Originally Posted by 11001011
Maybe I was not being clear, here is how a MOV works.
A MOV has a crystalline structure. As the voltage rises the crystals begin to align and it's resistance begins to drop. When the avalanche point is reached the majority of crystals are aligned and it's resistance is near 0. As the voltage drops the crystals begin to go back to random and it's resistance rises to near infinite.
As a MOV receives more and more surges more and more of the crystals do not return to the random state, they stay aligned. This causes it's avalanche point to drop.
A MOV fails either by a surge larger than it can pass or by the avalanche point becoming so low that normal operating voltage over heats the MOV.
A MOV fails in three stages...
The MOV will first become a short circuit.
Then the MOV will explode or just burn up due to thermal runaway.
Then the MOV will be an open circuit.
MOV's don't just quit working.
Trust me on this one, I have personally conducted surge testing on MOV's and various electronic devices at work using the surge generator pictured here.
On the left is a surge generator capable of applying surge pulses from 200V to 6.6kV at 1.2/50us (open circuit) and 8/20us (short circuit), as well as at 0.5us/100kHz damped oscillation, directly onto the power line to easily test for IEC, EN, Telcordia, or any custom profile.
On the right Electrical Fast Transient gererator (EFT) Burst test can test to 4.4kV, meeting and exceeding the IEC limits. This is for applying high frequency burst noise directly onto power lines or when used in conjunction with a "trench" EFT can be inductively coupled to power cords and interconnects.