Setting SPD is one of the most effactive measures to protect instrument from the impulsion of lighting, surge and current, but it is just a part of lightning protection engineering.
The signal loop of SPD should be set at the spot instrument terminal as well as at the instrument terminal in control room.
When the horizontal linear distance of the signal cable laid on the ground is larger than 100m or the vertical dimension is larger than10m, SPD should be set both terminal of spot instrument terminal and terminal in control room.
The following spot instrument terminals need to set SPD:
a) The insrtrument terminal of safety instrument system;
b) The spot terminal of transmitter spot terminal;
c) The electrical signal actuators’ spot instrument terminal like eletrical convertor, electropneumatic valve positioner, magnetic and so on;
d) The spot terminal of thermal resistance;
e) The spot terminal of electronic switch.
The following spot instrument terminals don’t need to set SPD when the protective disatance requirement is satisfied:
a) The spot terminals of thermocouple;
b) The spot terminals of contace switch ;
c) Distruibution room and pump signal from eletrical control room