I am designing a circuit that has a nominal 24VAC input. This circuit is protected by a 4A Fast-Acting fuse. I would also like to insert a transient voltage surge protection. For achieving that, I am willing to add a 26VAC varistor between the input terminals. I am well aware that a long duration over-voltage may lead to a catastrophic failure in the varistor. However, for this voltage there is no TMOV device available, not even THT. My question is: is the only solution to insert an extremely ugly, huge and Thermal Cutoff near the varistor terminals?
As suggested by Dwayne Reid, I think it would be better to use a TVS Diode instead of a varistor. Besides, he pointed that the 26VAC rating is underestimated. I agree, and think that a 11% variation in the mains 230V as well as a 25% voltage regulation from the transformer should be considered. A 35VAC rating makes much more sense.
Well, I have finally decided to use 2x 1812 35VAC varistors in parallel. The TVS diodes are a good option, but can only withstand much lower surges. As far as I have read, the fast acting fuse at the input should protect the varistor against catastrophic failure. Another worry with the TVS Diode is that most manufacturers do not specify failure condition. It could only damage the P-N junction in a way it appears to be an open circuit. In that way, the user would never notice that he is currently unprotected against surges.