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Often the overvoltage protection is what's referred to as
a "crowbar", meaning that it shorts out the field supply, like a crowbar across
the wires, to trip the field breaker, often an SCR, silicon
controlled-rectifier. A lot of transistors and SCRs have operating points that
change with temperature. A simple crowbar would have a 15V zener diode connected
from the alternator output to the SCR's gate. Once the voltage rises high enough
for the zener to conduct, the SCR conducts and stays on, putting a short on the
field input supply.
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