the MOVs from failure. The simple answer is use a large diameter
    | MOV, which gives the best of the both, a similar clamping voltage and a
    | higher energy rating. Some manufactures attempt to increase this by the use
    | of MOVs in parallel, unfortunately this does not normally work. For it to
    | work it requires matched MOVs, without this the lowest rated MOV takes all
    | the surge energy, while the other does very little.

    Apparently some "experts" think that today's home appliances can withstand
    some higher surge levels, and that the MOVs are being destroyed more often
    than desireable in the protectors.

    When I look at my computer SMPSUs and see "100-240V 50/60Hz" on them, should
    I assume these units can withstand the surge levels that would not be clamped
    by a 240V protector device in UK, when used in the USA whether on 120V or 240V?

    Surges that originate upstream on the power utility distribution lines might
    well have twice the voltage on a 240V connection compared to a 120V connection.
    But surges that originate after the utility transformer steps the voltage down
    to the utilization voltage, are going to be the same.

    The only reason I see to have any more protection in the USA compared to the UK
    is that we have areas of the country with more frequent lightning.


    | The basic circuits we have been discussing, do provide reasonable levels of
    | protection, but do have limitation, in relation to the energy they can
    | absorb.

    1. So maybe we don't need a lower clamping voltage in the USA, given that
    more and more appliances (especially computers) handle all of 100-240V.

    2. So maybe there is an advantage in the ability to handle more energy with
    the clamping voltage doubled. That and being destroyed less often.

    1+2 = maybe the "experts" are right.

    3. I want to use 240V, even though that means a different system configuration
    in the USA (compared to UK).

    1+2+3 = more devices available (but I have to be very careful to choose units
    that have this doubled clamping voltage else the MOVs will give up their magic
    smoke as soon as I plug them in).
     
  15. Guest

    On Sun, 21 Sep 2008 20:50:05 GMT wrote:
    | Hi Phil
    |
    | Lets try and answer your questions.
    |
    | On 21-Sep-2008, wrote:
    |
    |> If *they* go ahead and change the surge protector designs so that the
    |> clamping
    |> voltage is doubled for the 120V protectors, then maybe that creates a
    |> device
    |> that could be used on twice the voltage.
    |
    | The answer is no, you could not use it at twice the voltage. Only the
    | ratings of the MOVs between Live and Ground have been increased in voltage
    | rating. The MOV between live and live is still the same (300VAC). If you
    | tried to use the unit on a higher rated supply the MOV between live and live
    | would fail, as it is only rated for a 240 VAC supply.

    Your reference to "The MOV between live and live is still the same (300VAC)"
    isn't making any sense here. If you are talking about a device intended for
    120V as in USA, then there is no "live and live". There is only one live,
    and a neutral, and a ground.

    Unless you are referring to balanced power as in NEC 647. And that is quite
    a different beast.

    If the voltage of ALL the MOVs on a 120V power strip are doubled, then that
    means the L-N are doubled, the L-G are doubled, and the N-G are doubled. So
    then if I use that device on 240V (USA style) then we change L to L1, and N
    to L2. So are yoy trying to say that when the voltage of a surge suppressor
    is doubled, it's not really doubled on the L-N pairing (which becomes L1-L2
    when used for 240V)?


    |> If the clamping voltage in the USA is doubled, but this does not happen in
    |> Europe, then the devices in both would end up being about the same, right?
    |
    | Yes the MOVs would be the same.

    So why could the be used on 240V in Europe but not in USA?


    |> I'm looking for devices to protect against surges at point of use for 240V
    |> as wired in the USA. I could use a protective device from Europe, but it
    |> would not be optimal. But my big point is, if they double the clamping
    |> voltage of protective devices from USA, they would basically be the same
    |> and this gives me another choice to find a device that should work be will
    |> be less than optimal.
    |
    | If the device is a power strip the would it could have the same protection
    | circuit, but it would not meet US or Canadian requirements due to the
    | sockets been European and not meeting US or Canadian standards.

    That's a different issue. I might swap out the receptacle parts, plus other
    components that can only handle 120V.


    |> So the remaining question is, if they do double the clamping voltage on
    |> the
    |> the models for the 120V market, are they usable on 240V? Would there be
    |> any
    |> difference between these "doubled for 120V" compared to the "normal for
    |> 240V" models in UK (not considering the NEMA vs. BS outlets and plugs,
    |> fusing requirements, etc).
    |
    | The answer should be yes if you ignore the differences in the standard
    | requirements.
    | Most counties other than the US base their standard on IEC 60884-1 for the
    | electrical safety of the basic power strip and on IEC 61643-1 for surge
    | suppression. So what is designed for Germany can also be sold in Holland,
    | Spain etc. and what is designed for the UK can be sold in Hong Kong, Cyprus,
    | Malta etc. Although Japan use the same socket as Japan it must be tested to
    | Japanese standard and have a PSE approval before it can be sold there.

