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Re: Surge Protector


to keyboard and composed:

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I think the term "surge" is a misnomer in this case. "Surge
protectors" (aka metal oxide varistors) will soak up spikes, but not
sustained overvoltages, which is how I perceive the term.

While I have no faith in these devices, I do agree that it may be more
economical to purchase a Belkin surgeboard (ie, a lifetime insurance
policy bundled with a power board) than to upgrade an existing
household contents policy, especially when amortised over several
years.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


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We have a "protector" fitted to the 60 telly "
  with a close ground strike recently it tripped and I assume helped
save things as the same boards also serves the sounc system and I was
delighted to see the survival even though the rooms incadecant was blown
  , I expect the induction from a strike only 4 metres away was severe ?
( it was LOUD)

Re: Surge Protector


Yes, if it was a crash rather than a rumble it was close. The equipment will
have experienced EMPs through induction on both the aerial and power lines,
and probably a little through space as well. The zapping of an incandescent
lamp indicates a very powerful effect as those devices are much more
resistant to lightning EMPs than solid state devices in electronic domestic
equipment. However, not as bad as the alleged problem suffered in a case I
dealt with some years ago. The "victim" claimed that his lamps had been
"blown out of their sockets". A somewhat tall order (either for bayonet caps
or Edison screw fittings!). In fact I would suggest that in your case the
possibility that the strike concerned caused a major disturbance to power
lines and transformer gear and maybe the lamps suffered from that (the
intermix piggyback syndrome)

"a t e c 7 7" <"atec 77 at hotmail dot com"> wrote in message
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Re: Surge Protector


Incidentally, how did you know it was 4 metres away?

"a t e c 7 7" <"atec 77 at hotmail dot com"> wrote in message
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Re: Surge Protector


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   A tree stopped existing
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Re: Surge Protector


OK. It was either on the tree or within about 1 metre of it. The sap
literally explodes the tree, due to the sudden heat. (= energy, unlike the
suggestion of someone on here who contended that there was no energy in a
lightning strike!) For those keen to learn more Frydenlund makes interesting
reading. And Dr Steven Gumley, who survived despite what I think is
foolhardy stuff in taking his VW Combi into storm activity to do
experiments! Although both gentlemen were involved with various products
they were somewhat above keen salesmen! In fact Steven was a Rhodes Scholar
(but on the other hand so was Bob Hawke!)

"a t e c 7 7" <"atec 77 at hotmail dot com"> wrote in message
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Re: Surge Protector


BTW, ISBN 0-442-01338-8

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Re: Surge Protector


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  Incandescent bulbs will not be destroyed by lightning strikes.
However a massively higher energy source can cause that problem.
Lightning may construct electrically conductive paths where none must
exist.  That path constructed by same process that starts a
fluorescent lamp.  That surge finished in microseconds.  But the
plasma path remains.

  For example, lightning formed a circuit from utility transformer
primary to secondary.  Now AC utility 13,000 or 33,000 volt primary is
connected directly into your building.  That high voltage applies high
energy.  What might explode?  Well which items completed that
electrical path?  Not necessarily everything.

  Demonstrated is why utility earth ground is critical to surge
protection.  Surges that may overwhelm protection inside all
electronics should not be permitted inside the building.  Then
electronics internal protector circuits are not overwhelmed.

  How might a ground strike be a direct lightning strike to telephone
equipment?  Yes, I said direct strike. Strikes to earth or a nearby
tree can also be direct strikes to a human, cow, or building that does
not have effective protection.  An application note from Polyphaser
discusses a failure and what provides protection:
http://tinyurl.com/38v2dv
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  Polyphaser app notes are considered industry benchmarks for
understanding the principles:
  http://www.polyphaser.com/technical_notes.aspx

  Did surge enter on earth ground?  Then building's surge protection
'system' is defective.  Destructive surges enter via ground when an
earthing system - what provided protection - is improperly installed.
Begin with some of those Polyphaser app notes.

  Every electronic device - powered on or off - in home or business -
is connected to the equivalent of a radio antenna system.  Principles
pioneered by Franklin in 1752 on churches and by early 20th Century
Ham radio operators are also installed by Telstra so that their
switching computer (connected to overhead wires all over town) is not
destroyed (for four days due to surge damage).  Direct lightning
strikes (via overhead or ground strikes) without damage are that
typical.

