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There is an ultrasonic-windsensor which will be mounted on a bridge in winter. Maybe several of them.

It seems like for outdoor use this device should be earth grounded(probably through mains earth? since no earth on bridge). In the manual the reason is given as radio freq. interference and lightning.

People are asking why would there be RF interference on a bridge in the middle of no where and why would lightning affect this device.

Would a lightning not a direct strike on such device but nearby still affect the performance or damage it?

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  • \$\begingroup\$ Your extract appears to be from a different pdf file than the one you linked. \$\endgroup\$
    – Andy aka
    Commented Jul 25, 2018 at 13:42
  • \$\begingroup\$ I guess they had it in EMC testing and found that without grounding the measurement values were not stable and instead of getting them stable without grounding they opted for an instruction in the manual... \$\endgroup\$
    – Arsenal
    Commented Jul 25, 2018 at 13:45
  • \$\begingroup\$ @Arsenal Im wondering when they mean lightning protection do they mean a direct strike on the device or they also include indirect effect from a lightning nearby. \$\endgroup\$
    – pnatk
    Commented Jul 25, 2018 at 13:48
  • \$\begingroup\$ If one of those took a direct hit from lightning, it would be fried. But even a nearby lightning strike could induce a significant voltage in the casing, and if that isn't properly grounded, the voltage will propagate down the cable screens instead. \$\endgroup\$
    – Simon B
    Commented Jul 25, 2018 at 14:36
  • \$\begingroup\$ A bridge is a place where I'd certainly expect elevated EMI, depending on what kind of bridge we're talking about; but if it's across a larger river where naval vessels cross: don't underestimate the powers nautical radars put out. If it's a bridge for automotive traffic: you get millions of cars. a fraction of these has damaged spark plug connections. How often would you like to malfunction? \$\endgroup\$
    – Marcus Müller
    Commented Jul 25, 2018 at 17:35

1 Answer 1

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Products exposed to high E and H field noise may cause errors or damage the interface such as lightning. The level of immunity depends on the amount of Common Mode (CM) rejection mainly and Differential Mode(DM) rejection .

Power supplies or Ethernet may be exposed to conducted line transients, from indirect coupled strikes. Power lines use arc gap quenching contacts at some voltage to protect the Basic Insulation Level rating (BIL) of the equipment attached. Yet even in homes the protection in power meters is only BIL6 (kV) with arc gaps to earth gnd.

These SMPS and Ethernet interfaces will always use CM choke on these lines. They may use 2 for improved performance.

The function of the CM choke is by inductance to raise the CM impedance while not raising the DM impedance for power or data transfer. Then attenuation is greatly improved with HV caps and clamps.

  • It is not just a rotated 1:1 transformer , but one designed for a range of frequencies and as perfect a balance as quality tolerances permit.

    • It also by raising both Lines if CM impedance more effectively balances the impedance resulting in better rejection ratio or CMRR, thus attenuating both CM impulse and DM pulse levels.

    • line filters also include Clamps such as MOV, TVS and Gas Discharge Tubes (GDT) crowbar devices with series current limiting.

    • depending on the length of the cable the expected E and H fields induced are measured by V/m and I/m from nearby lighting strikes. No assumptions can be made that the bridge will act as a shield or a conduit to high current strikes to protect the cable.

To design such an interface one can rely on Corporate EMI Immunity standards or IEC stds or IEEE standards for impulse immunity, to validate the design.

But to solve this one must consider the interface device limits, the insulation ratings, yet expected transient levels to be much higher than industrial arc welding noise.

Solutions include:

  • CM chokes or BALUNs for this application, plastic X,Y caps., TVS clamps , MOV’s, TVS’s, GDT’s shielded twisted pair (STP) balanced DM signalling such as RS-422 or optical solutions.

For more detailed discussions I found many references such as; https://incompliancemag.com/article/designing-ethernet-cable-ports-to-withstand-lightning-surges/

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  • \$\begingroup\$ Does this power entry module docs-emea.rs-online.com/webdocs/1548/0900766b81548475.pdf does what you mentioned? Is this only used to choke RF noise? \$\endgroup\$
    – pnatk
    Commented Jul 25, 2018 at 14:37
  • \$\begingroup\$ This is to protect power entry cable only but also supports transient 2kV transient immunity as per many common standards \$\endgroup\$
    – D.A.S.
    Commented Jul 25, 2018 at 14:48

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