I heard that when installing a varistor for surge protection in an electrical installation, a HRC fuse should be connected in series with the varistor. I coudn't understand the requirement of this series HRC fuse. Anybody know the reason ?
Because a varistor can only absorb relatively small amounts of energy. If you get a sustained surge or multiple short surges in succession, it will undergo "catastrophic deconstruction". The idea is that the HRC fuse ruptures before the varistor spits its guts everywhere.