  Appreciate why each telephone switching center can suffer hundreds
of surges during each thunderstorm - and no damage.  First they earth
each surge before it can enter the building.  Then protection already
inside electronics is not overwhelmed.


Re: Surge Protector


Spot on w_tom! And I had given you up as a pontificating politiciser. Just
shows how wrong one can be...

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Re: Surge Protector


On Tue, 10 Jul 2007 18:34:01 +1000, a t e c 7 7 <"atec 77 at hotmail
dot  com"> put finger to keyboard and composed:

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Maybe I'm being unfair.

For example, Belkin's 4-way Surgeboard (the cheapest) boasts the
following specs:

 Part # F9A402au2M
 Protection: Level 1
 Joules: 714J
 Maximum Spike Current: 19,500A

Assuming that a 100W incandescent lamp experiences a cold current of
10 times its operating current for 2 mains cycles, then its energy
dissipation would be only 100W x 10 x 20ms x 2 = 40 Joules.

Comparing the two figures (714J vs 40J), is it any wonder that your
lamp failed while your MOV survived?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


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  Your post is based in some assumptions.  For example, a lamp
filament absorbs nearer to zero energy when cold.  Bulbs fail after
filament gets too hot - vaporizes.  Typical surges are microseconds.
Filament never gets hot enough fast enough to increase resistance,
then vaporize.

   For example, a 'whole house' protector rated for 50,000 amps will
have wires how large?  2 or 2.5 mm diameter copper.  A wire rated for
20 amps constant current is also sufficient to shunt 50,000 amps surge
current because that surge is not constant; typically in
microseconds.  Same reason why short transients surges would dissipate
so little energy in a light bulb.

  To have sufficient energy to blow a bulb, that current must conduct
longer.  Source of higher energy and source of current long enough may
be utility supplied electricity.

  Confused is how light bulbs dissipate energy verses what joules
measure for an MOV.  Same units measure different parameters.

  Joules for MOVs is a measure of how conductive the MOV may be and a
ballpark measurement of its life expectancy (size, pulse width, and
number of transients).  Joules for MOVs is equivalent to measuring
gauge of a wire from Ben Franklin lightning rod to earth.  If that
wire is too thin, then it will vaporize.  If MOV has too few joules,
then it will degrade.  MOV vaporizes only when current massively
exceeds what manufacturer intended - well beyond Maximum Permissible
parameters.

  More joules in an MOV mean MOV absorbs even less energy.  More
joules for a light bulb means a bulb dissipates more energy.   For a
constant current, higher joules in an MOV means more conductive; high
joules for a light bulb means less conductive.

  Conductivity of a wire is measured in amperes.  Conductivity of MOVs
is measured in joules.  The difference between wire and MOV: wire does
not degrade with use.  MOVs degrade with use.   Wires are for
continuous current.  MOVs are for rare and short current transients
that are typically measured in microseconds.  Whereas necessary wire
size is measured by wire gauge; MOV size is measured by joules and
lamp energy dissipation is measured in joules.  Joules for a lamp and
joules for an MOV are measuring two different parameters.

  What happens when an MOV is better - more joules?  Then it absorbs
even less energy.  MOV function is not to absorb energy.  It's
function is to shunt (connect, divert, clamp, bond) that electricity
elsewhere.


Re: Surge Protector


Spot on again. Saved me a long posting! He was of course considering orange
joules against apple joules.

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Re: Surge Protector


to keyboard and composed:

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I would have thought that the converse was true. For the first 2 or 3
mains cycles after switch-on, the lamp draws 10 times its normal
operating current, which means that it dissipates 10 times its rated
power. This energy impulse warms up the filament, after which time the
lamp begins to emit light. Presumably, when the lamp is emitting light
it would be absorbing less energy because it would be converting the
energy that would otherwise be absorbed as heat.

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I accept that the surge *duration* is only microseconds, but the surge
*current* can be 100s or 1000s of amps. Hence the surge *energy*
(VI.dt) could be far in excess of what the lamp experiences during
switch-on, which is itself a stressful transient. And to make the
situation worse, the surge voltage is not clamped (?) as it is in the
case of a MOV, so AFAICS the effective impulse is Isquared.R.dt.

