MOV Failure Mode?
Forrest Gehrke
I've had near-lightning hit. What is the failure
mode of MOV's I find in surge protectors?
Short-circuited? Open? Or either?
If open, (which is what I found) what is a good way to
test them?
The one's in question are rated at 400v.
Seems a bit high for 120v ac line.
The on/off switch on one of these protectors
has been blasted open-circuited.
//
k2bt
kilroy
As for testing, you would need a 'hi-pot' tester, definately not worth the
expense. Replace 'em if burnt or if there has been a near lightning strike.
73
bob
KC8HPG
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Forrest Gehrke wrote in message <37DFBE9A...@worldnet.att.net>...
John Woodgate
<fege...@worldnet.att.net> inimitably wrote:
>Hello,
>I've had near-lightning hit. What is the failure
>mode of MOV's I find in surge protectors?
>Short-circuited? Open? Or either?
Open. failing short-circuit would either be very dangerous or be
followed by failing open.
>
>If open, (which is what I found) what is a good way to
>test them?
You need a supply voltage greater than the breakdown voltage. No other
way.
>
>The one's in question are rated at 400v.
>Seems a bit high for 120v ac line.
Not really. Peak voltage is 170 V, after all, and could be +10% on that.
The MOV is designed to lop off the 1.5 kV spikes you get from load dumps
and network switching operations.
--
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Tony Williams
Forrest Gehrke <fege...@worldnet.att.net> wrote:
> Hello,
> I've had near-lightning hit. What is the failure
> mode of MOV's I find in surge protectors?
> Short-circuited? Open? Or either?
> If open, (which is what I found) what is a good way to
> test them?
The failure-mode of MOVs tends to be open circuit,
either partially or fully. The only diy low voltage
test that might be possible would be to measure the
capacitance and compare it with the quoted figure
for a new one.
After such an overload as you have described it would
be a cheaper/safer to simply replace all MOVs.
--
Tony Williams.
Murray Kelly
all shorted. The fuse was blown and all the smoke was out
of them.
If I saw any open, then I never knew!
Forrest Gehrke
> >Hello,
> >I've had near-lightning hit. What is the failure
> >mode of MOV's I find in surge protectors?
> >Short-circuited? Open? Or either?
--
******************************************************************
* Murray Kelly vk4aok mke...@powerup.com.au *
* 29 Molonga Ter. / Graceville/ QLD. 4075/ Australia *
* ph/fax Intl+ 61 7 3379 3307 *
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Max Imo
2,000) suffered from severe molecular displacement, and without fail all had
emitted every bit of their magic smoke. The device is open until the
applied voltage rises above the designed avalanche voltage.
LOL
Max
Forrest Gehrke <fege...@worldnet.att.net> wrote in message
news:37DFBE9A...@worldnet.att.net...
> Hello,
> I've had near-lightning hit. What is the failure
> mode of MOV's I find in surge protectors?
> Short-circuited? Open? Or either?
>
> If open, (which is what I found) what is a good way to
> test them?
>
> The one's in question are rated at 400v.
> Seems a bit high for 120v ac line.
>
Forrest Gehrke
>
> >Hello,
> >I've had near-lightning hit. What is the failure
> >mode of MOV's I find in surge protectors?
> >Short-circuited? Open? Or either?
>
> followed by failing open.
> >
> >test them?
>
> way.
> >
Ok. Thanks to all, in addition to John who responded.
So it's OPEN that I should find these MOV's.
Now I am curious. I have a 500 dc supply. Suppose I connect
it to a variac, connect the dc supply through a milliammeter
in series with an MOV under test. I raise the dc voltage
gradually and note if current drain begins around 400 volts.
If that occurs is the MOV OK?
MOV's are rated for a given number of joules. When subjected
to their rated voltage, is it possible they would only partially
give up their joule rating, or is it all or nothing? If not all,
would that condition be evident by a significantly higher voltage
where current would begin to be drawn?
//
Gary Coffman
>Hello,
>I've had near-lightning hit. What is the failure
>mode of MOV's I find in surge protectors?
>Short-circuited? Open? Or either?
MOVs fail in basically two different ways. Every spike
that exceeds the MOV's joule rating will damage it. This
damage takes the form of a depressed voltage at which
it becomes a good conductor. In other words, the current
versus voltage curve shifts to the left.
The failure mechanism is local microwelding of the sintered
MOV material, forming locally good conducting regions. If this
happens enough times, the MOV will will be "bridged" by the
microwelds, and become a good conductor at your working
voltage, though it may or may not still read "open" at a lower
voltage. So it fails "shorted" (sort of).
