What physical consequence occurs to a Metal-Oxide Varistor (MOV) every time it successfully clamps a surge event?

Its internal structure changes, and its threshold voltage slightly decreases

The core protective mechanism of the Metal-Oxide Varistor (MOV), the workhorse of consumer surge protectors, is inherently destructive to the component itself. Every time an MOV clamps an electrical surge by shunting the excess energy, physical changes occur within its internal structure. This repeated clamping action causes a gradual degradation, manifesting as a slight decrease in its threshold voltage. This means that a protector used frequently, or one that has absorbed significant energy from a major event, may no longer provide the advertised level of protection, even if the device remains physically intact. This degradation is why protection is not permanent, and why a protector's total Joule rating is considered a budget that gets spent over time rather than an infinite capacity.