Short reply - No, use a bigger (larger diameter) varistor.
Detailed reply - Varistors are voltage dependant devices that have a VI
characteristic that means the voltage across them remains constant(ish) for
a wide range of currents. Varistors also have manufacturing tolerance. If
you put a two varistors in parallel, and one is at the top of its tolerance
range and the other is at the bottom, virtually all the current will go in
to one of them, and hardly any onto the other - hence the short reply of no.
This is the same problem that occurs if you parallel diodes etc.
If you fully understand the tolerances and VI curve charicteristics, and are
using varisors from the same batch, and have statistical infomaton on
manufacturing tolerance, you can parallel varistors, BUT you will not be
able to simply double the current rating, you will have to apply a de-rating
factor to allow for the matching of currents between the varistors
Joules - Note that I have carefully avoided mentioning joules above. It is
much better to work in volts, amps and time when dealing with varistors. You
can then derive joules if you need to, but never loose sight of the raw
electrical information of volts, amps and time. Joules confuse the issue!
Joules are more usefull for dieting for the metrically inclined! Let me
explain: if you have two varistors, and want to double the joule rating,
just put them in series. Your joule rating will be just about doubled, no
problem! The joule rating hides the fact that this not a very clever thing
Most manufacturers of surge suppressed socket outlets (power strips in the
USA) ignore this fact and just add up the MOV rating in Joules when quoting
the performance of the units. Therefore the figures they quote have no
relevance in the performance rating of their units, but it looks good in the
Ok, thanks for the info, but im a little confused here.
I thought MOV's conducted at a certain voltage, example if the voltage gets
to 130 or above, and the MOV is rated at 130 volts, the MOV conducts, or
If you connected two in series, would this not double the votage at which
they pass current?
Absolutely! Connecting two varistors in series will double the voltage
across them (not actually what you want) and hence double the joule rating
(sort of what you asked for!).
I was trying to encourage you to not use joules, as they do not help you.
Just work with volts, amps and time and you won't get caught out.
What prompted your original question? What current rating are you trying to
pass and for what time?
Well, I am making a "surge protector" out of a standard 15 amp 120 VAC
duplex receptacle with a MOV connected across the hot and neutral terminals.
Both mounted in a handy box.
I got the MOV at radio shack, all they had was a 700 joules one, so I was
thinking I could get more protection by adding another one in paralell.
700 joules - or 70 joules? Big difference. Furthermore, fire
safety issues should be addressed - as defined by UL1449 2nd Edition.
MOV inside a fire resistant box. But then which type of surge are you
trying to protect from? You are installing for normal mode transients.
Meanwhile destructive transients are longitudinal mode. IOW a surge
seeking earth ground now has two wires (not just one) to find earth
destructively via adjacent electronics because of that MOV.
Effective shunt mode protectors divert a surge to earth. You are not
doing that - as well as making what could be a potential fire hazard.
Maybe look at an MOV from Littelfuse that has internal protection. Or
put that box in a location where a small flame will not be dangerous.
Ever see an MOV burn or vaporize? Do it to learn - but in a location
where human life would be threatened.
Meanwhile some MOV manufacturers provide ball park numbers for
estimating joules ratings for MOVs in parallel. Since MOVs, working
properly, do not vaporize or blow out, then increasing joules by
paralleling them is rather easy. But two MOVs do not exactly achieve a
double joules rating. Another post that discusses voltage, amperage,
and time rather than joules is accurate. A 70 joule MOV does not mean
it is stopping or absorbing up to 70 joules as others participating in
this thread have falsely assumed in the past. Do you calculations in
terms of voltages, amps, etc to eventually appreciate its actual joules
Again, I suspect you do not have a 700 joule MOV.
Let me support W_thom's post. Making a DIY surge protector is a scary thing
to do! You need to ask yourself "what happens when the varistor comes to the
end of its life?, what disconnects the varistor from the circuit ?". If you
don't know the answer, it could be that your house burns down as a result!
This is what UL 1449 2nd issue addresses. Using 130V varistors on a 120V
supply, though common practice, is also a cause of many failures as there is
not a lot of headroom for supply voltage variation. If your supply is over
8% high, then you could be heading for a nasty thermal runaway failure.
