Metallic oxide varistors (MOV's) behave much like a capacitor except the dielectric insulator between the 2 tin plates has a fixed 'soft' clamp voltage, above which the MOV begins to conduct current in either direction. As the voltage rises so does the current flow.
At twice the MOV's rated voltage they can absorb (briefly) several 10's of thousands of amps. That is why they are so popular in surge suppressors for AC or DC power feeds. Because of their high capacitance they are not used to protect data feeds. Tranzorbs and Sidacs and gas tubes are better for those applications.
An MOV's capacitance is not affected by voltage (the same for Tranzorbs, Sidacs and gas tubes) changes until the voltage exceeds the clamp voltage of the MOV. Often the maximum safe AC/DC voltage allowed is printed on the MOV. Its size and the datasheets offer details about the maximum surge current it can handle one time, and what it can handle with 5,000 or so 'small' surges, so its lifespan can be predicted in real-world conditions.
Because the MOV is basically two metal plates spaced by a dielectric, it acts much like a capacitor in the nF range. The larger the size the more capacitance, but it is not enough to affect AC or DC power feeds, as they have a low drive impedance so MOV's are 'ignored' until a surge event happens. For that reason MOV's must be fused or have a circuit breaker in series with them in case the surge is so intense the MOV fails (shorts out).
For more details and graphs see the following link:https://en.wikipedia.org/wiki/Varistor
This is a few paragraphs from the Wiki link that summarize some important details.
Composition and operation
Varistor current-voltage characteristics for zinc oxide (ZnO) and silicon carbide (SiC) devices:
The most common type of
varistor is the metal-oxide varistor (MOV). This type contains a
ceramic mass of zinc oxide grains, in a matrix of other metal oxides
(such as small amounts of bismuth, cobalt, manganese) sandwiched
between two metal plates (the electrodes). The boundary between each
grain and its neighbour forms a diode junction, which allows current
to flow in only one direction. The mass of randomly oriented grains is
electrically equivalent to a network of back-to-back diode pairs, each
pair in parallel with many other pairs.
When a small or
moderate voltage is applied across the electrodes, only a tiny current
flows, caused by reverse leakage through the diode junctions. When a
large voltage is applied, the diode junction breaks down due to a
combination of thermionic emission and electron tunneling, and a large
current flows. The result of this behaviour is a highly nonlinear
current-voltage characteristic, in which the MOV has a high resistance
at low voltages and a low resistance at high voltages.
Electrical characteristics:
A varistor remains non-conductive as a shunt-mode device during normal operation when the voltage across it remains well below its
"clamping voltage", thus varistors are typically used for suppressing
line voltage surges. Varistors will almost always eventually fail for
either of two reasons.
A catastrophic failure occurs from not successfully limiting a very
large surge from an event like a lightning strike, where the energy
involved is many orders of magnitude greater than the varistor can
handle. Follow-through current resulting from a strike may melt, burn,
or even vaporize the varistor. This thermal runaway is due to a lack
of conformity in individual grain-boundary junctions, which leads to
the failure of dominant current paths under thermal stress when the
energy in a transient pulse (normally measured in joules) is too high
(i.e. significantly exceeds the manufacture's "Absolute Maximum
Ratings"). The probability of catastrophic failure can be reduced by
increasing the rating, either by using a single varistor of higher
rating or by connecting more devices in parallel.
Cumulative degradation occurs as lesser surges happen. For historical reasons, many MOVs have been incorrectly specified allowing
frequent swells to also degrade capacity. In this condition the
varistor is not visibly damaged and outwardly appears functional (no
catastrophic failure), but it no longer offers protection. Eventually,
it proceeds into a shorted circuit condition as the energy discharges
create a conductive channel through the oxides.
The main parameter affecting varistor life expectancy is its energy
(Joule) rating. Increasing the energy rating raises the number of
(defined maximum size) transient pulses that it can accommodate
exponentially as well as the cumulative sum of energy from clamping
lesser pulses. As these pulses occur, the "clamping voltage" it
provides during each event decreases, and a varistor is typically
deemed to be functionally degraded when its "clamping voltage" has
changed by 10%. Manufacturer's life-expectancy charts relate current,
severity and number of transients to make failure predictions based on
the total energy dissipated over the life of the part.