The other way that the MOV can fail is if it gets a massive
overload, ie the joule rating is grossly exceeded. Then it
will fail open. In many cases, all you'll find of the MOV is a
pair of wire stubs in this case. This is typically what happens
when the MOV is asked to snub lightning currents.
MOVs aren't suitable for that job. They are transient clippers,
not snubbers for an 18 kA lightning stroke. Gas tubes are
designed for that job.
>If open, (which is what I found) what is a good way to
>test them?
The only way to test them effectively is to use a current
limited high voltage to trace out the MOV's characteristic
voltage/current curve. (The same sort of tester you'd use
to test a zener, or to determine the reverse breakdown
voltage of an ordinary diode, is what you need, though
generally you need it to source a higher voltage than
what is needed to test zeners.)
>The one's in question are rated at 400v.
>Seems a bit high for 120v ac line.
Remember that it must not conduct on the
*peaks* of the operating voltage. 120 VAC
is RMS. Peak is about 170 volts, and peak to
peak would be 340 volts. Remember also that
a MOV doesn't have as abrupt a knee as a
zener. The curve is more S shaped, so you
have to stay further away from the corners.
>The on/off switch on one of these protectors
>has been blasted open-circuited.
In that case, I'd be pretty suspicious that the MOV
is open too.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Roy Lewallen
definitely include a series resistor in the circuit.
Roy Lewallen, W7EL
Roy McCammon
>
> On Wed, 15 Sep 1999 15:43:23 +0000, Forrest Gehrke <fege...@worldnet.att.net> wrote:
> >Hello,
> >I've had near-lightning hit. What is the failure
> >mode of MOV's I find in surge protectors?
> >Short-circuited? Open? Or either?
>
> MOVs fail in basically two different ways. Every spike
> that exceeds the MOV's joule rating will damage it. This
> damage takes the form of a depressed voltage at which
> it becomes a good conductor. In other words, the current
> versus voltage curve shifts to the left.
>
> The failure mechanism is local microwelding of the sintered
> MOV material, forming locally good conducting regions. If this
> happens enough times, the MOV will will be "bridged" by the
> microwelds, and become a good conductor at your working
> voltage, though it may or may not still read "open" at a lower
> voltage. So it fails "shorted" (sort of).
Seems that if that progressed far enough, the MOV would
conduct at line voltage, get real hot, maybe short some
more, and then blow open. Eh?
John Woodgate
<fege...@worldnet.att.net> inimitably wrote:
>Now I am curious. I have a 500 dc supply. Suppose I connect
>it to a variac, connect the dc supply through a milliammeter
>in series with an MOV under test. I raise the dc voltage
>gradually and note if current drain begins around 400 volts.
>
>If that occurs is the MOV OK?
Yes: that's how you test them.
>
>MOV's are rated for a given number of joules. When subjected
>to their rated voltage, is it possible they would only partially
>give up their joule rating, or is it all or nothing? If not all,
>would that condition be evident by a significantly higher voltage
>where current would begin to be drawn?
No, the voltage does not normally go up. They do degrade if subject to
heavy discharges, but this damage is not easily detectable by
measurements AFAIK.
--
Regards, John Woodgate, Elector of Rayleigh, Grand Four-cusped Astroid of the
First Order of Magnitude. OOO - Own Opinions Only. Phone +44 (0)1268 747839
***PLEASE DO NOT E-MAIL COPIES OF NEWSGROUP POSTS TO ME***
John Fields
> Hello,
> I've had near-lightning hit. What is the failure
> mode of MOV's I find in surge protectors?
> Short-circuited? Open? Or either?
>
> If open, (which is what I found) what is a good way to
> test them?
>
> The one's in question are rated at 400v.
> Seems a bit high for 120v ac line.
>
> The on/off switch on one of these protectors
> has been blasted open-circuited.
---
The initial failure mode of an MOV is _always_ a short circuit,
and that is why it should be preceded by a series fuse.
If the fuse isn't there and and there's enough energy available
to cause the MOV to melt/explode the final failure mode will be
an open and the downstream circuitry will then be subjected to
the transient causing the open or to future transients which will
then cause questions to be asked about whether the downstream
circuitry will fail open or shorted.
Test the MOV by measuring the current it will allow to flow when
its threshold voltage is exceeded or whether its threshold
voltage has changed with rated current flowing through it.
Don't let the test last for too long, since every "hit" the MOV
takes will further degrade it!
---
John Fields, Austin Instruments, Inc.
El Presidente Austin, Republic of Texas
"I speak for the company" http://www.austininstruments.com
Peter Zechmeister
(I think the knee of the MOV was about 200V) the only failures I've
seen are when the installing electrician hooks it up to 240VAC.
The failure mode falls into two classes:
1. Fuse blown and MOV vaporized. The stub of the leads get
welded to the PCB copper traces!
