How to test a varistor?

Thread Starter

xchcui

Joined May 12, 2014
272
I think he means that you can simply connect the MOV across the probes of the megger. With the 250V range you should measure around 250V across the MOV and the current will be very low. In the 500V range it will probably current limit and the voltage across the MOV should drop to the actual avalanche voltage at that limit current. The knee is fairly sharp so you won’t see much difference in the avalanche voltage between 1mA or even 5mA. You could probably put the DMM in the line to measure current but this will either be very small if the voltage is below 430V or the current limit if the voltage is high enough. I was trying to explain a way of testing without using a DMM but as you have one then it’s easier and more accurate
When I want to test a Varistor I want to know if it still works like when it was new.
In many data sheets, they are commonly tested for 20uS at high current and at 1mS at a lower current. I have machines that do that, and you don't.
Some tests are done at 1mA. I think your part will test, 387V, 430V, 473V with the current held at 1mA. (min/typ/max)
At 430V and 1mA the part will get hot but will survive for a short time. Don't leave the device on for a minute.
Connect a volt meter across the Varistor. Set to 500V or 1kv.
Your megger makes about 2mA. So the voltages will be slightly higher. And the heat will be 2x more. Do not send power for more than one second.
Apply the megger across the varistor and apply power for just long enough to read the volta meter. (not the megger meter!)

Now you will know the 2mA voltage clamp level. Look at the graph. The 1mA voltage is almost the same as the 2mA voltage. So if your megger makes 1ma or 3mA the voltage reading will be with in 1%.

I know this megger + voltmeter test does not test all numbers but it is a good test. A number in the 380 to 475 range indicates the part is good. A very low number show a leaky and damaged part. A high number suggests an open part.

View attachment 277170
Your part is good to 0.6Watts and at 2mA you are past that. Test fast.
ronsimpson
After i looked into your attached graph and after you mentioned the power dissipation rating of the MOV(0.6Watts),i figured out that,during the test,the varistor will experience(as you mentioned)an higher-value than 0.6W,indeed.
And if i wish to test,later,smaller diameter varistors with the megger,it will be an issue,since their power dissipation rating are even lower.
As i would like to reduce the risk of damaging/degrading the new varistor,i thought to reduce the testing current to 0.5mA instead of 1mA by adding a resistor in series(maybe 140K ohm),as Jerry-Hat-Trick explained before,and then compare the voltage result to the voltage/current in the graph.Is it okay?
will there be a significant change in the voltage at 0.5mA vs 1mA?
The graph doesn't shows the max clamping voltage at 0.5mA,it only shows the voltage at the"max leakage current"part.
But,according to the tendency of the straight line(at the clamping voltage part)close to the 1mA,can i estimate that the line of the clamping voltage will be still straight(same voltage) until 0.5mA?
 

Jerry-Hat-Trick

Joined Aug 31, 2022
544
I thought to reduce the testing current to 0.5mA instead of 1mA by adding a resistor in series(maybe 140K ohm)
Yes, I think that would work. Depending on the actual avalanche voltage which can be +/- 10% of 430V your current will vary but you can maybe measure that by measuring the voltage across the resistor and then adjust the value of resistance to target 0.5mA more closely if that is what you want. Maybe just try it!
 

Thread Starter

xchcui

Joined May 12, 2014
272
Yes, I think that would work. Depending on the actual avalanche voltage which can be +/- 10% of 430V your current will vary but you can maybe measure that by measuring the voltage across the resistor and then adjust the value of resistance to target 0.5mA more closely if that is what you want. Maybe just try it!
Okay,i understand that the clamping voltage around the 1mA,
(+5mA or -0.5mA)is about the same,so i can reduce the testing current to 0.5mA,while the power(Watt)on the varistor will be in the safe level.
In regards to the resistor in series:
***According to your previous calculation,i understand that
the calculation for choosing resistor value to the varistor is the same
as the calculation for choosing resistor value to a Led,right?
(Voltage source minus Led's avalanche voltage/current target=resistor value)
***You mentioned before:
...Be sure that you use a resistor that can handle a high voltage, if not sure make up the 560K with ten 56K resistors in series.
I always thought that the wattage is what matter.
Since when i bought the resistors,i saw that they only mentioned wattage and resistance,not voltage.
I have 0.25W resistors,Do you mean that if i will connect,for example,an 10M ohm resistor to 1000VDC,while the wattage will be less than 0.25W(0.1W),the resistor may be damaged,even though that the wattage on the resistor is less than its wattage value?

