Thermal cutoff switches are common in all Tektronix oscilloscopes starting in the mid-1950s. If the chassis gets above a certain temperature, typically about 128-133° Fahrenheit (53-56°C), the switch opens. Typically this interrupts the mains power coming into the scope and the scope will cool down. Thermal cutoff switches used in Tek scopes are self-resetting. After the scope cools down sufficiently, the thermal switch will close, allowing power into the instrument.
The 310A has an unusually high 165°F (74°C) thermal cutoff. This is due to the fact that the 310A is a compact tube scope without a fan.