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

["w_tom" <w_tom1@xxxxxxx>]

  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.

On Apr 12, 11:52 am, and...@xxxxxxxxxxxxxxxx wrote:
> 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?