Connect with us

what is the joule rating on surge protectors?

Discussion in 'Electronic Basics' started by jjoensuu, Jul 22, 2006.

Scroll to continue with content
  1. jjoensuu

    jjoensuu Guest


    I have a question about the joule rating on surge protectors. From some
    information I found on the net, this is the amount of energy a surge
    protector can absorb before it gets saturated, after which it must be

    If this is correct, how does the surge protector get saturated? Is it
    like a battery or something?

    Thankful for a clarification...
  2. Eeyore

    Eeyore Guest

    If it gets a really big surge it'll 'burn out'. For obvious reasons ( heating ).

    I've never seen it happen myself. What's your reason for asking ?

  3. Ralph Mowery

    Ralph Mowery Guest

    You may think of the joule rating as similar to a gas bottle. It is rated
    in how many PSI of gas it can hold. Once you go past this (not allowing for
    any safety factory and such) , the bottle blows apart. The surge protector
    will absorbe so much power and then it will start to over heat. If you are
    lucky it will do like some did to me and short out and blow a fuse before
    the protected circuit gets overloaded. If hit with a very large surge, it
    will blow apart and open up and not provide any more protection.
  4. A surge protector absorbs energy from the surge and turns it into
    heat. It is assumed that this process takes place so fast, that none
    of the heat has time to escape from the component absorbing the
    energy, so all of that energy goes into raising the temperature of the
    component (called an MOV for metal oxide varistor). The joule rating
    is based on the mass of the MOV, its specific heat (how much the
    temperature of a gram of that material rises per joule of energy
    added) and its peak temperature rating. Higher joule rating devices
    are physically larger.
  5. w_tom

    w_tom Guest

    Joule rating is similar to a life expectancy number. However little
    relationship exists between MOVs joule rating and how many joules may
    actually be shunted. For example, an MOV with higher joules rating
    will 'absorb' less joules over a lifetime that shunts many times more
    joules; when compared to an MOV that is smaller.

    MOVs that vaporize or disintegrate were used beyond what the
    manufacturer intended. An unacceptable and catastrophic MOV failure is
    not what joules are measuring. MOVs fail by degrading. A degraded MOV
    has no physical burning, but has a voltage change of typically 10%.
    But this does not promote protector sales. Many protectors that are
    undersized will then promote more such sales.

    To determine how long an MOV may last, consult charts provided by
    manufacturers. For example, a 180 joule MOV may shunt 1 million 20
    amps transients, 1,000 300 amp transients, 10 600 amp transients, or 1
    4000 amp transient. In each case, the MOV has no observable damage.
    Any burn marks on MOV is totally unacceptable operation.

    As MOV joules increase, joules that it may shunt to earth increases
    exponentially; increased life expectancy. Those joules shunted to
    earth are vastly more than the MOV joule rating because the function of
    an MOV is not to absorb energy. Its function is to shunt as much
    energy as possible to earth.

    As MOV degrades, its varistor voltage changes. MOVs degrade; must
    not blow out or open circuit. Some mistake that open circuit MOV as
    normal operation. That open MOV as demonstrated in these scary
    pictures is an unacceptable condition often found in plug-in protectors
    that may have been grossly undersized and are a fire threat: Protectors.pdf

    Above third URL demonstrates what a protector's indicator lamp
    reports. All active components (the MOVs) were removed and indicator
    lamps still reported MOVs as OK. Those indicator lamps would only
    report a type of failure that is unacceptable and should never occur.
    Indicator lamp actually implies a protector that was grossly undersized
    (had insufficient joules) and may have been a threat to human life.
  6. John  Larkin

    John Larkin Guest

    Then there must be two "joule" ratings: how much energy it can absorb
    in any one shot, and how much it can gobble over its working lifetime.

    Do the datasheets make this clear?

  7. Rich Grise

    Rich Grise Guest

    That's OK, John. My Dad taught me many years ago to just consider
    the source.

  8. John  Larkin

    John Larkin Guest

    As they say in Texas, call me anything you like, just don't call me
    late for dinner.

  9. Rich Grise

    Rich Grise Guest

    Ah! Another pseudonym for goof-ball posts: "Late For Dinner". ;-P

  10. John Fields

    John Fields Guest

    It doesn't.
    It doesn't.