    I have some "wall warts" that are rated for 100-240V 50/60Hz, but the plugs
    are NEMA 1-15 (e.g. standard for 120V w/o ground pin).


    |> Apparently some "experts" think that today's home appliances can withstand
    |> some higher surge levels, and that the MOVs are being destroyed more often
    |> than desirable in the protectors.
    |
    | They can, but they will last longer with the lower clampng voltage, It is
    | the marketing department like to use the higher energy rating as a marketing
    | tool, the engineer still prefer the lower clamping voltage.

    But on 240V, the clamping voltage would be "just right" (doubled from 120V).


    |> The only reason I see to have any more protection in the USA compared to
    |> the UK
    |> is that we have areas of the country with more frequent lightning.
    |
    | Not true some area of the UK have similar lighting strikes frequency as the
    | US.

    Some areas of the US have substantially more than most of the US. How does
    the UK compare to say, Florida?


    |> 1. So maybe we don't need a lower clamping voltage in the USA, given that
    |> more and more appliances (especially computers) handle all of
    |> 100-240V.
    |
    | As I have already said the lower the clamping voltage the longer the life of
    | the protected device will be.

    Of course. But I want to run the device at 240V for power efficiency.


    |> 1+2+3 = more devices available (but I have to be very careful to choose
    |> units
    |> that have this doubled clamping voltage else the MOVs will give up their
    |> magic
    |> smoke as soon as I plug them in).
    |
    | NO any device rated for 240 VAC should give acceptable life both for your
    | equipment and the protection device itself, if purchased from one of the
    | major manufacturers. Do not be tempted to buy a cheap no name unit,
    | irrespective of the manufacturers claims.

    I never do that. I always want to know who to sue (even if that would be a
    very unrealistic thing) :)
     
  16. Guest

    | On Sep 21, 1:00 pm, wrote:
    |> 1+2 = maybe the "experts" are right.
    |
    | Doubling a varistor's voltage does nothing to double its surge
    | energy handling abilities. That energy number is more related to
    | current - not voltage. Voltage is a symptom of that current.
    | Increasing a varistor's voltage mean a varistor absorbs more energy
    | which means the varistor fails (degrades) faster.

    The surge energy handling is not the issue I am asking about.

    The issue I am asking about is using double the supply voltage when the MOVs
    are doubled in their clamping voltage.


    | Protector does not work by absorbing surges. It works by acting
    | more like wire during a surge. A protector that absorbs less energy
    | is more effective and lasts longer.
    |
    | How does a varistor absorb less energy? Increase its joules rating.

    An increase of joules rating is MORE energy, not less.


    | Doubling voltage is about getting a varistor to absorb no energy -
    | conduct near zero current during normal operation. For example, a 185
    | volt varistor for 120 VAC operation will conduct 800 volts during a
    | more destructive surges. Increase varistor voltage to 370 volts for
    | 240 VAC operation and that varistor may conduct at 1500 volts. The
    | resulting higher voltage for same current means the varistor absorbs
    | more energy during that surge which is not desirable. See varistor
    | manufacturer datasheets for V-I charts.

    What I have read is that the varistors used for 120VAC operation conduct at
    330V and posts here have said that is 400V. I just verified that one of my
    surge suppressors lists 330V. The theory is, if they double that voltage to
    at least 660V, then these could be used on 240VAC circuits. Originally I
    was expecting to use German Schuko surge suppressor strips. These are
    designed for 230V or the full 220V-240V European range. Complications with
    this would be plugging in wall warts that have US plug prongs. Once it is
    mentioned that power strips in USA should have the voltage rating of MOVs
    doubled, that opens up the possibility of power strips that can run on 240V
    and have the right outlet type.
     
  17. Guest

    On Mon, 22 Sep 2008 20:40:58 GMT wrote:

    | As I have said previously manufacturers sell there power strips using the
    | Energy rating as a mean of trying to say their unit is better than the
    | competitor, as it has a high energy rating whereas in real life this makes
    | no difference to the actual protection they provide to the connected
    | equipment, as it is the clamping voltage that makes the difference.

    Unless the MOVs are destroyed and go open circuit before the surge is
    complete.


    | MOV only fail in a correctly designed power strip if they are subject to
    | repeated high energy surges, this is rarely true in real life conditions.

    Or a substantial available current sufficient to heat the MOV to the point of
    vaporization.
     