Transients can occur between any pair of conductors. So you need to protect
between line and neutral, neutral and ground, line and ground. If you are on
the type of supply where neutral and ground are bonded together in your
building, and are very close to the bonding location, then you can obviously
In your context, adding a second varistor in parallel will at worst do next
to nothing, and at best, if you are really really lucky, it will double the
Above is one type of transient - also called normal mode. This is
the type of transient that plug-in protectors are designed to protect
from. However the other and typically destructive type of transient is
not between two wires. Longitudinal mode transients come down any one
or all wire seeking a complete path to earth ground. Plug-in
protectors do not claim to protect from this type since the transient
is not a voltage between two wires. The transient is a voltage
difference between any one wire and earth.
So that confusion remains, plug-in protectors don't discuss the
various transient modes. Properly earthed 'whole house' protectors
make both types of transient irrelevant. Longitudinal mode transients
- an electrical path from cloud to earth, then miles through earth to
earthborne charges - is why single point earth ground is the most
critical component in a protection system.
It's not the MOV that is protection. That MOV is equivalent to a
switch. The effective MOV makes a temporary connection from every
incoming wire to earth. Therefore those MOVs shunt the typically
destructive type of surge.
Normal mode transients - between wires - are not the typically
destructive transient. Protection inside appliances makes normal mode
transients irrelevant. But the longitudinal mode transient can
overwhelm that internal protection. Longitudinal mode transients are
best earthed where they would enter the building with a short (low
impedance) connection to a single point earth ground. A longitudinal
mode transient can come down one wire or all wires. The effective
protector shunts longitudinal mode transients to earth - best many
meters away from transistors. That many meter separation then
contributes to appliance protection.
I'm in agreement with your reply, but I'm looking at
a different aspect than you: two separately housed
MOV's plugged into the same circuit. The positioning
of my reply is for comparison, not disagreement.
Short reply - Yes, but a little different than discussed.
Individual point of use surge protectors, each with their
own MOV's, plugged into the same circuit, increase the
joule rating. If the first one to conduct blows open,
the second one conducts, so the total joule rating is
additive. If the first one to conduct blows shorted, the
second one is irrelevant, until and unless the short
blows open. (The typical failure mode is shorted. However,
if the 120 Vac supply or the surge energy is still present,
they open.) Since they are in separate housings, the heat
dissipated in one does not affect the other. But within a
single housing, use a bigger varistor, just as you say.
And even if they work in that configuration, it's not too
good - the clamping voltage will be doubled. And when
the first one opens, the second one is useless.
.> I'm in agreement with your reply, but I'm looking at
You make an interesting point. Conventional practice/wisdom says that a
single shot current rating corresponds to a current that a device can endure
with no more than a defined amount of degradation, which is usually quite
low. Hence if I buy a device that is rated at 10kA, and I test it with10kA,
I don't expect to see bits fly off, acrid black smoke, or flames!
You could indeed have a new definition: Single shot current rating WITH
PERMITTED PARTIAL CATASTROPHIC FAILURE (the capitals are for emphasis, not
shouting!) and what you say then might work and make sense. Users would need
to be fully aware of the difference. Marketing men would need to avoid the
temptation to mislead the unwary.
You stated "Since they are in separate housings, the heat dissipated in one
does not affect the other." During the transient event, any heat dissipated
is actually absorbed (adiabatic behaviour), ie the heat raises the
temperature of the varistor itself, and does not have time to be conducted,
convected or radiated anywhere else. In this case it makes no difference if
the varistors are separate or not. On the other hand, a varistor suffering
from thermal runaway may well fail in a much slower manner if suitable
disconnection is not provided. Your separate boxes may indeed isolate the
mess caused by the failing device from a healthy device
Sorry, I'm missing the point in the above paragraphs.
I'm thinking of two physically separated in different
enclosures devices. Each would be tested independently,
if testing were done. It is my understanding that
MOVs that are tested are then discarded.
What happens in a real world surge is problematic
A C E
O | | |
U | | |
R MOV1 MOV2 Equipment
C | | |
B D F
Assume a source surge V of 10,000.
Assume MOV1 conducts at 330 and clamps to 130
Assume MOV2 conducts at 332 and clamps to 130
A-B will go to 130, keeping MOV2 from operating
and holding C-D and E-F to 130, until the surge
stops or MOV1 opens. If MOV1 opens, C-D rises to
332 and MOV2 clamps to 130.
