I'm tasked wich building a test rig for electrical machines which consists of

  • Machines mounted on top of a large ground plate which is connected to PE through a thick cable

  • A torque transducer: The torque signal GND is internally connected to the chassis and thus also to PE

  • An oscilloscope which records the signals from the torque transducer.

  • A resolver which also internally has the signal ground directly connected to the chassis and thus also to PE

  • A digital controller (e.g. dSpace/RT Box). From Datasheet: "PE and signal GND are coupled internally by means of a high impedance RC network, consisting of a 1 MΩ resistor, a 1 µF capacitor and a 5V zener diode (SMAJ5.0CA), all connected in parallel". The resolver signals are connected to the signal GND of the controller and thus the signal GND is pulled to PE with two 3 meter long coax cables.

  • Oscilloscopes, controller and PC communicate via Ethernet: The ethernet connection also pulls everything to PE through the Ethernet socket in the wall.

    enter image description here

Now I'm wondering about the ground/PE concept. I want to have everything as failure proof as possible, because much hardware and software used in the test rig is self-made. I see 3 problems:

  • There are multiple parallel PE loops. Supposing a high voltage hits the ground plate until the RCP fires.
    • There is the desired low impedance connection from the ground plate to PE through the grounding cable
    • There is an undesired current path from the ground plate to torque transducer signal ground(=PE) to the oscilloscope to PE
    • There is an undesired current path from the resolver (signal GND=PE) to the controller through the "high impedance RC network" (which I assume will block the fault current) to PE
    • There is an undesired current path from resolver/torque transducer to controller/oscilloscope and then through all Ethernet devices to the Ethernet socket in the wall which connects to PE

My questions:

  1. Are my concerns valid? I'm concerned about a fault working through the entire test rig and destroying everything.
  2. How is such a test rig usually built fail-safe (e.g. in industry)?
  3. What do you think about (1) putting all Digital electronics behind an isolation transformer which breaks the direct PE connection and (2) using a WLAN stick to break the PE connection via the Ethernet socket? Then the only connection that remains is through the grounding cable

I apologize if the description isn't clear enough. I'm not a trained electrician, but more of a "theory idiot". Thank you!

  • \$\begingroup\$ Ethernet is an isolated interface. There should be no connection to PE. If you do have a wall socket that provides a PE connection to a shielded cable via shielded connector, use unshielded cable or connector so you don't have PE connection via Ethernet. \$\endgroup\$
    – Justme
    Aug 4, 2021 at 10:58
  • \$\begingroup\$ There may be a coupling capacitance to PE on ethernet built in \$\endgroup\$ Aug 4, 2021 at 11:15

1 Answer 1


For best noise immunity, use common PE connections with overvoltage and current limit protection and/or CM choke filter on signal lines.

Use STP cables for signals with source ground only to shield.

It is good to realize impedance paths for ground faults and use a GFCI and follow installation guides in manuals.

Industry standards were defined for immunity inhouse and gradually found their way into IEEE and CE standards.

PE is a standard practice for all equipment with PE plugs. Noise immunity depends on sources, cables, orientation and coupling and desired levels of signal/noise ratio.

  • \$\begingroup\$ Thank you. I'm not concerned with noise at this stage, but more with making sure that as many of my electronics survive in a fault. I'm wondering about the possible current paths that the fault current takes until the GFCI trips. Am I correct in worrying about fault current also flowing from (1) grounding plate->torque transducer->Oscilloscope->PE, (2) grounding plate->torque transducer->Oscilloscope->Ethernet network->PE and (3) flowing in ground loops in case of a ground fault? \$\endgroup\$ Aug 4, 2021 at 12:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.