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

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

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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


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Steve Y

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:

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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,

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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

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Jake Brodsky

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:

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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

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Common mode or longitudinal mode: point is that current on any or both wires will even pass through that datalogger to obtain earth ground. When discussing these currents, resulting voltages can make linoleum, wood, concrete, and wall paint electrically conductive. Even if all inputs have galvanic isolation, still, that only becomes a capacitor to longitudinal surges. Isolated to AC electric is not isolated to surges. Everything becomes conductive - including the best electrical insulator: air.

All appliances (including datalogger) contain internal protection. So that internal protection is not overwhelmed, earth that Semitron. An earthing connection that is as short as possible ('less than 3 meters'). Same electrode that everything else (including AC power to the datalogger) uses. If the datalogger uses two wire AC, then it is not isolated from earth - when discussing transient voltages and currents. What is isolated for human safety is not isolated for transistor safety. Why? Different voltages define same material 'as and as not' conductive.

Again, refer to that application note:

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the building (ie datalogger) and antenna tower (sensor) have earthing. So that sensor is not harmed, a Semitron or something equivalent (see also Sidactor and see warning below about its operating voltage) earths sensor end of 4/20 ma wires. As long as the sensor and its wires remain at voltages same as earth beneath, then sensor is not damaged. With a protector on sensor end, then surges from the earth or surges via wire (from other sensors) are not destructive.

Telco does same thing. Lightning to any other building in town is equivalent to a lightning rod connected to their $multi-million computer. Telco does what you are doing - earths wire as it leaves that other building and again earths wire as it enters their building. Earths via a 'whole house' type protector - because destructive surges will find earth ground even destructively through isolated (ungrounded) equipment.

Currently Telcordia (the standards organization for all telcos) is upgrading GR-1089 - their standards for surge protection. For example, gigahertz ethernet cable new standard requires an earthed protector from each wire to earth ground AND no protector between twisted pair ethernet wires. A protector between wires is for surges that typically are not destructive which is why new standards do not call for a protector between wires. Standards are quite blunt about what is necessary for communication wires (such as ethernet): a protector from each wire and short (less than 10 feet) to a single point earth ground.

Appreciate the circuit: a constant current source (not a voltage source) creates destructive surges. Your concern is not voltage. Your concern is current. From first year EE: voltage will increase, as necessary, to conduct that current (which is also how pre-1970s cars created 20,000 volts on spark plugs). If attempting to stop a surge, then voltage will increase as necessary to overwhelm that "dam" (galvanic isolation, the isolated datalogger, or spark plugs). Protection can never stop that current because even 4 kilometers of air could not. Protection means shunting (conducting, diverting, clamping) current to earth so that near zero voltage occurs on datalogger or sensors. This shunting so that component or datalogger protection is not overwhelmed.

Even with an isolated datalogger, voltage will increase as necessary to conduct to earth ground. Again, all electronics have internal protection. Earth a protector so that current will not overwhelm that protection. That datalogger sitting on a table remains a potentially destructive path to earth - via table top.

Why did Ben Franklin invent the lightning rod? Because wood (in church steeples) is an electrical conductor (like a tabletop) and because that electricity cannot be stopped using isolation. Best isolation - 4 kilometers of air - did not stop it.

How would I create an earth ground? Datalogger is AC powered? Then the same 3 meter earth ground rod required electric power source must also be connected 'less than 10 feet' to the Semitron earth ground terminal. Ground wire must have no sharp bends, no splices, not inside metallic conduit, separated from all other non-earthing wires, and remain independent of other earthing wires until all meet at the earthing electrode.

At the sensor end, a one meter ground rod would probably be sufficient. But grounding electrode must contact soil below the frost line. If wire is deep enough, then a one wire solid and bare copper wire could be laid in the trench as an earthing electrode. That wire should be solid, copper, and 6 AWG or larger. Water is not considered sufficiently conductive. Earthing electrode must be in earth - either under water or located where wire enters water. If this is water that people swim in, then that earthing should also connect to anything adjacent that is electrical and earthed (ie overhead lightning).

Appreciate a lesson from scuba diving. When lightning struck the surface, then divers floating free were unaffected. But those touching the bottom were connected to the better conductor - and felt lightning currents.

Generally, 4/20 ma sensors operate at voltages less than 30 volts. Your Semitron conducts nothing until voltages exceed 200. Can your sensors withstand 200 volts applied between any one wire and all other wires (or its body) without damage? Best check those specs for longitudinal mode voltages. Would a 60 volt protector be a better choice? Semitron is the right idea. But its operating and sparkover voltage may be excessive. Telephone appliances can withstand 600 volts without damage. Semitron is for telephone appliance protection. Consult sensor voltage specs. That protector voltage must be below internal sensor protection voltage (max voltage between any two points).

Some first dig a hole, drive an earth> First of all, maybe I should stress that the multiple sensors and the

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In gratitude for God not electrocuting him for flying kites in thunderstorms?


-- Bobby G.

Reply to
Robert Green

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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Paul E. Bennett

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