Surge protection for 4-20mA sensors and data logger?

Hi all!

For my grandpa's country I am going to install tens of 4-20mA sensors, which will be routed through some hundreds meters of good cable to the data logger, at the border of the house. To give some (yes, I know it's impossible to give total) protection against nearby lightings, etc.. I thought about adding surge protection.

If I understand it right, I should put one device in parallel with each sensor, and another device in parallel with each data logger input (i.e. the two ends of each cable), right?

Varistors, gas-dischargers, etc.. to me it seems that a Transil diode may be a very cheap solution but a very effective one nonetheless. Am I wrong?

And, being there two in parallel for each cable (one at the sensor and the other at the data logger input), are they going to false the results by much? I am aiming at 16bit resolution, with a full scale precision of about 0.1%

Thank you! Andrea

This was posted in an electrical engineering forum where knowledge of differential and longitudinal mode currents is assumed. Again, zener in parallel is for differential mode currents. Destructive surges are longitudinal mode. A surge voltage is same on both wires as current passes destructively through datalogger to earth ground. What would a zener diode between wires see? 5000 volts on one wire and 5000 volts on the other wire (relative to earth) means zener diode sees zero volts. Zero voltage as thousands of volts pass destructively through a datalogger.

Protection means each wire must connect to a service entrance ground. Each wire inside each cable must make that connection directly or via a protector as demonstrated in an application note written for laymen:

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Protectors between wires do nothing - see no voltage - as longitudinal surges find earth ground, destructively via the datalogger. Your solution is for differential mode transients which are not typically destructive.

Is total protection impossible? Telco shuts down and disconnects from overhead wires everywhere in town during each thunderstorm? Or course not. Telco can suffer hundreds of surges during each storm. They also don't put protectors between wires - in parallel. Telco also earths each wire; a protector from each incoming wire to the single point earth ground. Your low voltage data collection network needs the exact same protection system. Telco suffers direct lightning strikes without damage. Telco does not "put one device in parallel with each" phone, "and another device in parallel with each" telephone computer input. You need what telcos have installed for almost 100 years without damage.