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

Discussion in 'Electronic Design' started by [email protected], Apr 10, 2007.

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  1. Guest

    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
     
  2. Steve Y

    Steve Y Guest

    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
    www.mtlsurge.com
     
  3. w_tom

    w_tom Guest

    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.
     
  4. Jake Brodsky

    Jake Brodsky Guest

    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
     
  5. 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.

    --
    ********************************************************************
    Paul E. Bennett ....................<email://>
    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..
    ********************************************************************
     
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