INTRODUCTION

The process control instruments and measurement devices usually belong to the equipment of important strategic value in a production plant. Often when a computer system or a PLC stops working the whole process or production is stopped.

Industrial PCs, PLCs, and probes are electronic equipments which are on one hand very sensitive to surge voltages, on the other hand they are very exposed to suffer the impact of surge voltages through their data lines, both by conducted and induced surge voltages.

The surge voltages in the data or signal lines are often originated from switching of large machines, electrostatic discharge or lightning strokes in the proximity of electrical conductors.

Selection of the protector:

The protector for data communication and signal lines may be selected without knowing the exact communication standards by consideration of the transmission mode, as shown in the following:

Protection series DIN-232: In a system of data wires the data transmission is carries out by comparing the voltage in each wire with a common reference point, usually called ground.
Protection series DIN-xx-3,DIN-xx-5,DIN-xx-8: In a system of pairs of wires with referencd potential (ground) a voltage difference within a pair of wires is used in order to transmit a signal.
Protection series DIN xxV-2C: A system of pairs of wires without reference potential uses the difference of the voltage between the pair of wires for data transmission, where one of the wires is used as reference (ground).

Protection based on communication protocol: In the following a quick selection guide is shown in order to select the protector for the most used communication standards. For other types of signals the protection is selected according to the number of wires and the voltage magnitude of the signal.