2. Fuse blown and MOV shorted. MOV is always burned, and usually
has ejected jets of molten or vaporized zinc. Can put a neat mirror
finish on the PCB! Hard to remove.
What happens depends on where in the AC cycle power is applied,
low part of cycle blows fuse before MOV disappears.
Peter Zechmeister - zech...@gold.tc.umn.edu - A University of Minnesota Alumni
> <37DFBE9A...@worldnet.att.net>, Forrest Gehrke
> <fege...@worldnet.att.net> inimitably wrote:
> >Hello,
> >I've had near-lightning hit. What is the failure
> >mode of MOV's I find in surge protectors?
> >Short-circuited? Open? Or either?
>>.....
>>
Dennis C. O'Connor
doing this... The transient spikes that MOV's are designed to shunt to
ground when they fire, tend to be of short duration... DC power supplies
don't tend to be short duration...
Denny
Forrest Gehrke wrote:
> John Woodgate wrote:
> >
> > >Hello,
> > >I've had near-lightning hit. What is the failure
> > >mode of MOV's I find in surge protectors?
> > >Short-circuited? Open? Or either?
> >
> > Open. failing short-circuit would either be very dangerous or be
> > followed by failing open.
> > >
> > >If open, (which is what I found) what is a good way to
> > >test them?
> >
> > You need a supply voltage greater than the breakdown voltage. No other
> > way.
> > >
> > >The one's in question are rated at 400v.
>
> Ok. Thanks to all, in addition to John who responded.
> So it's OPEN that I should find these MOV's.
>
> Now I am curious. I have a 500 dc supply. Suppose I connect
> it to a variac, connect the dc supply through a milliammeter
> in series with an MOV under test. I raise the dc voltage
> gradually and note if current drain begins around 400 volts.
>
> If that occurs is the MOV OK?
>
> MOV's are rated for a given number of joules. When subjected
> to their rated voltage, is it possible they would only partially
> give up their joule rating, or is it all or nothing? If not all,
> would that condition be evident by a significantly higher voltage
> where current would begin to be drawn?
> //
Gary Coffman
>> >Hello,
>> >I've had near-lightning hit. What is the failure
>> >mode of MOV's I find in surge protectors?
>> >Short-circuited? Open? Or either?
>>
>> that exceeds the MOV's joule rating will damage it. This
>> damage takes the form of a depressed voltage at which
>> it becomes a good conductor. In other words, the current
>> versus voltage curve shifts to the left.
>>
>> The failure mechanism is local microwelding of the sintered
>> MOV material, forming locally good conducting regions. If this
>> happens enough times, the MOV will will be "bridged" by the
>> microwelds, and become a good conductor at your working
>> voltage, though it may or may not still read "open" at a lower
>> voltage. So it fails "shorted" (sort of).
>
>Seems that if that progressed far enough, the MOV would
>conduct at line voltage, get real hot, maybe short some
>more, and then blow open. Eh?
Yep.
Luke Seubert
>
> On Thu, 16 Sep 1999 14:05:34 -0500, Roy McCammon <rbmcc...@mmm.com> wrote:
> >Gary Coffman wrote:
> >>
> >>
> >> MOVs fail in basically two different ways. Every spike
> >> that exceeds the MOV's joule rating will damage it. This
> >> damage takes the form of a depressed voltage at which
> >> it becomes a good conductor. In other words, the current
> >> versus voltage curve shifts to the left.
> >>
> >> The failure mechanism is local microwelding of the sintered
> >> MOV material, forming locally good conducting regions. If this
> >> happens enough times, the MOV will will be "bridged" by the
> >> microwelds, and become a good conductor at your working
> >> voltage, though it may or may not still read "open" at a lower
> >> voltage. So it fails "shorted" (sort of).
> >
> >Seems that if that progressed far enough, the MOV would
> >conduct at line voltage, get real hot, maybe short some
> >more, and then blow open. Eh?
>
> Yep.
>
not use MOVs. They are expensive, but they provide true protection over
the long run. Check them out on the web - zerosurge.com
--
Luke Seubert
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
-- Benjamin Franklin, Historical Review of Pennsylvania, 1759
kilroy
consensus seems to be...replace it if there is any doubt!
MOV's are a lot less pricey than replacement cost to your new Yaesu!
73
KC8HPG
kilroy
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John Fields wrote in message <37E174D4...@austininstruments.com>...
>Forrest Gehrke wrote:
>
>> Hello,
>> I've had near-lightning hit. What is the failure
>> mode of MOV's I find in surge protectors?
>> Short-circuited? Open? Or either?
>>
>> If open, (which is what I found) what is a good way to
>> test them?
>>
>> The one's in question are rated at 400v.