Now,when i am looking into the data sheet,i see that there are three specs:
"minimum dielectric withstanding voltage"(300V),
"maximum working voltage"(250V)and
"maximum overload voltage"(500V)specs.
I assume that the maximum working voltage(250V) is the important one.
Why did you choose to make ten 56K resistors in series to get 560K ohm
(when the voltage source is 1000VDC)and not less amount of resistors in series?
If i will connect 10 resistor,the voltage drop will be less than 100V on each one.
Can't i use less resistors?
For example:to connect four 140K ohm resistors in series instead,that will drop
on each resistor 250V?(as the maximum working voltage on datasheet).
 
Last edited:

Jerry-Hat-Trick

Joined Aug 31, 2022
544
Not sure I understand your first question. The point is that if you want to target 0.5mA then if the avalanche voltage of the varistor is 430V and your megger is supplying 500V then the resistor sees 70V so for a current of 0.5mA you'd need a 140K resistor. But if the avalanche voltage is actually 450V then a 140K resistor is 0.36mA. Just Ohms law.

In addition to a power rating properly specified resistors have a maximum operating voltage specified - above which they may arc across. Seldom an issue with most electronic circuits but important when playing with high voltages. Good 1/4 watt resistors can withstand 200V (so long as their values are high enough that they aren't dissipating to much power) so with only 70V across the resistor on the 500V megger range you'll probably be fine. But if you are using the 1000V megger range you should string resistors together in series so that none of them sees more that 200V, probably 100V to be absolutely sure.

So a single 1/4W 1M resistor across 1000V will probably arc across. Maybe I'm wrong, but I wouldn't try it!
 

Thread Starter

xchcui

Joined May 12, 2014
272
Not sure I understand your first question. The point is that if you want to target 0.5mA then if the avalanche voltage of the varistor is 430V and your megger is supplying 500V then the resistor sees 70V so for a current of 0.5mA you'd need a 140K resistor. But if the avalanche voltage is actually 450V then a 140K resistor is 0.36mA. Just Ohms law.
Yes,my first question wasn't clear indeed,but your explanation actually answered it.
I admit,my first question was not clear enough,since
by mistake i wrote diode instead of led(i corrected that),
but now you clarified this issue.
In addition to a power rating properly specified resistors have a maximum operating voltage specified - above which they may arc across. Seldom an issue with most electronic circuits but important when playing with high voltages. Good 1/4 watt resistors can withstand 200V (so long as their values are high enough that they aren't dissipating to much power) so with only 70V across the resistor on the 500V megger range you'll probably be fine. But if you are using the 1000V megger range you should string resistors together in series so that none of them sees more that 200V, probably 100V to be absolutely sure.

So a single 1/4W 1M resistor across 1000V will probably arc across. Maybe I'm wrong, but I wouldn't try it!
You right about the arc across on 1000V and i also wouldn't
try it either.Besides damaging the resistor,at the megger manual,there is even a warning about avoiding arcs as they could damage the megger itself.So the reason of string resistors together in series when using 1000V is more clear to me now.
In fact,thanks for all your great explanation,warning and guidance,all the aspects of this issue are much more understandable and i have a way to test the varistor now.
Thanks again for your great help and thanks for all
the other participants in this thread. :)
 
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Thread Starter

xchcui

Joined May 12, 2014
272
Hi.

I would like to verify one more thing about the duration
of the test refer to the megger and the varistor wattage.

In regards to the varistor wattage:
...Some tests are done at 1mA. I think your part will test, 387V, 430V, 473V with the current held at 1mA. (min/typ/max)
At 430V and 1mA the part will get hot but will survive for a short time. Don't leave the device on for a minute.
Connect a volt meter across the Varistor. Set to 500V or 1kv.
Your megger makes about 2mA. So the voltages will be slightly higher. And the heat will be 2x more. Do not send power for more than one second...
...Your part is good to 0.6Watts and at 2mA you are past that. Test fast
According to this,i understand that i need to make the test fast enough(one second)if the wattage,during the test will be more than the"power dissipation rating"of the varistor,but if during the test the wattage will be less on the varistor,can i extend the test to about 3-5 seconds without degrading much the new varistor or it is too much time.(as i would like to have enough time to see the results)

