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# Varistor test

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#### xchcui

##### Member level 2

Hi.
The above circuit contains:megger,mov,resistor and DC
voltmeter and it for checking the clamping voltage of the varistor at 1mA.
My question is not about the purpose of this circuit or if it give an accurate
results,but to the phenomenons that i got,while pressing the 500VDC on the megger.
My questions:
1)When i pressed the megger(500vdc),the voltmeter showed 0V
and rised-up gradually,but after 10 seconds it didn't reach yet to the expected voltage(391V),so
i had to stop the testing(i am allowed to turn-on the megger up to 10 seconds).
I assume that after 10 seconds,it was reach the target.
Why did the voltmeter start from 0v and rise up grdually,while it takes so much time(more than 10 seconds)to reach the target voltage?I thought,at first,that it is related to the capacitance of the varistor,but the varistor has very small capacitance.

2)At the same time,(when the megger is pressed 500vdc),the total resistance of the circuit(read on the megger)gradually increase while the current is decreased.
Is it related to the battery of the megger,that lost power or to the"capacitance charging current effect"during the test?
The strange thing is that the megger shows low starting resistance and then gradually increase,
while the voltage(on the voltmeter)at the same moments gradually increased.
I was expecting that that the voltage on the varistor will start high(391v),since the varistor resistance is low(relativity)and when the resistance of the varistor starts to increase the voltage should decrease.But it showed the opposite.
May you explain those phenomenons?
Thanks.

Hi,

What happens without the MOV?
What value is the resistor?
Is the wiring, the resistor and the voltmeter designed for 500V DC operating voltage?

Klaus

Hi,

What happens without the MOV?
What value is the resistor?
Is the wiring, the resistor and the voltmeter designed for 500V DC operating voltage?

Klaus
I don't know what happens without the MOV,since the purpose of the
circuit is to test the MOV.
The value of the resistor is changing,depend on how much i want to drop the
voltage on the resistor in order to get the correct voltage on the MOV,but let
say 110K ohm.
The wiring and the resistor designed for 500v dc.
The voltmeter can test up to 1000VDC.

I think that the problem is related to my DMM.
My DMM is suitable to measure direct voltage up to 1000VDC,
but apparently,it is not suitable more this task.

Thanks for your response.

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