What happens when a surge protector fails?

Hatch surge protectors use metal oxide varistors (MOV) as the primary surge protection technology. Metal oxide varistors protect downstream electronics by clamping overvoltage transients, dissipating the excessive energy in the transients in the MOV itself. The degradation and failure of a MOV is characterized by it conducting conducting more and more current which in turn heats up the MOV. This eventually will result in over-current circuits engaging to interrupt current flow through the MOV.