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

If you want to protect the datalogger and the sensor then you will need protection at both ends. Just using diodes will not give you sufficient protection, you will need gas discharge tubes as well.

If you want to protect the sensor and the electronics are not earthed, a parallel connected device will be OK but for the datalogger you will better with a series connected device, this will give you more protection and react quicker.

Any true industrial product will not introduce any errors into your signal, especially as you are using 4/20mA signals rather than voltage

Steve

snipped-for-privacy@NOSPAMPLEASE.com wrote:

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:

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 protecti> For my grandpa's country I am going to install tens of 4-20mA sensors,

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

Which is why there are two distinct names for the functions (Shield and Screen). On things like Type 23 frigates the shield was grounded at each and every bulkhead it passed through while the screens were grounded only at the instrument rack ends. However, Grounding of Shields is a matter for individual installation analysis to suit the environmental conditions appertaining to the installation. [%X]

What I use as standard. It precludes consideration of using equipment from the likes of National Instruments for fast data aquasition in many of the environments I deal with because of their lack of isolation and their propensity to poor-man's differential input (use of two inputs combining to provide the differential input).

Galvanically isolated, Energy Managing, Surge Clamped inputs and outputs for everything is fairly normal practice for me (analogue and digital). In short, attention to detail (such as ensuring enough terminals for each and every conductor, decent signal earthing scheme, decent protective earthing strategies, isolation strategies, termination management, design for safe testing) is very important.

Not only the tel-co's but also the Oil and Gas Industry, and the Railway Industry have large amounts of information on this topic.

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