New scenario, except this time MOV2 conducts at 330
and MOV1 conducts at 332. Now we need to know the
voltage drop A-C. MOV2 will draw a bucket of
current through that wire, and there will be some
inductance as well, so it is quite possible that
MOV1 will see in excess of 332 while MOV2 is clamping
C-D at 130. So - MOV2 absorbs part of the surge, and
causes MOV1 to conduct. MOV1 now clamps to 130, turning
In either case, a surge in excess of rating that causes
MOV destruction will cause both MOV's to participate.
In the second case, a non-MOV-destructive surge *may*
cause both MOV's to participate.
But, bottom line, in either case, the total MOV joule
rating is increased by parallel MOV's. What is not
increased is the amperage, except for the brief time in
the second case where both MOV's conduct at the same time.
The parallel MOV's extend the duration of protection,
but would seem to have little effect on the total current
they can conduct.
That goes back to something you mentioned, advising to look
at the amperage rating.
Right - during the non-destructive to the MOV transient
period, the heat in one doesn't affect the other. I'm
referring to an event that raises the temperature in the
MOV to the point of destruction. The flames (sometimes)
in MOV 1 will affect MOV 2 ! I don't know if the kinetic
energy of an exploding MOV (speculated) would adversely
affect a second MOV in the same housing. I have no data
on "exploding MOV's" - don't know if it happens, nor even
what the definition of "exploding" would be in that case.
What usually happens with MOVs mounted on a PCB, is that the track burns off
the board before the MOV blows, i.e. the track acts as a fuse during high
When an MOV explodes it normally destroys anything in the area around it,
wiring, LEDs and other devices etc.
All MOVs must have protection fitted to prevent this happening.
Aside from the point you are trying to make, I think you may have
how varistors work and muddled them up another suppression technology:
Varistors are voltage limiting devices with a continuous VI characteristic
where V progressively increases with I, but in a non-linear manner. A
130V varistor NEVER clamps at 130V.
This is in contrast to voltage switching devices such as gas discharge
tubes, spark gaps and thyristor based devices that switch to a lower voltage
once a threshold voltage is reached, which is the behaviour you describe.
The VI characteristic (mid-tolerance) of a 130VAC rated varistor is very
220V @ 10mA
240V @ 100mA
250V @ 1A
And about 10 micro amps at 130V!
Perhaps my point make more sense now.
In addition, are you under the misapprehension that varistors are single
like fuses? Well the aren't, unless abused. In general, you should select a
varistor so it will give
you a reasonable lifetime, and so failure or overstressing is unlikely.
Coming back to your way of looking at things, if I buy a car a twin turbo
car, and the manufacturer tells me you can drive it at 200 mph for no more
than 10 minutes, I would expect it to fully work afterwards, but wouldn't be
surprised if it had suffered some very minor effects as a results. Perhaps
there might me some minor increse in emmisions etc. You would be
happy to have one of the turbos fail in the same circumstances, providing
200 mph was achieved for 10 minutes. I hope this
illistrates the point I am making.
On a technical level, your point is entirely dependant on the failure mode
of the varistors, which is indeterminate. Never the less, it is an
interesting way to look at things.
Finally, it may amuse you to know that an exploding varistor (if safely
is actually a comparatively benign form of failure compared to a slow
Well, thanks for everyone's help and opinions.
I feel any advice done in hosnesty is helpful.
Let me address the issue of burning down my house first:
The MOV will be enclosed in a metal "handy box", with a duplex receptacle,
the whole thing will be covered with a metal duplex cover, factory made to
fit the duplex receptacle and box. The wire entry into the handy box will be
with a metal cable clamp (romex clamp is what Ive always called them). This
wire will then go to a 10 amp button type breaker rated at 600 volts.
(breaker is on the hot wire).
Im not worried about catching anything on fire. Fire is always my first and
most important concern.
As fas as attracting lightning, I dont see how this could be, since the
electronic equipment will be plugged into the duplex receptacle, how could
adding a MOV "attract" more lightning than the electronic equipment alone?
(I could be wrong, not trying to start an argument, just trying to get
I could see where attaching two more MOV's from hot and neutral to ground
could be benificial.
But you still have the ground wire going back to the panel (house fuse box),
looks like any current going throug the MOV's will take this path or the
neutral, since they both go back to ground.