And regards to the megger itself the specifications of the megger says:
that if the resistance result during the test is less than 1.5M ohm
when using 500V or less than 5M ohm when using 1000V,measurement-time must not exceed 10 seconds.
Does it mean that if i get a result of short circuit(zero resistance),
the measurement-time can be,also,maximum 10 seconds or at this situations the measurement-time must be shorter?
(the manual doesn't mention anything about this situation).
Thanks.
 

ronsimpson

Joined Oct 7, 2019
2,987
megger says:.................,measurement-time must not exceed 10 seconds.
This makes me think that there is a current limit resistor inside the Megger. (some current limit circuit) This circuit heats up in 3 or 10 seconds, under a short. (less than 1.5M or 4M = too much internal heat)
 

drjohsmith

Joined Dec 13, 2021
852
Okay,i understand that the clamping voltage around the 1mA,
(+5mA or -0.5mA)is about the same,so i can reduce the testing current to 0.5mA,while the power(Watt)on the varistor will be in the safe level.
In regards to the resistor in series:
***According to your previous calculation,i understand that
the calculation for choosing resistor value to the varistor is the same
as the calculation for choosing resistor value to a Led,right?
(Voltage source minus Led's avalanche voltage/current target=resistor value)
***You mentioned before:

I always thought that the wattage is what matter.
Since when i bought the resistors,i saw that they only mentioned wattage and resistance,not voltage.
I have 0.25W resistors,Do you mean that if i will connect,for example,an 10M ohm resistor to 1000VDC,while the wattage will be less than 0.25W(0.1W),the resistor may be damaged,even though that the wattage on the resistor is less than its wattage value?

Now,when i am looking into the data sheet,i see that there are three specs:
"minimum dielectric withstanding voltage"(300V),
"maximum working voltage"(250V)and
"maximum overload voltage"(500V)specs.
I assume that the maximum working voltage(250V) is the important one.
Why did you choose to make ten 56K resistors in series to get 560K ohm
(when the voltage source is 1000VDC)and not less amount of resistors in series?
If i will connect 10 resistor,the voltage drop will be less than 100V on each one.
Can't i use less resistors?
For example:to connect four 140K ohm resistors in series instead,that will drop
on each resistor 250V?(as the maximum working voltage on datasheet).
Ive only had a quick scan of the posts, so sorry if its already covered

Do keep in mind, these protection devices "degrade" with use ,
Dont use one that you have "tested" in a bit of equipment,
as it wont meet specs.
 

Thread Starter

xchcui

Joined May 12, 2014
272
This makes me think that there is a current limit resistor inside the Megger. (some current limit circuit) This circuit heats up in 3 or 10 seconds, under a short. (less than 1.5M or 4M = too much internal heat)
Yes,it looks like it is related to the internal heat and the manual is warning not to exceed 10 seconds in order not to damage the megger circuit due to over heating.
But the manual is warning not to exceed 10 seconds if the resistance at 500V/1000V is less than 1.5KΩ/5KΩ,
so my question was: Does the measurement-time can be,also,maximum 10 seconds at short(zeroΩ) situation
or at this situations the measurement-time must be shorter
than 10 seconds?

Ive only had a quick scan of the posts, so sorry if its already covered
Do keep in mind, these protection devices "degrade" with use ,
Dont use one that you have "tested" in a bit of equipment,
as it wont meet specs.
But this is all the purpose of the testing.
I need to test the new varistor before i will use it.
And as i understood from other explanation,at 1mA test current
the degrading will be negligible.won't it?
I was also talking about make the test at 0.5mA
to reduce the possiblity of degradation and that the wattage
on the varistor will be less than its rated wattage.
This the reason why i am asking the following question
from post #26:
According to this,i understand that i need to make the test fast enough(one second)if the wattage,during the test will be more than the"power dissipation rating"of the varistor(0.6W),but if during the test the wattage will be less on the varistor,can i extend the test to about 3-5 seconds without degrading much the new varistor or it is too much time.(as i would like to have enough time to see the results)
 

drjohsmith

Joined Dec 13, 2021
852
Yes,it looks like it is related to the internal heat and the manual is warning not to exceed 10 seconds in order not to damage the megger circuit due to over heating.
But the manual is warning not to exceed 10 seconds if the resistance at 500V/1000V is less than 1.5KΩ/5KΩ,
so my question was: Does the measurement-time can be,also,maximum 10 seconds at short(zeroΩ) situation
or at this situations the measurement-time must be shorter
than 10 seconds?


