A surge protection circuit should not influence the normal operation of the protected
system. That is, series impedance should be very small (Z1 Z2) and shunt
impedance should be very large (Z2
ZL) for normal signal voltage and frequencies.
Let ZL be the load impedance. During abnormal conditions (during a surge) the series
impedance should be very large (Z1
ZL) to limit the surge current, and shunt
impedance should be very small (Z2 ZL) to divert the surge current. Note that Z1,
Z2 and ZL may be functions of frequency, voltage or current. Also, surge protection
circuits should not be damaged by the surge themselves