Protection of the temperature input against external voltage

Question:
How can the temperature input (T1/T2) be protected against external voltage?

Answer:
By means of the additional switching presented by the figure below the temperature input of the frequency inverter can be protected against external voltage, which can result from insulation mistakes of the temperature monitoring.
Following components are required:

• R1, R2 : Metal oxid varistor S20K11 (11 V)
• F1, F2 : Fuse 250 V / F0,1 A (5 * 20 mm)

The varistors become conductive, as soon as their rated voltage (here 11 V) is exceeded. This enables a current flow on earth potential leading to the activation of the fuse.