But this is all the purpose of the testing.
I need to test the new varistor before i will use it.
And as i understood from other explanation,at 1mA test current
the degrading will be negligible.won't it?
I was also talking about make the test at 0.5mA
to reduce the possiblity of degradation and that the wattage
on the varistor will be less than its rated wattage.
This the reason why i am asking the following question
from post #26:
The talk Started to cover MOVs
Which in particular do degrade with uage.
Varister conduction is a non linear effect they might only change a little with a small current , but you will have to determin if this is ok.
For instance ,if a unit ends up being checked multiple times.
We only fit the MOVs etc at the end of the line in production
 

Jerry-Hat-Trick

Joined Aug 31, 2022
544
If you have money burning a hole in your pocket, maybe buy https://webstore.iec.ch/publication/63834

IEC 61643-331:2020
Components for low-voltage surge protection - Part 331: Performance requirements and test methods for metal oxide varistors (MOV)

In the year 2001 I was working for a large PLC in the UK who were divesting non-core businesses. I was put in charge of a company in Ivybridge in Devon which made varistors. I arranged for the sale of the business to the management. I think they sold out in around 2016 and the company is now https://www.equipment10.com/GB/South-Brent/382617535185041/PD-Devices-Ltd#gsc.tab=0

Maybe you could give them a call? My experience with smaller companies is that you can often find helpful people if you explain what you want and ask nicely.

I regret, I don't recall exactly how they tested the, but I do remember that MOVs deteriorate with the energy they absorb and obviously they were 100% tested! I'm pretty sure you'll be okay at 0.5mA for a few seconds, but maybe you could test one multiple times, or at 0.5mA over a period of time to see if you notice degradation?

May I ask what you need MOVs for?
 
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Thread Starter

xchcui

Joined May 12, 2014
272
Jerry-Hat-Trick

The talk Started to cover MOVs
Which in particular do degrade with uage.
Varister conduction is a non linear effect they might only change a little with a small current , but you will have to determin if this is ok.
For instance ,if a unit ends up being checked multiple times.
We only fit the MOVs etc at the end of the line in production
I understood,as i mention before,that the varistors degrade each time a current pass through them.This is the reason why i was asking 3 time about the measurement-time duration,the current and the wattage but i didn't get an answer for it yet?
you said,that i need to determine if the varistor is affected by my test,but all the idea is to get an answer here in order to avoid another tests which will degrade the varistor more.
I don't want to spend the varistors on experiments,since i am going to buy only 10 pcs and i just need (in regards to jerry-hat-trick question),several varistors to replace/backup the ones that burned on some home appliances and some important electronic boards.
If you have money burning a hole in your pocket, maybe buy https://webstore.iec.ch/publication/63834

IEC 61643-331:2020
Components for low-voltage surge protection - Part 331: Performance requirements and test methods for metal oxide varistors (MOV)

In the year 2001 I was working for a large PLC in the UK who were divesting non-core businesses. I was put in charge of a company in Ivybridge in Devon which made varistors. I arranged for the sale of the business to the management. I think they sold out in around 2016 and the company is now https://www.equipment10.com/GB/South-Brent/382617535185041/PD-Devices-Ltd#gsc.tab=0

Maybe you could give them a call? My experience with smaller companies is that you can often find helpful people if you explain what you want and ask nicely.

I regret, I don't recall exactly how they tested the, but I do remember that MOVs deteriorate with the energy they absorb and obviously they were 100% tested! I'm pretty sure you'll be okay at 0.5mA for a few seconds, but maybe you could test one multiple times, or at 0.5mA over a period of time to see if you notice degradation?

May I ask what you need MOVs for?
As i mention before,i just need several varistors to replace/backup the ones that burned on some home appliances and some important electronic circuit boards.
Thanks for your suggestion,but i don't have money burning a hole in my pocket and i just needed to know a good quick test
without degrade much the new varistor,thats all.
So i understand that the 0.5mA for a few seconds will
be okay.
What about my question about the megger?
I have not gotten any answer for this yet:
And regards to the megger itself the specifications of the megger says:
that if the resistance result during the test is less than 1.5M ohm
when using 500V or less than 5M ohm when using 1000V,measurement-time must not exceed 10 seconds.
Does it mean that if i get a result of short circuit(zero resistance),
the measurement-time can be,also,maximum 10 seconds or at this situations the measurement-time must be shorter?
(the manual doesn't mention anything about this situation).
Thanks.
ronsimpson said that it is related to the internal heat in the megger circuit(current limit circuit)and he apparently right.
This makes me think that there is a current limit resistor inside the Megger. (some current limit circuit) This circuit heats up in 3 or 10 seconds, under a short. (less than 1.5M or 4M = too much internal heat)
but what i would like to understand if the 10 seconds measurement-time limit is valid also to a 0Ω short?
or rather at 0Ω the measurement-time should be less than 10 seconds.
ronsimpson defined the short as less than 1.5M/5M,which then
the circuit heats up in 3-10 seconds,i refer to the different
between"short under 1.5M/5M(500V/1000V)"and a zeroΩ
short?
 