    W = qV

    P = W/t
    I think that what's much more likely is that you've misinterpreted
    what you've read.
    No, it doesn't. It shunts thousands of _coulombs_ to ground for
    microseconds, and dissipates power during the transient depending on
    its clamping voltage and the charge flowing through it for the
    duration of the transient.
    No, It doesn't. A switch is designed to drop negigible voltage when
    it's turned on, while an MOV is designed to clamp to a certain
    voltage when it's "turnrd on". That voltage drop and the charge
    flowing through the MOV is what causes it to dissipate power.

    No, that 4500 amps will only last for microseconds, so the only work
    the earth will be doing will be:

    W = IVt

    same like the MOV.
    Now that you've seen the numbers, (equations, anyway,) you can
    dispel the "myths" by getting some MOV data sheets, plugging the
    numbers you get there into the equations, and solving them.
    Because that 4500 amperes wasn't. it was 4500 coulombs times the
    voltage dropped across the MOV times the time the spike lasted.
    Never mind Micheal, I've seen no math from you, so on what are you
    basing your tirade?
    They aren't. see above,
    I'm sure they don't. What's happened is that you've misinterpreted
    what they meant because of your ignorance.
    Their "speculations" are much closer to reality than yours.
  11. John  Larkin

    John Larkin Guest

    If it did, it would vaporize.
    As JF says, a 39 j MOV is rated to absorb 39 J in a single-shot event.
    "Earth", as in a good ground, doesn't absorb the transient energy. The
    transient is partially reflected and partially dissipated *in the
    generating source*, and the MOV absorbs a bit of it.

    Earth absorbs transient energy only in proportion to how bad a ground
    connection you make.
    If "earth" is zero volts and you dump, say, 10,000 amps into it,
    what's its power dissipation?

  12. w_tom

    w_tom Guest

    Earth is zero volts when a 10,000 amp current travels four miles to
    earthborne charges? Not true. When earth is conducting lightning, a
    four legged animal in that path may become a victim. A tree is struck.
    Shortest path to earthborne charges is down tree, through earth, up
    that cow's hind legs, down its fore legs, then on through miles of
    earth to earthborne charges. Why is the cow killed? If earth is zero
    volts, then electricity would never travel through and kill a less
    conductive cow. Earth is not zero volts because earth is not zero
    ohms. Earth is dissipating lightning energy. A cow's body makes a
    more conductive path; therefore cow is killed. Earth is so
    non-conductive that energy in those 10,000 amps is dissipated in many
    miles of earth and takes an electrically conductive path through cow.
    Earth is where energy is absorbed because earth is not a zero ohm
    conductor. Earth is not zero volts. Miles of earth is where energy
    from 10,000 amps is absorbed.

    Effective protection means a low impedance (as possible) shunt to
    earth so that little energy is absorbed by the shunt (MOV, GDT,
    avalanche diode, or lightning rod) and so that significant energy is
    absorbed by earth. A low impedance shunt does not absorb the energy
    just like wire does not absorb energy. Better protectors are even
    lower impedance when to provide even better protection. Shunt energy
    to earth where energy is absorbed. Franklin demonstrated same in 1752.
    Did his ligthning rods, instead, absorb the energy of a lightning
    strike? Of course not. Earth absorbs the energy. Just like MOVs, the
    lightning rod make a low impedance connection to earth.

    MOV joules is how life expectancy is measured. Just like wire, a 39
    joule MOV absorbs little energy while shunting massive energy into
    earth. Unlike wire, the MOV eventually degrades with use. We measure
    this MOV life expectancy in joules. A more conductive and superior MOV
    absorbs less joules. Therefore a more conductive MOV will shunt more
    energy to earth, absorb less energy, and has a longer life expectancy.

    They are shunt mode protectors. To protect by absorbing energy, a
    protector would be a series mode device - in line with electronics.
    MOVs are not series mode devices. MOVs are shunt mode devices. MOVs
    are effective when energy is diverted to and absorbed by earth. If
    MOVs absorbed energy, then voltage on protected appliances must
    increase - completely undesireable. Effective MOVs make a low
    impedance connection to earth just like a switch. That means less
    voltage confronting protected appliances, less energy absorbed by the
    MOV, and more energy into earth. That means even better protection.
    Notice: better protection means even less energy absorbed by the MOV;
    more energy absorbed in earth.
  13. John  Larkin

    John Larkin Guest

    Shortest path? A standing cow is a shorter path than no cow?