  18. Guest

    | On Sep 22, 4:40 pm, wrote:
    |> Regarding your comments, varistor do not conduct voltage, they conduct
    |> current, the voltage is the resistance produce by the flow of current
    |> through the device.
    |> As I have said previously manufacturers sell there power strips using the
    |> Energy rating as a mean of trying to say their unit is better than the
    |> competitor, ...
    |
    | Correct about a surge; it is current. References to conducting
    | voltage are a misnomer used only to explain a concept - not an
    | accurate engineering term.
    |
    | In North America, joules must be listed per safety standards.
    | Meanwhile, many protectors only use 1/3rd and never more than 2/3rd of
    | their joules when doing protection. Number of joules actually used
    | may be even lower depending on what surge is where. IOW joules number
    | on a power strip protector really provides no useful information for
    | engineering purposes - at best only provides a ballpark absolute
    | maximum that will never be achieved. Joules is a number that must be
    | published per North American standards.

    The clamping voltage would be more useful. But it would help to also know
    the clamped impedance (should be very very low, but it would not be zero).


    | Many also see the word 'joules'; then assume those joules will absorb
    | all of a surge. Again, a conclusion based only on assumptions; not
    | based in learning what joules actually measure.
    |
    | Surges are current - as stated by both authors. Voltage is only a
    | symptom of that current. A surge properly diverted (by a protectors
    | or other methods) means a massive current creates 'near to zero'
    | voltage.

    Right. And we want that voltage drop to be below the supply voltage so the
    surge is not any greater.


    | Another way to reduce voltage during a surge? Increase varistor
    | joules. More joules means less voltage and less energy gets
    | absorbed. A concept that many have difficulty grasping.

    Joules _is_ energy. So that doesn't make sense UNLESS you are saying that a
    higher joules RATING has a lower impedance, and that results in a lower voltage
    drop when conducting.
     
  19. bud--

    bud-- Guest

    ..
    From previous threads, w_ seems to have the idea that high joule
    ratings magically reduce the actual clamp voltage.

    In fact, high joule ratings come with high surge current ratings.
    Comparing a high joule MOV with a lower joule MOV (with the same nominal
    clamp voltage) at the same surge currents, the high rated one will be
    operating at a lower percentage of its rated surge current. The current
    density through the MOV will be correspondingly lower. That produces a
    lower actual clamp voltage. There is no magic involved. And the
    difference isn't that great.

    High current/joule rated MOVs are used because they give long life. The
    joule rating for a MOV is a single event rating. As the individual
    energy hits on a MOV become a smaller percentage of the rating, the
    cumulative energy rating of a MOV goes up rapidly (not linearly).
     
  20. bud--

    bud-- Guest

    ..
    Francois Martzloff was the US-NIST guru on surges with many published
    papers. From one of them:
    "The fact of the matter is that nowadays, most electronic appliances
    have an inherent immunity level of at least 600 V to 800 V, so that the
    clamping voltages of 330 V widely offered by [surge suppressor]
    manufacturers are really not necessary. Objective assessment of the
    situation leads to the conclusion that the 330 V clamping level,
    promoted by a few manufacturers, was encouraged by the promulgation of
    UL Std 1449, showing that voltage as the lowest in a series of possible
    clamping voltages for 120 V circuits. Thus was created the downward
    auction of "lower is better" notwithstanding the objections raised by
    several researchers and well-informed manufacturers. One of the
    consequences of this downward auction can be premature ageing of [surge
    suppressors] that are called upon to carry surge currents as the result
    of relatively low transient voltages that would not put equipment in
    jeopardy."

    The paper dates back to 1995. Suppressors with very high ratings are now
    readily and cheaply available which may make the argument less relevant.

    For the 250V world, one would have to know the immunity level. It may be
    closer to the normal voltage than in the US, making an increase in clamp
    voltage less practical.
    ..
    ..
    Matching would be better. At least you should sequentially go through
    the paralleled MOVs, with some sharing along the way.
    Or can you drive one into failure (conduction at normal voltages) before
    the others are 'used up'?

    How do manufacturers get ratings of 1000+J for a single MOV in a plug-in
    suppressor? Paralleling? There seem to be high rated single MOVs that
    can be used in service panel suppressors.
    ..
    ..
    Another Martzloff paper looks at the energy absorption for a MOV at the
    end of a branch circuit. It is surprisingly small for 2 reasons:
    1. At about 6000V (US) there is arc over from panel busses to
    enclosure(+neutral+ground+earthing system). After the arc is
    established, the voltage is hundreds of volts. That dumps most of the
    energy coming in on hot power wires to earth. (Receptacles (US) also
    arc-over at about 6kV.)
    2. The impedance of branch circuit wiring for surges greatly limits the
    surge current, and thus energy, that can reach a MOV.

    The maximum energy dissipated in the MOV was 35 Joules for a 10 meter
    branch circuit. In 13 of 15 cases it was 1 Joule or less. That was with
    surges source currents from 2,000 to 10,000A (the maximum likely for a
    home). Surprisingly, the highest energies were for some of the lower
    surge currents because the MOV could hold the service panel voltage
    below arc-over.
     
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