I've been lurking for some time on the board here - but hadn't had occasion to ask a question until now:

Does anyone have experience with power distribution networks and their impacts on sensitive instrumentation?

I've got a 12.5kVolt line within a few feet (4) of a dielectric (aka capacitive or E-field) sensor used for detecting moisture. What sort of shielding is standard for this type of line? (I don't know total power through the line.) I'm concerned that transients on the line may cause improper behavior in the sensor.

Nick answered below about sensitive instrumentation in the face of interference - but I'd like to better define the interference itself.


1 Answer 1


First, here are a few questions for more information. Do you have a general idea on what's the max current in the power line? Is it 50/60Hz frequency? If you know the model of the sensor, could you post it too?

Capacitively coupled interference is effectively blocked by a Faraday cage, which is a grounded metal box or cage.

Magnetically coupled interference is blocked by ferrous alloys with high permeability. Steel might work for this purpose. There is also an alloy especially useful for shielding called Mu-metal. Magnetically coupled interference disappears more quickly with distance than capacitively coupled.

On sensor's susceptibility to interference. Well-designed industrial sensors reject 60Hz (at least to some extent). Make sure that it's connected by a good shielded cable, and the shield is grounded properly.

You could make an experiment to check the susceptibility of your sensor. Put your sensor (or a sensor of the same model) in a sealed container, so that it always sees the same humidity. Bring it closer or further from the source of interference. Observe the signal.

  • \$\begingroup\$ Nick - thanks for the response. I'm guessing that power is probably on the order of 1MW on this line (large industrial plant). I work for the manufacturer of the sensor: agricheminc.com/pdfs/FP-24C%20Sensor.pdf. \$\endgroup\$
    – JDB
    May 8, 2012 at 4:39
  • \$\begingroup\$ Noise/interference in this case is not periodic (hence why I asked about transients on high voltage lines) - nothing at 50/60Hz. The device essentially has a Faraday cage built around the sensing area. The mu-metal is something I'd forgotten about - I don't think magnetic interference is a big issue since the falloff is much greater (as you stated). \$\endgroup\$
    – JDB
    May 8, 2012 at 4:51
  • \$\begingroup\$ As far as interference, past experience has shown that skin effect noise transmission from poorly wired VFDs is received by the sensor pickups - bypassing any but the best grounding/drains (not much we can do when it is at the same frequency the sensors operate at). That is, interference is not on analog signal lines (since the device has digital comms), but sensed by the instrumentation (which has notch filters set up to reduce extraneous noise - only so much you can do to increase SNR). For more on the device I'm working with see: google.com/patents/US6388453 \$\endgroup\$
    – JDB
    May 8, 2012 at 4:53

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