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Let's assume the surge lasts for 1us and that the current is 1000A. A
100W 240V lamp would have a resistance of around 600 ohms when hot and
about 60 ohms when cold.

The surge energy would then be at least 1000 x 1000 x 60 x 1E-6 = 60J.

As this is more than what the lamp experiences during a normal cold
start (40J ?), one would expect that the strike would very quickly
warm it up. In any case, my Internet research suggests that the
duration of a typical strike is at least 20us, so the energy figure
would be more like 1200J.

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Sorry, but I can't make any sense of this.

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Conductivity is something else altogether. You are confusing me by
redefining your own terms.

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I can't see it.

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Sure, it shunts the surge *current* through itself in an attempt to
protect the attached equipment, but in so doing it must dissipate an
amount of energy equivalent to Vclamp x Isurge x dt.

To put this energy rating in perspective, 714J = 171 calories, which
means that the MOV can absorb that amount of energy required to
increase the temp of 171 grams of water by 1 degC, or 1 gram of water
by 171 degC. As for how much this would raise the temperature of a
typical MOV disc, I don't know, but it shouldn't be too hard to
estimate.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


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  Time to study datasheets.  Learn what the curves are reporting.  How
you we make an MOV more conductive?  We increase it joules.  How do we
make an MOV dissipate less energy?  We make it larger - more joules.
It is right there in datasheets.

  Again, what is the current rating of a 2mm wire?  20 amps?  Then why
is that same wire also sufficient for conducting a 50,000 amp surge?
Same reason why a hundred amp surge inside the building does not blow
out filaments.  Same reason why an MOV rated for a 2000 amps surge
will dissipate less than 2 watts AND why its wire leads are 0.8 mm
diameter.  0.8 mm conducts 2000 amps and does not vaporize?  Read
datasheets.

  Do some numbers.  What is the resistance of a cold 100 watt bulb?
15 ohms?    300 amps squared times 15 ohms times 30 microseconds is
maybe 40 joules.  Well MOVs that are rated at 70 joules must not
dissipate more than 2 watts after a 2000 amp surge.  Why would the 100
watt light bulb be dissipating any more than 2 watts?

  But again, read the MOV datasheet.  Even a 0.8 mm wire on that MOV
conducts 2000 amps and does not vaporize.

  How do we make a MOV more conductive?  We increase its joules.  What
makes a protector better?  More joules.  How much energy does an MOV
dissipate during same a size surge when we increase its joules?  Less
energy is absorbed.

  Again, joules that measures MOV performance measures parameters
completely different in a light bulb.   Do the numbers.  To make a
protector absorb less energy; to make it more conductive - then we
increase its joules.

  We don't want these shunt mode protectors to absorb any energy.
However we make neither perfect conducting MOVs nor perfect conducting
wires.  Both MOVs and wires dissipate energy.  Better means less
energy dissipated by a wire and by an MOV.

  MOVs don't work productively by absorbing surges.  MOVs work by
doing what wires also do.

  Finally back to that 70 joule MOV.  While conducting its maximum
2000 amp surge, it may absorb 70 joules.  Meanwhile maybe 2400 joules
from that same 2000 amp surge is being absorbed in earth.  2000 amps
times 40,000 volts times 30 microseconds.   40,000 volts may be across
4 kilometers of earth between the earthed MOV and distant earthborne
charges.  That same surge did not dissipate over 1000 joules
destructively inside the building because the 70 joule MOV shunted
energy in 2000 amps elsewhere.

  How do we increase energy dissipated in earth?  We enlarge that 70
joule MOV to 300 joules.  Now the MOV may absorb only 40 joules during
that 2000 amp surge.  That is even better protection.  By increasing
joules, we absorb less energy in the MOV - the MOV has been made more
conductive.


Re: Surge Protector


to keyboard and composed:

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The reason that it makes no sense is that both the filament and the
MOV are resistive devices, the difference being that the MOV has a
knee shaped IV characteristic. Below its rated voltage, the MOV's
resistance is extremely high, but above this point the resistance
falls sharply. Nevertheless, both devices still dissipate power
according to the formula P=VxI, where V is the voltage appearing at
the terminals of the device, and I is the current passing through the
device. The energy dissipated during a surge would be VI.dt.

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Resistivity and conductivity refer to an intrinsic property of a
material.

