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
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.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
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?