Hi all!
Thank you for all the replies, they were truly enlightning.
First of all, maybe I should stress that the multiple sensors and the single, multiplexed data logger, are completely isolated with respect to the ambient (and the latter is battery powered, and powers all of the sensors), although the sensors and the data logger aren't galvanically isolated each other (I am using MIC2982 chip to power on/off each sensor at will, of course respecting their "warm up" specs, and an ADC with multiplexer to select the input from each channel, which has its own sense low-tempco resistor. The data logger has its own memory, which I will read every n months or maybe I may add a GPRS later).
This "system is isolated from Earth, but components aren't from each other" probably was already clear, so I'm not going to change a bit, but I wanted to make it clear since none of the sensor nor the data logger is connected to Earth ground.
Anyway, I understand now that I have to use a device such as this one:
(SEMITRON SL1122A200)
which combines the strenght (5KA) of a gas discharge tube with the quick reaction of two bidirectional tranzorbs, integrated in the same package. Thus my sensors protection would become:
sensor 4-20mA power ------*-----*--------- to data logger 4-20mA power | | Tranzorb | | | | Earth Ground ---*----Gas | | Tranzorb | | | | sensor 4-20mA signal ------*-----*--------- to data logger 4-20mA signal
after some hundreds meters of cable, each data logger input would get:
from sensor 4-20mA power ------*-----*-------- datalogger 4-20mA power | | Tranzorb | | | | Earth Ground ---*----Gas | | Tranzorb | | | | from sensor 4-20mA signal ------*-----*-------- datalogger 4-20mA signal
thus the GDT/TVS hybrid will divert to Earth longitudinal currents/voltages (well, I'm not really sure what "longitudinal" means here though: common mode?), so that a really high (billions of volts?) common mode transient doesn't try to reach Earth through the datalogger (which *currently* was thought to be completely isolated from Earth, and battery powered. What if I mount it 30 cms above Earth? why would a transient "want" to pass through the data logger, when it is quite isolated from Earth anyway?).
I guess I better still keep ALSO the unidirectional 24V tranzorb to protect from possible "transversal" overvoltages, since the two bidirectional TVS's would do it too but are rated a too high voltage.
But, what if in the site where I have to place the data logger and/or each sensor (water level sensors) there's no ground connection? Should I bury e.g. one meter of copper wire to create the ground? Some sensors will be immersed under 100+ meters of water.
Also, one thing that worries me is that, being the sensors put far away with respect of each other (its like a "star" system where the data logger is in the center) a lightning somewhere will create a big difference of potential in a sensor versus another opposite sensor, and the cables will bring this difference right into the data logger.. thus in theory I would also need to "gas discharge / tranzorb" (meant as a tense here) each wire with each other wire, where the combinations would explode.
Finally, I understand that the SL1122A200 will be totally ineffective if I don't connect it to a low impedance, true, Earth ground. But there's none currently.. so what would be a good way to do it? Burying how much ground wire underground?
Thank you very much for all your support, each opportunity like this one is great to learn new things.
-- Andrea