OTOH, resistance is measured in ohms, conductance in siemens. It
doesn't matter whether you have a MOV or a wire filament.

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I still don't see it.

The amount of energy that a MOV dissipates would be given by VI.dt, ie
it is determined by the event, not by the device. For example, a 20mm
275V 1000J MOV and a 1m 275V 1MJ MOV (does it exist?) will both
dissipate the same amount of energy when hit by the same surge.

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That makes no sense. Assuming that the MOV has a 275V rating, and
assuming that the duration of the surge is 20us, then ...

 Power  = I.V  = 2000A x 275V   = 550kW
 Energy = P.dt = 550E3 x 20E-6  = 11J

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Can you show me one?

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Power = V.I =  V x 2000A = 2Watts

Therefore V = 1 millivolt.

Clearly this is absurd.

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Because it is a 100W light bulb?

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No, the same amount of energy is absorbed, it's just that the bigger
MOV will experience a lesser increase in temperature.

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No, the amount of energy dissipated in the MOV is determined by
V.I.dt.

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But the MOV is designed to *clamp* the surge voltage, therefore it
must absorb an amount of energy given by Vclamp.Isurge.dt.

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Energy = V.I.dt.

If Energy = 0, then V=0, or I=0, or dt=0.

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It shunted the 2000A through itself, and clamped the surge voltage to
275V.

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The MOV still clamps the surge to 275V. The rest of the surge voltage
(40kV - 275V) still dissipates in the earth, or the air, or wires, or
wherever. Increasing the rating of the MOV doesn't alter the amount of
energy that it is called upon to dissipate.

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- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


  I am troubled that you are posting without reveiwing MOV
datasheets.  My posts repeatedly referenced datasheets for good
reason.
 http://www.nteinc.com/specs/10to99/pdf/movs.pdf

  Review a 2V250.  Seventy joules.  Conducts 4500 amps.  Dissipates
how many watts?  Less than 1 watt.  Left for you is to get another
number from those datasheets - the maybe 0.8? mm diameter wire
conducts 4500 amps without vaporizing. For reasons defined below, I
have intentionally not provided enough numbers for doing arithmetic.

  Myths proclaim MOVs protect by absorbing energy.  If MOV's purpose
is to absorb energy, then the protector must be a series mode device.
It would connect between appliance and wall receptacle like a switch
or fuse.  But MOV protectors are shunt mode devices.  MOV connects
just like a light bulb - in parallel.

  Series mode devices block (high resistance) to stop surge current.
Shunt mode devices shunt (short, conduct, clamp, near zero resistance)
to divert current.  Clamping means that conductor (the MOV) is better
when resistance is lower.  Better shunt mode protector absorbs less
energy.

  Example: light bulb and MOV connect same way in a circuit.  But
light bulb conducts current without affecting voltage fed to adjacent
appliance.  MOV shunts current so that voltage to the adjacent
appliance is reduced.

  Again conductivity of a wire is defined in amperes (discussing
siemens is only arguing semantics).  Conductivity of an MOV is defined
in joules.  That example accurately defined the underlying concept.

  A 20mm 275V 1000J MOV and a 1m 275V 1MJ MOV will not dissipate same
energy when conducting the same surge.  But again, study charts in
those datasheets.  And that is the point.  You are speculating without
first learning the technology.  That 1MJ MOV will dissipate less
energy. Saying otherwise means you did not study charts in datasheets
or manufacturer application notes.

  You also do not know how many watts are dissipated because
datasheets were not read.   Speculating without first learning
manufacturer facts which is why I am explaining less with each post.

 If confused, then numbers from that datasheet can say why I have
posted in error.  Instead you want me to read manufacturer  datasheets
and application notes for you?  If you need me to do that, then
experience says this thread will 'waste on' forever.  Posted are
bottom line facts.  If you don't do the work from datasheets, well, I
have already spent too much time explaining what manufacturers have
already written.  Your calculations are invalid because you did not
first learn information.

  BTW, I intentionally shorted some facts.  Each reply says you
ignored what was required - study of charts in manufacturer
datasheets.  You did not first consult manufacturer datasheets or
their application notes.  Therefore arithmetic, performed without
information from those datasheets, is in error.  Repeat again ...
consult those datasheets.