    The energy of a lightning bolt is mostly converted into light and heat
    within the thousands of feet of length of the bolt itself. Where else
    would it go? The fraction of a bolt's energy that winds up heating the
    earth is minute; just do the math. If the earth at the point of
    contact spikes up to 10KV (not a grossly unreasonable number) and the
    cloud potential is a few hundred megavolts (up to a gigavolt for
    positive lightning) then the earth dissipates well under 0.01% of the
    total energy of the bolt.

    In a series circuit, the lowest resistance dissipates the least power.
    The function of the MOV is to keep this piece of equipment from
    absorbing enough energy to fry it, but it actually diverts more energy
    *upstream* than it does into ground.

    Nobody has yet answered my question: what are typical one-shot versus
    lifetime joule ratings for MOVs?

    Collecting folklore can add up to a lot of contradictions.
    Understanding the physics has no such problems.

  14. John Fields

    John Fields Guest

  15. John  Larkin

    John Larkin Guest

    Does he? Why didn't he say so? "Electricity follows the shortest path"
    is qualitative folklore. I wonder what the inductance of a cow is.

    So any MOV can absorb a megajoule?

  16. Well, convert a mega joule of electric energy to heat, if given enough
    time to do it. 4 weeks should be long enough for most.
  17. John Fields

    John Fields Guest

    _Cubic_ miles, which means there are zillions of _parallel_
    resistive paths through which the charge travels until it's
    That's not true, and you just can't seem to get it through your head
    that MOVs are _not_ voltage-triggered zero ohm switches, they're
    designed with specific clamping voltages in mind, and are also
    designed to be able to maintain that clamping voltage for a certain
    amount of time and with a certain quantity of charge flowing through
    them for that amount of time.

    Visit a data sheet and do the math.
    Sure it does, and they _both_ absorb energy and dissipate it as

    Say you've got an MOV with 4500 amperes flowing through it while
    dropping 200 volts. It'll be converting electrical energy into heat
    at the rate of:

    W = IVt = 200V * 4500A * t = 900,000 joules per second

    A piece of #10AWG wire 12" long, with a resistance of about 1
    milliohm per foot would drop about:

    E = IR = 4500A * 0.001R = 4.5V

    and dissipate:

    W = EIt = 4.5V * 4500A * t = 20,250 joules per second

    Interestingly, the resistance of the MOV, at that current, would be:

    E 200V
    R = --- = ------- = .044 ohms
    I 4500A

    Which is far short of the zero ohm shunt you think it should be.
    That lower impedance only comes about because they're bigger, can
    supply more parallel conduction paths and, consequently can absorb
    more energy than smaller units before they go into "meltdown".
    They absorbed part of it, since the rods and the wire connecting
    them to earth were resistive.
    No, earth only absorbs _some_ of the energy.
    No. the MOV is designed to drop a certain voltage at whatever charge
    flows through it up to a specified maximum for a specified time.
    The lightning rod is designed to shunt all of the current in the
    strike to earth.
    No. A more conductive and superior MOV will be larger, so absorbing
    more energy will raise its temperature less and cause less upheaval
    in its internal structure.

    No. A larger MOV will be damaged less than a small one for the same
    internal dissipation. That's why it will last longer than a smaller
    In some cases that's true, and good examples are any of the 78XX
    No. MOVs are effective when they absorb and dissipate the energy in
    a destructive overvoltage transient.
    Yes, but MOVs aren't linear devices. They're designed to clamp the
    voltage across themselves to a particular value regardless of the
    current they're carrying, up to some specified maximum.

    Effective MOVs keep the voltage across the device they're protecting
    from rising to destructive levels.
    Nope. Sorry Charlie, your earlier argument had earth being so
    non-conductive that a cow was killed, but now you want to switch
    sides and have a perfectly conducting earth absorbing charge from a
    perfectly conducting MOV?

    If you're serious, let's see some numbers...
  18. John  Larkin

    John Larkin Guest

    Yeah, gotta protect against those slow lightning bolts.

  19. John  Larkin

    John Larkin Guest

    Yeah, but the big one clamps at about 7 kilovolts!

    Some of the curves sort of imply a lifetime of 1 to 1e6 shots,
    depending on the energy per shot, with a fuzzy disclaimer.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day