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

[Jake Brodsky <ab3a@xxxxxxxxxxxxxxxxx>]

andrea@xxxxxxxxxxxxxxxx wrote:
> 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
>
>
There are two reasons to install protectors:  First, you need to protect
the nearby structure from fire caused by the discharge.  Second, you
might want to continue working through the discharge and maintain
minimum noise pickup.  These goals often conflict with each other.

The goal of fire safety is to limit the damage by grounding the cable
shield frequently.  However, this can introduce quite a bit of noise.
Some multi-conductor cables have two shields: One around the bundle, for
grounding everywhere, and one around the pair for grounding locally.

The problem you get in to with long runs of 4-20 mA current loops is
that the local ground potential at each end can be very different.  If
you put a surge protector at each end, then one or both are almost
guaranteed to fire with any nearby lightning strike.

My suggestion is to use an isolated differential instrument.  Ground
everything to ONLY ONE ground and put the protectors there.  Use
whatever discharge devices you can afford. In my experience, what
matters more is that these things get replaced right away.

It's also important for you to use isolated analog inputs.  Having one
input fire and not the others will result in a lot of damage unless the
inputs are isolated.

People write books about this stuff.  This is just a very quick
overview.  I suggest you read up on some telephone company grounding
manuals.  They have experience with this sort of thing...

Jacob Brodsky, PE