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

["w_tom" <w_tom1@xxxxxxx>]

  Same protection that permits your telephone CO to operate without
damage during every thunderstorm is also your solution.  Remember the
difference between differential and longitudinal mode currents?  You
are installing differential mode protection.  But destructive surges
are longitudinal mode.  A surge voltage is same on both wires
(overhead or underground) as current passes destructively through
datalogger to earth ground.  What would a zener diode between wires
see?  Zero voltage as thousands of volts confront a datalogger.

  How does a telco everywhere operate during every thunderstorm
without damage?  Bennison, Ghazi, and Ferland measured surges on
telephone wires during thunderstorms in 1968 and 1969 in IEEE
Transactions on Communications.  Hundreds of transients occurred in
each thunderstorm.  Yet damage is unacceptable.   Protection during
thunderstorms is that routine in every day in every town even 70 years
ago.

  An application note from one industry professional demonstrates the
technique:
   http://www.erico.com/public/library/fep/technotes/tncr002.pdf
Each wire that enters a structure must connect to a single point earth
ground - either directly or via a protector.  In your case, a
protector may be a transzorb, gas discharge tube, or MOV.  Protector
is not protection.   Protector is a connecting device to protection.
What did Bennison et al demonstrate?  Destructive transient is
longitudinal. It seeks earth ground either via a data logger OR safety
earthed before entering a building.  Protector must dump a surge into
earth long before it can get to the datalogger.

  Essential to protection is single point earth ground.  Incoming
wires inside every cable must connect to that same earthing electrode
at the service entrance.  No earth ground means no effective
protection.

On Apr 10, 7:53 am, and...@xxxxxxxxxxxxxxxx wrote:
> 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%