As fas as the regulations, I took apart a power strip, all it has is a MOV
between hot and neutral buss bars, and a reset button breaker on the hot
wire. Plus the case of the power strip is plastic, may be fire resistant, I
dont know, but I do know the metal handy box is.
Now, to address the 700 joules rating: I messed up, it is 70 joules, not
700, those typos and decimal points will get you everytime! LOL
Someone mentioned testing the MOV, how is that done? Except by visually
One MOV test is to feed a 1 milliamp current source and measure its
voltage. Obviously voltage will rise as necessary to conduct only 1
mA. MOVs don't fail by vaporizing - an unacceptable failure mode.
MOVs fail by degrading. From an MOV manufacturer's datasheet:
This manufacturer defines a functional MOV as "delta Vb / Vb <10%".
That power strip breaker is required because a power strip must not
serve a load of more than 15 amps. IOW that breaker (or fuse) is for
other human safety issues. Typically an MOV is thermally fused
separately so that too much heat or too much current (much less than 15
amps) will disconnect the MOV.
To appreciate what an MOV does to be effective, learn a difference
between normal mode and longitudinal mode transients. Longitudinal
mode might not be seen by an MOV across two AC wire while transient
passes through and damages an adjacent appliance; with a very severe
MOVs don't attract lightning. But, for example, a longitudinal mode
transient only on a black (hot) wire can be shunted to green wire by an
MOV. Now a transient that would normally have been made irrelevant by
a computer's power supply (appliance internal protection), instead, is
directly connected to computer motherboard; and seeking earth ground.
This failure mode has been demonstrated even across an entire network
of computers. The adjacent protector shunted a direct lightning strike
to utility wires into and damaging three powered off computers that
were in that path to earth.
How do MOVs work in parallel? MOVs don't conduct at a fixed
voltage like zener diodes. For example an 80 joule MOV rated at 150
volts AC will conduct a microamp at 108 volts, a milliamp at 210 volts,
1 amp at 310 volts, and 1000 amps at 500 volts. One MOV wired in
parallel and having a slightly lower voltage rating will not take the
full brunt of a transient.
Meanwhile this 80 joule MOV is rated for a 'Withstand current' of 5,500
It was a hypothetical, which should be obvious.
I was trying to force fit something as gently as I could
to point out that your statement could use a little work.
It is clear that my hypothetical only confused things.
I'll be blunt so the point won't be missed this time.
You said: "Varistors also have manufacturing tolerance.
If you put a two varistors in parallel, and one is at the
top of its tolerance range and the other is at the bottom,
virtually all the current will go in to one of them, and
hardly any onto the other - hence the short reply of no."
My stupid example was intended to illustrate that
even if one varistor operated as you say, with virtually
all of the current going through it, the overall rating
is in fact increased. The rating would be essentially
doubled, give or take innaccuracy in the rating.
Additionally, unless there is a large manufacturing
variation, both MOV's will conduct appreciable current
in a surge event that causes one of them to conduct heavily.
Right. I did not specify a 130 v varistor. I
provided a bad hypothetical to try to illustrate
a point. You are right to challenge it - it stunk.
Not at all. Nothing you posted here addresses it.
No. Are you? Some surges events are
destructive to MOV's, some are not.
Well the aren't, unless abused. In general, you should select a
Well, I don't know what your example has to do with
my way of looking at things. My way of looking
at MOV's in the real world is that some surge events
are destructive and some are not. Some surge events
may damage an MOV, without destroying it. Such events
degrade and therefore subtract from the capacity of
the MOV. Some surge events don't damage the MOV.
Testing (as opposed to real world) is assumed to
degrade the MOV, while a real world surge may not.
That just dodges the issue. Putting MOV's in series is
useless as a means to double the joule rating, as you
pointed out. Putting them in parallel is effective
at doubling the rating. (Increase would be a better
term than double, because there may be some variation
from rating in the actual devices.) It is also increases
the current that can be drawn during a surge event,
because it will split between the two MOV's, but
in the case of current, the word increase does not
indicate essentially double the amount. The differences
between the MOV's would determine the proportion
of total current each MOV would draw.
I *think* we agree that joule rating may be misleading to
Can you spell it out? Seems to me that if we apply
the same limitation - "safely contained" - to both
cases, they would be equally benign. So I'm not sure
what you have in mind.
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