drjohsmith

Joined Dec 13, 2021
852
Jerry-Hat-Trick


I understood,as i mention before,that the varistors degrade each time a current pass through them.This is the reason why i was asking 3 time about the measurement-time duration,the current and the wattage but i didn't get an answer for it yet?
you said,that i need to determine if the varistor is affected by my test,but all the idea is to get an answer here in order to avoid another tests which will degrade the varistor more.
I don't want to spend the varistors on experiments,since i am going to buy only 10 pcs and i just need (in regards to jerry-hat-trick question),several varistors to replace/backup the ones that burned on some home appliances and some important electronic boards.

As i mention before,i just need several varistors to replace/backup the ones that burned on some home appliances and some important electronic circuit boards.
Thanks for your suggestion,but i don't have money burning a hole in my pocket and i just needed to know a good quick test
without degrade much the new varistor,thats all.
So i understand that the 0.5mA for a few seconds will
be okay.
What about my question about the megger?
I have not gotten any answer for this yet:

ronsimpson said that it is related to the internal heat in the megger circuit(current limit circuit)and he apparently right.

but what i would like to understand if the 10 seconds measurement-time limit is valid also to a 0Ω short?
or rather at 0Ω the measurement-time should be less than 10 seconds.
ronsimpson defined the short as less than 1.5M/5M,which then
the circuit heats up in 3-10 seconds,i refer to the different
between"short under 1.5M/5M(500V/1000V)"and a zeroΩ
short?
You have hit the nail on head
If you test a MOV, it will be degraded.

Only you can tell if your test degrade it to much , for you.

It a hard engineering decision . I've mentioned how we cover it. To recap , we test a pre production environment , and convince ourselves the design meets requirements.
Once we're in production , we do not test MOVs in shipped equipment .
You decision might be different.
We work on legal advise on lines of if we degrade in any way , and a unit fails, we will need to prove that we did not damage the MOVs , and we know we can't say that .
Real world engineering is a hard set of compromises.
 
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Thread Starter

xchcui

Joined May 12, 2014
272
Okay.We can summarize that any test will actually degrade
the varistor,though this fact was known to me,i just tried to figured out the best test with minimum degradation.
I think we have done with that issue.
But i see that still,no one,besides ronsimpson,is referred to my
question about the megger.Which was:
The manual is warning not to exceed 10 seconds if the resistance at 500V/1000V is less than 1.5KΩ/5KΩ,
so my question was: Does the measurement-time can be,also, 10 seconds at short(zeroΩ) situation
or at this situations the measurement-time must be shorter
than 10 seconds?
I would like to understand if the 10 seconds measurement-time limit is valid also to a 0Ω short?
or rather at 0Ω the measurement-time should be less than 10 seconds.
ronsimpson defined the short as less than 1.5M/5M,which then
the circuit heats up in 3-10 seconds,i refer to the different
between"short under 1.5M/5M(500V/1000V)"and a zeroΩ
short?
 

drjohsmith

Joined Dec 13, 2021
852
Okay.We can summarize that any test will actually degrade
the varistor,though this fact was known to me,i just tried to figured out the best test with minimum degradation.
I think we have done with that issue.
But i see that still,no one,besides ronsimpson,is referred to my
question about the megger.Which was:
If I read that correct , your asking the community if you use an expensive mega outside its defined limits , is it ok ?
May be the company that makes your mega can answer that ?
 

