Tips to protect your equipment from Lightning surge.

Typical structure

Signal Level Down

Signal Delay

SPD for signal lines connects on signal line and signal goes through SPD and it will have probability to affect signal quality, such as level down, delay etc.
Majority of SPD for signal lines are break into 3 blocks.
A: By-pass surge currnt(usually Gap device) ,
B: Decoupling device(resistance, inductance etc.),
C: Keep voltage steady(Usually Ziner Diode).

It is true that surge by-pass capability and voltage protection level are important fucntion for SPD, but select SPD which does not affect signal is also important. Decoupling device will be one of key parts for this.
Direction of connection is also important. Euipment should be connectd "C" side on the above SPD. SPD may be broken if your connection was wrong, since surge current directly come into "C" parts which is Usually Ziner Diode which can not take care of big surge current.
　You should be careful of specification of the equipment you want to protect from surge current. As we mentioned the above, decoupling device may affect signal quality. You also be careful specification of the equipment to protect, since some equipment uses modified specification. It is a good idea that you ask the equipment manufacture or once connect the target equipment whether you have any problems or not. Connector shape also have to be checked.