@erik_squires
The element most often used is Metal Oxide Varistor. Siemens uses multiple large 150VAC 40mm MOVs. Here is data sheet for large MOVs: https://www.mouser.com/datasheet/2/240/Littelfuse_Varistor_HA_Datasheet_pdf-596397.pdf
The problem with MOVs is, that their clamping voltage is temperature dependent - lower at high temp. Because of that 150VAC MOV at 25degC will start conducting (1mA) at as high as 264V (187VAC). At 200A clamping current it is 405V (287VAC), but with multiple parallel MOVs current will divide - lowering clamping voltage of each MOV (stronger protection is better). 150VAC is just rating at 85degC (including element tolerance) to make sure they stay off in normal conditions. IMO whole house protection is necessary, since today everything contains electronics. I use surge suppressing breaker (Siemens panel), that replaced double 20A breaker. My gear is plugged into Furman Elite 20PFi conditioner with tight non-sacrificial over/under voltage protection. I still unplug during thunderstorms or when I travel. I'm moving to Florida now and plan to install protection as big as I can find - perhaps Siemens FS140 or even couple of them. Still, I would highly recommend even basic protection, like mentioned breaker: https://www.amazon.com/dp/B0052MG5K0/?coliid=I2G7DXSO9P4HAH&colid=41JEAXYNOO5S&psc=1&ref...
The element most often used is Metal Oxide Varistor. Siemens uses multiple large 150VAC 40mm MOVs. Here is data sheet for large MOVs: https://www.mouser.com/datasheet/2/240/Littelfuse_Varistor_HA_Datasheet_pdf-596397.pdf
The problem with MOVs is, that their clamping voltage is temperature dependent - lower at high temp. Because of that 150VAC MOV at 25degC will start conducting (1mA) at as high as 264V (187VAC). At 200A clamping current it is 405V (287VAC), but with multiple parallel MOVs current will divide - lowering clamping voltage of each MOV (stronger protection is better). 150VAC is just rating at 85degC (including element tolerance) to make sure they stay off in normal conditions. IMO whole house protection is necessary, since today everything contains electronics. I use surge suppressing breaker (Siemens panel), that replaced double 20A breaker. My gear is plugged into Furman Elite 20PFi conditioner with tight non-sacrificial over/under voltage protection. I still unplug during thunderstorms or when I travel. I'm moving to Florida now and plan to install protection as big as I can find - perhaps Siemens FS140 or even couple of them. Still, I would highly recommend even basic protection, like mentioned breaker: https://www.amazon.com/dp/B0052MG5K0/?coliid=I2G7DXSO9P4HAH&colid=41JEAXYNOO5S&psc=1&ref...