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

[andrea@xxxxxxxxxxxxxxxx]

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:

http://it.farnell.com/jsp/Passive+Components/Suppressors/SEMITRON/
SL1122A200/displayProduct.jsp?sku=772859

(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