Thermal cutoff
Thermal cutoff switches are common in all Tektronix oscilloscopes starting in the mid-1950s. If the chassis gets above a certain temperature, typically about 53 − 56 °C (128 − 133°F), 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.
Instruments without thermal cutoff switches are the exception. They are the 511, 512, 513, the 514, and possibly others.
Certainly all of the 500-series scopes that take letter-series and 1-series plug-ins have thermal cutoff switches. This also includes the 127, 132, and 133 powered plug-in enclosures.
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.