Surge protection device having two part ceramic case for metal oxide varistor with isolated thermal cut off

A surge protection device, comprising:

• a ceramic housing having a first portion with a box-shape and an open side, the first portion comprising two gas-tight chambers separated by a ceramic partition, the housing including a second portion comprising a ceramic lid configured to fasten onto the first portion and cover the open side, forming a gas-tight container;
• an uncoated, metal oxide varistor located in a first one of the gas-tight chambers, the varistor having a first conductive plate connected to a first electrical input lead that passes through a wall of the housing and a second conductive plate connected to an electrode that passes via the ceramic partition, into the second one of the gas-tight chambers, the electrode being terminated in a fixed contact in the second chamber;
• a thermal cutoff device located in a second one of the gas-tight chambers, the ceramic partition being configured to shield the thermal cutoff device from the metal oxide varistor, the thermal cutoff device comprising a spring connector connected to a second electrical input lead passing through a wall of the housing, the spring connector being terminated in a moveable contact, the spring connector having a spring bias that biases the moveable contact away from the fixed contact, the spring connector being configured to superimpose the moveable contact over the fixed contact of the electrode when the spring connector is urged toward the fixed contact;
• and
• a low melting-temperature solder bond between the fixed and moveable contacts, the solder bond being configured to melt and release the moveable contact from being bonded to the fixed contact when temperature of the fixed and moveable contacts rises above a melting point of the solder bond, the spring connector being configured with the bias to thereby separate the contacts and interrupt electrical connection of the thermal cutoff device to the metal oxide varistor.