Thread Starter

xchcui

Joined May 12, 2014
272
If I read that correct , your asking the community if you use an expensive mega outside its defined limits , is it ok ?
May be the company that makes your mega can answer that ?
No,i really don't understand what is not clear in my question.
Manual says:don't exceed 10 seconds measurement-time
(don't push the ON button on the megger)if the measured resistance will be less than 1.5MΩ/5MΩ(500V/1000V).
Apparently,it is due to internal heat.
Now,less than 1.5MΩ can be 1MΩ,can be 500KΩ,can be
100Ω and can be 0Ω.
From this warning i can understand that i can push the ON button for 10 seconds if the measured resistance will be 0Ω.
But my logical tell me that at 0Ω the internal heat might be bigger than when the resistance will be 1MΩ,for example.
So maybe i need to push the ON button for less
than 10 seconds when it is 0Ω,don't i?
If i could ask the company that made my megger,this was the first thing that i were do,but i can't do that(nevermind why).
Honestly, I really think it’s time to try it. We’ve done what we can to explain the theory. Do let us know your results
Yes,all your explanation were very helpful to understand the
theory and i thank you very much for this.
The problem is that i had 1 important thing to verify about the megger before i start the testing,but i don't get exactly the answer for that.
I want to do things right,i don't want to ruin things,even
that my megger is not expensive one,it is not smart to do things
like that,if there is any doubt.
From your knowledge about electronic circuits,of those meggers(emphasis the not expensive one),
can you answer this question,as i can't make a contact with the megger's company.
 
The problem is that i had 1 important thing to verify about the megger before i start the testing,but i don't get exactly the answer for that.
I want to do things right,i don't want to ruin things,even
that my megger is not expensive one,it is not smart to do things
like that,if there is any doubt.
You are considering using a Megger for something it was not designed to do, so I am reluctant to say for certain that there is no risk at all as I don't know the internal schematic of the instrument.

My final suggestion is that if you connect the varistor in series with a 1.5M resistor (made up with say five 300k resistors in series) and use the megger to find the resistance on the 500V range I believe it would think the overall resistance is around 11M Ohm and on the 1000V range it would read something like 2.7M Ohm. If you put a DMM across the varistor it should read around 430V +/- 10% and it should read higher on the 1000V range because the current is higher. The current will be very small but it will probably give you an indication of the health of the varistor.

If you are so concerned about your megger you could go back to the solution using a 380VAC transformer, full wave rectified, large capacitor and a resistor in series with the MOV to limit the current to 0.5mA (assuming the varistor is functional) and connect a DMM across the varistor. There should be about 100V across the resistor so it'd need to be about 200K Ohm - maybe 3 x 68K Ohm in series. Worst case is that the MOV is shorted so the 530V is just across the resistor so the current will be around 2.7mA so the resistors should be 2W to be safe.

But if you don't fully understand this and the dangers of playing with high voltage DC then I urge you to find someone to help you do this.
 

Thread Starter

xchcui

Joined May 12, 2014
272
You are considering using a Megger for something it was not designed to do, so I am reluctant to say for certain that there is no risk at all as I don't know the internal schematic of the instrument.
No,i never meant to ask,if there is a risk or can i do that,since
i know that you can't guarantee for that.
I meant to ask only your opinions,from your experience in electronic:
How can the manual warning,as i mentioned before,be interpreted in your opinions?That's all.
But okay,I leave it aside.

My final suggestion is that if you connect the varistor in series with a 1.5M resistor (made up with say five 300k resistors in series) and use the megger to find the resistance on the 500V range I believe it would think the overall resistance is around 11M Ohm and on the 1000V range it would read something like 2.7M Ohm. If you put a DMM across the varistor it should read around 430V +/- 10% and it should read higher on the 1000V range because the current is higher. The current will be very small but it will probably give you an indication of the health of the varistor.
As for the megger,those resistance seems to be a safe values,but
it made me confuse:confused:If the total resistance is 11MΩ,it means that the current
through the varistor is about 46uA.(500V/11MΩ).
At this current the voltage on the varistor(according to some varistor's V-I Characteristic graphs that i checked on several manuals)should be 300V-350V.
So,why should i read 430V +/-10%,while at this
voltage,the current should be around 1mA?:confused:
 
If the total resistance is 11MΩ,it means that the current
through the varistor is about 46uA.(500V/11MΩ).
At this current the voltage on the varistor(according to some varistor's V-I Characteristic graphs that i checked on several manuals)should be 300V-350V.
So,why should i read 430V +/-10%,while at this
voltage,the current should be around 1mA?
Ok, well done for doing the detailed research, I just simplified by assuming the avalanche voltage was fairly sharp at 430V. Perhaps you could do the maths based on voltage at different currents to target a better resistance value.
 
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