Hitachi C1421R/T Wartungshandbuch Seite 16

Although it basically functions as protection of VIN terminal against over-voltage, since VIN terminal is usually supplied
from the drive winding of the transformer and the voltage is proportional to the output voltage, it also functions against
the over voltage of the secondary output which occurs when the control circuit becomes defective or in some other cir-
cumstances.
15-UK