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I have been testing an industrial system. I have developed a test device to properly test this industrial system. I am currently finalizing the test device and have put it in an enclosure to EMC proof it from external sources.

When probing around the industrial system with my multimeter I discovered that in the industrial system the 0V is connected to the EARTH(enclosure) via a 200k resistance. I don't know where the connection occurs, but I was a little surprised.

It does not seem to make sense cause this way, you loose the EMC proofing, right? Since the test device is also connected to the 0V it also looses its EMC proofing... The industrial systems' main components are a AC Drive, PLC and Power Supply. But I would suggest these components are designed to act as floating components if not EARTHed by the user.

Since I'm not the designer of the Industrial system and due to company rules, I can't go in depth on its workings and function. Though I am surprised that the EARTH connects to 0V.

Maybe an experienced engineer can enlighten me with some information on EARTHING, 0V reference points and EMC in industrial systems?

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  • \$\begingroup\$ youtube.com/watch?v=eGeFVIJ2u0w youtube.com/watch?v=u9l203U950M \$\endgroup\$ May 21 '14 at 15:06
  • \$\begingroup\$ All I can say is that the returns that people often call zero, are current carrying wires and by definition can't be at zero potential compared to Earth. I find that in industrial systems that use more than one power source, there can be a difference between "grounds" of 40V or even more. \$\endgroup\$ May 21 '14 at 15:18
  • \$\begingroup\$ @IgnacioVazquez-Abrams Those video's are really usefull!! Thank you! \$\endgroup\$
    – Matthijs
    May 21 '14 at 18:03
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Most of the time you will either have a direct connection at a single point connecting the internal system ground to the chassis ground, or a ground to chassis connection with filtering like resistance, capacitance, and ferrite beads. It's up to the product designer to determine the most appropriate approach, and many times it's adjusted during EMC testing.

There's no hard-and-fast rule-- it's a complex subject and usually the design engineer can't know for sure whether the grounding scheme will meet EMC requirements until it's tested.

There are many books written on the topic, for example

http://www.amazon.com/Introduction-Electromagnetic-Compatibility-Clayton-Paul/dp/0471755001/

Of course, safety is also a factor when dealing with Earth ground, so any approach has to be compatible with those goals too.

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  • \$\begingroup\$ Thank you. I now understand there can be some circuitry between the reference and the EARTH which might cause the resistance i measured. But this means the circuit is not 100% isolated from the other machinery on the indistrial environment, right? \$\endgroup\$
    – Matthijs
    May 21 '14 at 18:06
  • \$\begingroup\$ I don't know what kind of equipment you're talking about, but isolation from earth ground is usually the exception rather than the rule. \$\endgroup\$
    – Daniel
    May 21 '14 at 19:18
  • \$\begingroup\$ And a disclaimer: this is a VAST generality. Don't take any action that would jeopardize safety based on this information. You need SPECIFIC information about your design and experience in that sphere of engineering in order to make that kind of determination. \$\endgroup\$
    – Daniel
    May 21 '14 at 19:20

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