An overvoltage protection device comprising: a first electrode member; a second electrode member; a varistor (652, 654, or 656) interposed between and electrically connected to each of the first and second electrode members; an electrically conductive meltable member, wherein the meltable member is responsive to heat in the overvoltage protection device to melt and form an electrical short circuit path across the first and second electrode members; and a void filling member surrounding at least a portion of the varistor, wherein the void filling member is formed of an electrically insulating material; wherein: the overvoltage protection device includes a sidewall defining a chamber, the chamber including a first subchamber and a second subchamber in fluid communication with the first subchamber; the meltable member is disposed in the first subchamber; the varistor is disposed in the second subchamber and a gap volume is defined between the varistor and the sidewall in the second subchamber; the void filling member is disposed in the gap volume to limit a flow of the meltable member into the gap volume; and the varistor is a first varistor wafer, characterised in that the overvoltage protection device includes: a second varistor wafer (654 or 656) formed of a varistor material; and an electrically conductive interconnect member (660 or 662) connecting the first and second varistor wafers in electrical parallel between the first and second electrode members (622, 624); wherein the first and second varistor wafers are axially stacked between the first and second electrode members (622, 624); the void filling member includes a receiver recess; and a portion (660B or 662B) of the interconnect member extends outwardly beyond the first and second varistor wafers and is disposed in the receiver recess. a first electrode member; a second electrode member; a varistor (652, 654, or 656) interposed between and electrically connected to each of the first and second electrode members; an electrically conductive meltable member, wherein the meltable member is responsive to heat in the overvoltage protection device to melt and form an electrical short circuit path across the first and second electrode members; and a void filling member surrounding at least a portion of the varistor, wherein the void filling member is formed of an electrically insulating material; wherein: the overvoltage protection device includes a sidewall defining a chamber, the chamber including a first subchamber and a second subchamber in fluid communication with the first subchamber; the meltable member is disposed in the first subchamber; the varistor is disposed in the second subchamber and a gap volume is defined between the varistor and the sidewall in the second subchamber; the void filling member is disposed in the gap volume to limit a flow of the meltable member into the gap volume; and the overvoltage protection device includes a sidewall defining a chamber, the chamber including a first subchamber and a second subchamber in fluid communication with the first subchamber; the meltable member is disposed in the first subchamber; the varistor is disposed in the second subchamber and a gap volume is defined between the varistor and the sidewall in the second subchamber; the void filling member is disposed in the gap volume to limit a flow of the meltable member into the gap volume; and the varistor is a first varistor wafer, a second varistor wafer (654 or 656) formed of a varistor material; and an electrically conductive interconnect member (660 or 662) connecting the first and second varistor wafers in electrical parallel between the first and second electrode members (622, 624); wherein the first and second varistor wafers are axially stacked between the first and second electrode members (622, 624); the void filling member includes a receiver recess; and a portion (660B or 662B) of the interconnect member extends outwardly beyond the first and second varistor wafers and is disposed in the receiver recess.