Surge Arrester

Although you don't define define the type of station class arrester, it is not an open circuit because regardless of construction, a grading current flows through the arrester as long as it is connected between line and ground. If it is a gapped type silicon carbide, grading current flows through the grading circuit and if gapless, grading current flows through the metal oxide varistors (MOV). The arrester rating may be based on the maximum phase to ground operating voltage (probably assuming 10% regulation), the maximum temporary overvoltage that can exist at that location, system grounding, etc.

Remember, surge arresters are designed to protect against impulse overvoltages not power frequency overvoltages. They have a TOV rating which is essentially a volt-time curve that defines the magnitude and duration of power frequency voltage above their maximum continuous operating voltage (MCOV) they can be expected to survive without damage.