Both types of over-voltage protection are effective, with the chosen approach largely a function of cost and the type of devices protected.
Deploying MOVs
Metal oxide varistors are small, dime-shaped switches that physically block incoming surges. Depending on the level and overall incoming surge voltage, MOVs can do this countless times prior to failure. Due to their passive blocking state, speed is not an issue. Upper-end surge protection manufacturers view MOVs as superior to fuse and relay-based protection schemes, since the response time of an MOV is much higher. These devices are normally stacked in parallel, creating a barrier of multiple layers that a high-intensity spike must pass through prior to causing significant damage.
RC Protection Schemes
Resistor/capacitor-based surge protection uses a capacitor to absorb current inrush. The attached resistor controls the speed at which the capacitor dissipates and shunts the absorbed current to the neutral line. Based on another passive concept, these are very effective designs. The limiting factor of an RC surge protection device rests solely with the value of the capacitor(s) and resistor(s) in the protection chain. Many manufacturers use a bank of small caps and resistors to save space, equaling the value of a single larger capacitor for higher voltages.
MOV Construction
Metal oxide varistor construction partially explains its effectiveness. Zinc is combined with ceramic and other metals, only coming into the circuit when a pre-defined level of voltage enters. The zinc allows for a high level of conductivity, with the ceramic insulating the resistor from excessive heat contributing to its ability for reuse. Zinc is a metal that responds to a phenomenon called current channeling, which allows the proper amount of voltage through up to the varistor's clamping level. This is the level at which the varistor diode shunts or redirects harmful excessive current away from more sensitive components down the chain.
RC Build
Resistor/capacitor configurations are often components connected to a larger relay. These relays are frequently used as mini circuit breakers, disconnecting at the first sign of significant over-voltage. The capacitor and resistor often absorb and dissipate much of this excess current prior to the relay tripping, effectively creating two layers of protection. Resistor/capacitor combinations are made with high-capacity metal film capacitors and ceramic resistors.