  Wire, light bulbs, and MOVs are resistive devices.  Using your
logic, then all must do exact same thing.  Reality - joules determines
MOV conductivity.  As MOV joules increase, the MOV becomes more
conductive, it becomes an even better shunt mode protector, AND it
absorbs less energy.  What happens when a shunt mode protector is even
better?  It absorb less energy.  You will never understand that
without first learning from those manufacturer charts and application
notes.

  Read datasheets before doing any arithmetic.  Having first not read,
then you even got watts wrong.

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Re: Surge Protector


to keyboard and composed:

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Left for you to understand is what the "Transient Power Dissipation"
figure actually means. In fact it most likely refers to the average
(not instantaneous) power that can be dissipated within the MOV (yes,
*within* the MOV) in response to *repetitive* transients. In other
words, while the instantaneous surge power (watts = joules per second)
can be of the order of hundreds of kilowatts, the "Transient Power
Dissipation" rating is a reflection of the MOV's recovery time.

For example, let's assume that a 20mm MOV has just absorbed a 20us 70J
surge. If it is rated at 600mW, then this means that it can tolerate
0.6 joules per second. Therefore the next transient must not arrive
within 70/0.6 = 117 seconds, ie about 2 minutes.

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The reason is that you clearly don't have a grasp of basic electrical
theory.

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Your NTE datasheet defines the MOV's energy rating as "the maximum
electrical energy which can be dissipated *within* the varistor by a
single impulse of 10 x 1000Ás current waveform with continuous voltage
applied".

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If the device to be protected can tolerate 300V, say, then all that a
shunt protector needs to do is to clamp the surge to 300V. It does
this by diverting the surge current through itself. In so doing, it
absorbs an amount of energy given by Vclamp.Isurge.dt.

I suggest that you study other shunt mode protectors such as
transorbs, zener diodes, spark gaps, MOSORBs, etc.

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Here is a Vishay datasheet.

 High Surge Suppression Varistors:
 http://www.vishay.com/docs/29082/23815825.pdf

Note 4 on page 5 confirms what I have been saying about energy
absorption, namely that ...

"If Vp is the clamping voltage corresponding to Ip [the peak current],
the energy *absorbed* in the varistor is determined by the formula:"

 E = K x Vp x Ip x t2

The "Peak Current as a Function of Pulse Width" drawing on page 6
shows a non-square current pulse. For t2 = 20us, we have K=1, and the
formula essentially reduces to E = Vclamp.Isurge.dt.

For the sake of convenience I have assumed that a varistor has a
perfect knee characteristic, and that the clamp voltage is the same
for all surge currents. However, the characteristic is shaped more
like an ice hockey stick with a finite slope.

Even so, if we take just one example, a particular 20mm MOV that has a
voltage of 300V at 400A will have a voltage of ~310V at 800A. This
means that if we connect two small MOVs in parallel, thereby making a
larger MOV with twice the energy rating, then an 800A surge that was
clamped to 310V by the single MOV will now be clamped to 300V by the
two MOVs (since each will carry 400A). Therefore your contention about
joule ratings and significantly reduced energy absorption is clearly
false.

You can also directly compare a 320V 7mm MOV with a 320V 20mm MOV. The
first clamps the surge at 780V while the second clamps it at 740V.
That's hardly any difference at all.

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That's exactly my point, only I would use the term "clamped".

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That makes no sense. You cannot redefine existing electrical terms to
suit yourself.

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See the above examples. I suggest you study those same datasheets
yourself.

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You have not understood what the datasheets were telling you. Learn
the difference between instantaneous power and average power.

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Your "information" is invalid because you do not understand basic
electrical principles. Datasheets will not teach you the meaning of
energy, power, amperes, joules, volts, etc. That knowledge is assumed.

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Done and ditto to you.

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See the examples above. Learn what a "clamp" does.

 Energy = Voltage x Current x time

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See the examples above. Learn what a "clamp" does.

 Energy = Voltage x Current x time

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See the examples above. Learn the difference between average and
instantaneous power, and the difference between watts and joules.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


On Sun, 15 Jul 2007 10:03:41 +1000, Franc Zabkar

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<snip>

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I should have said that the surge current used in this example was
1000A.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Re: Surge Protector


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  Voltage numbers provided by Franc for a 320V 7mm and 20mm MOV are
for 1 ampere; not 1000 A.  Voltage numbers for a 1000 A spike would be
1060 volts and just over 1000 volts; not 780V and 740V.

