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

Discussion in 'Electronic Design' started by [email protected], Apr 12, 2007.

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  1. Guest

    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:

    http://it.farnell.com/jsp/Passive+Components/Suppressors/SEMITRON/
    SL1122A200/displayProduct.jsp?sku=772859

    (SEMITRON SL1122A200)

    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.
     
  2. w_tom

    w_tom Guest

    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:
    http://www.erico.com/public/library/fep/technotes/tncr002.pdf
    Both 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 earthing electrode into hole's
    bottom, then use a 6 inch plastic pipe to form a 'manhole'. Earthing
    connections (and maybe sensor) can be inspected, protected, and
    repaired. Connection points being the most frequent failure point and
    where diagnostic testing is best conducted. A cap atop that pvc pipe
    means the whole assembly can be buried or covered so as to be
    transparent to casual observers and so that connections are protected
    from earth and water; and yet be easily accessed.
     
  3. Robert Green

    Robert Green Guest

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

    (-:
     
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