Can anyone explain to me the

Hi Suzan, The behavior of silicon diode surge protector is different than a metal oxide varistor. A metal oxide varistor shows a decrease in resistance over the rated Voltage. See the curve supplied by the manufacturer. For high power, like lightening protection, high power varistors are available.  A small overvoltage will cause a relative small current through the varistor.  This way the amplitude of the spike is reduced but fuses are not blown. Most appliances are designed not to be damaged by small overvoltage spikes. A silicon diode surge protector is a special zenerdiode. For AC two zenerdiodes are connected back to back. For DC a zenerdiode will protect against overvoltage and reverse polarity. A zener diode shows a more abrupt level between non conducting and conducting. This might cause high currents and blown fuses at only small overvoltages. Most silicon diode surge protectors cause a permanent short circuit when overloaded. This way a relative small diode can be used. The equipment is not damaged, but the protector has to be replaced when an overvoltage has occurred. For protection against lightening on power lines, I prefer a combination of a high power varistor and a spark gap. Silicon diode surge protectors might cause unwanted short circuits in this application.  For protection inside sensitive electronic equipment silicon diode surge protectors can be used. However for repetitive spikes, silicon diode surge protectors might cause radio frequency interference. In that case a varistor might be better. The metal oxide varistor and the silicon diode surge protector both have their benefits depending on the application.

Hi Suzan, The behavior of silicon diode surge protector is different than a metal oxide varistor. A metal oxide varistor shows a decrease in resistance over the rated Voltage. See the curve supplied by the manufacturer. For high power, like lightening protection, high power varistors are available.  A small overvoltage will cause a relative small current through the varistor.  This way the amplitude of the spike is reduced but fuses are not blown. Most appliances are designed not to be damaged by small overvoltage spikes. A silicon diode surge protector is a special zenerdiode. For AC two zenerdiodes are connected back to back. For DC a zenerdiode will protect against overvoltage and reverse polarity. A zener diode shows a more abrupt level between non conducting and conducting. This might cause high currents and blown fuses at only small overvoltages. Most silicon diode surge protectors cause a permanent short circuit when overloaded. This way a relative small diode can be used. The equipment is not damaged, but the protector has to be replaced when an overvoltage has occurred. For protection against lightening on power lines, I prefer a combination of a high power varistor and a spark gap. Silicon diode surge protectors might cause unwanted short circuits in this application.  For protection inside sensitive electronic equipment silicon diode surge protectors can be used. However for repetitive spikes, silicon diode surge protectors might cause radio frequency interference. In that case a varistor might be better. The metal oxide varistor and the silicon diode surge protector both have their benefits depending on the application.