  More reasonable numbers for these Vishay VDRH07K320 (45 joule) and
VDRH20X320 (382 joule) MOVs would be 400A and 100A; in next two
paragraphs.

  A 45 joule 7mm MOV conducting a 400 A current is 1040 volts.  A 382
joule 20 mm MOV conducting same 400 A is only just above 1000 volts.
As joules increase, then the MOV becomes more conductive; absorbed
energy decreases by more than 3%.

For 100A, voltage numbers for that 45 and 382 joule MOVs are 970V and
910V.  Energy absorbed by a higher joule MOV decreases by 5%.

 As previously stated, energy absorption decreases when MOV joules
increase. More MOV joules means that MOV becomes more conductive.
Better is a more conductive MOV.  Why?  Better shunt mode protectors
absorb less surge.  Shunt mode protector means more conductive (less
power absorbed) is better.

  Now to complete an analysis using those datasheet charts.  What
happens due to that 3% and 5% reduction in energy absorption?   Fact
from the datasheet that Franc did not grasp.

  For a typical 30 microsecond 400A transient, the 45 joule MOV has a
life expectancy of slightly more than 10 surges.  But a more
conductive 382 joule MOV has a life expectancy of 900 surges.  With 8
times more joules, better conductivity means the MOV lasts 50 times
longer.  Just another reason why we want MOVs with better
conductivity; to absorb less energy. Life expectancy increases
massively.

  For the 100A surge, life expectancy of a 45 joule MOV is 2,000
surges compared to 80,000 for the 382 joule MOV. Again, 5% less energy
absorbed due to 8 times more joules results in a 40 times increase in
life expectancy.

  Demonstrated:
1) As MOV joules increase, then MOV conductivity increases.
2) More MOV joules means less energy absorbed.
3) Less energy absorbed also means less degrading of an MOV's
crystalline structure, less degrading, and therefore a significant
increase in MOV life expectancy.
4) MOV joules increased 8 times caused a 40 or 50 times increase in
number of surges; increased MOV life expectancy.

  More joules, better conductivity, and less energy absorbed means
less MOV degradation during each surge.  That is what we want from
MOVs.  More surge energy shunted elsewhere.  Less energy absorbed by
an MOV.

  Returning to the original point.  MOV are not for absorbing surges.
We install MOVs to shunt (divert, clamp) that surge elsewhere.   As
demonstrated earlier, for every joule absorbed by an MOV, maybe 10 or
30 joules are shunted elsewhere.  Increased MOV joules makes a better
(more conductive) protector.  Less energy absorbed by MOVs also
results in significantly longer MOV life expectancy.

  Using Vishay datasheets, MOVs are not installed to absorb surges.
That directly contradicts popular myths.  Unfortunately, it does
absorb some energy because an MOV is not perfect.  Increasing MOV
joules means an MOV becomes more conductive and degrades less during
each surge.  Install more joules in a shunt mode protector for better
conductivity and longer life expectancy.


Re: Surge Protector


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I agree that as the energy hit per event becomes a smaller percentage of
the MOV energy rating, the cumulative dissipation capacity is increased
out of proportion. Buying a suppressor with high energy ratings can
greatly increase its life.

But it is not credible that a 3% lower voltage at the MOV (and a 3%
lower energy hit per event) results in an 890% increase in the number of
hits the MOV can withstand. Because a higher energy rated MOV is larger,
the energy dissipated per unit area is lower - a decrease far more than
3%. A low energy dissipation per unit area just does far less damage to
the MOV.

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Ditto.


In the examples by 3 or 5% - almost trivial as Franc noted.

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Not because of a 3-5% decrease.

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I don't think it was demonstrated earlier. It depends on the circuit and
the event.

  Increased MOV joules makes a better
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I don't remember that in the Vishay datasheets. Sometimes, like
connecting a MOV across a relay coil, a MOV is installed to absorb
surges. In the case of service panel or plug-in suppressors I agree that
  absorbing surges is incidental.

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It has nothing to do with perfect. It has to do with physics. If there
is a voltage across a black box and a current through it for a given
time, energy is dissipated in the box.

I agree with Franc.

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