WACKER Group IRFUN 38/115 Operator's Manual page 25

Repair instructions
This looks very similar to a small plate capacitor in its design. It is
characterized by a characteristic similar to a Z diode and a very high
load capacity. Its resistance of more than 1 MΩ collapses during
overvoltage in less than 50 nano seconds to less than 1 Ω in extreme
cases.
This protects the electronics against excessive input voltage.
The output frequency is adjusted to 200 Hz using the potentiometer.
Secure the adjustment screw of the potentiometer with locking paint.
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