Ian White, G3SEK
>At any rate, there has been a whole lot of experience shown here and the
>consensus seems to be...replace it if there is any doubt!
>MOV's are a lot less pricey than replacement cost to your new Yaesu!
One other hint, from a previous discussion on this topic: if the
performance of MOVs is degraded by repeated large surges, it makes sense
to connect the MOV *after* the mains switch, in equipment that is not
running continuously.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
John Woodgate
<G3...@ifwtech.demon.co.uk> inimitably wrote:
>kilroy wrote:
>>At any rate, there has been a whole lot of experience shown here and the
>>consensus seems to be...replace it if there is any doubt!
>>MOV's are a lot less pricey than replacement cost to your new Yaesu!
>
>One other hint, from a previous discussion on this topic: if the
>performance of MOVs is degraded by repeated large surges, it makes sense
>to connect the MOV *after* the mains switch, in equipment that is not
>running continuously.
>
enough current through the mains switch to explode it, or weld it 'on'.
Whether this is a bad thing or nor depends on circumstances. It may be
preferable to risking the quicker degradation of the MOV if it is on-
line continuously.
Fred E. Davis
>Hello,
>I've had near-lightning hit. What is the failure
>mode of MOV's I find in surge protectors?
>Short-circuited? Open? Or either?
According to "Transient Voltage Suppression, 3rd Edition" from General
Electric:
"4.2.1 Failure Modes
Varistors initially fail in a short-circuit mode when subjected to
surges beyond their peak current/energy ratings. They also
short-circuit when operated at steady-state voltages well beyond their
voltage ratings. This latter mode of stress may result in the eventual
open-circuiting of the device due to melting of the lead solder joint.
When the device fails in the shorted mode the current through the
varistor becomes limited mainly by the source impedance. Consequently,
a large amount of energy can be introduced, causing mechanical rupture
of the package accompanied by expulsion of package material in both
solid and gaseous forms. Steps may be taken to minimize this potential
hazard by the following techniques: 1) fusing the varistor to limit
high fault currents, and, 2) protecting the surrounding circuitry by
physical shielding, or by locating the varistor away from other
components."
rst...@ihug.com.au
Consequently,
> a large amount of energy can be introduced, causing mechanical rupture
> of the package accompanied by expulsion of package material in both
> solid and gaseous forms.
When the package does rupture the MOV can then go open circuit.
Sometimes this can be difficult to spot, you might only get a hairline
crack in the package. So as far as the user is concerned you get both
types of failures, although technically a short is the primary failure
mechanism.
Regards
Rob
Sent via Deja.com http://www.deja.com/
Share what you know. Learn what you don't.
Mikhail Fridberg
Roy McCammon wrote in message <37E13F7E...@mmm.com>...
>Gary Coffman wrote:
>> The failure mechanism is local microwelding of the sintered
>> MOV material, forming locally good conducting regions. If this
>> happens enough times, the MOV will will be "bridged" by the
>> microwelds, and become a good conductor at your working
>> voltage, though it may or may not still read "open" at a lower
>> voltage. So it fails "shorted" (sort of).
>
>Seems that if that progressed far enough, the MOV would
>conduct at line voltage, get real hot, maybe short some
>more, and then blow open. Eh?
Shouldn't a properly designed and coordinated fuse/curcuit breaker
protection system prevent that from happening (Murthy law aside)?
Mikhail Fridberg
John Woodgate wrote in message ...
>>One other hint, from a previous discussion on this topic: if the
>>performance of MOVs is degraded by repeated large surges, it makes sense
>>to connect the MOV *after* the mains switch, in equipment that is not
>>running continuously.
>>
>If you do that, a big surge while the equipment is switched will pull
>enough current through the mains switch to explode it, or weld it 'on'.
>Whether this is a bad thing or nor depends on circumstances. It may be
>preferable to risking the quicker degradation of the MOV if it is on-
>line continuously.
>--
>Regards, John Woodgate
Main switch should never end up being welded on. It has to be rated to
interrupt maxim available supply current, even though normal working current
might be orders of magnitude lower. This is not a MOV problem but an issue
of proper coordination of breakeras/fuses.
Mike.
Mike.
Gary Coffman
It isn't rated to take the 18 kiloamperes of a lightning surge.
Mikhail Fridberg
Gary Coffman wrote in message <1unmN3l0TOOHjTPG21OaU7evpo=J...@4ax.com>...
If you want to protect against lightning, you do not put MOV after the main
switch. You put a lightning arrestor to where lightning could strike. If you
can not put arrestor where lightning will strike, then you have to use main
rated at 18kA interrupt current. Main switch should NEVER end up being
welded "ON". As I said, it's a matter of proper coordination and design.
Mike.