Surge protection for 4-20mA sensors and data logger?
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Posted by on April 10, 2007, 7:53 am
Please log in for more thread options 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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Posted by Steve Y on April 10, 2007, 9:45 am
Please log in for more thread options 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 www.mtlsurge.com andrea@NOSPAMPLEASE.com wrote: | ||||||||||||||||
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Posted by w_tom on April 10, 2007, 12:50 pm
Please log in for more thread options 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...@NOSPAMPLEASE.com 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% | ||||||||||||||||
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Posted by Jake Brodsky on April 12, 2007, 8:51 am
Please log in for more thread options andrea@NOSPAMPLEASE.com wrote:
> Hi all!
There are two reasons to install protectors: First, you need to protect
> > 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 > > 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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Posted by Paul E. Bennett on April 13, 2007, 5:10 am
Please log in for more thread options Jake Brodsky wrote:
> 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. 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] > 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. 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). > 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. 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. > 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... 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. -- ******************************************************************** Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ******************************************************************** | ||||||||||||||||
>
> 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
>
>