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In a DC power system like the one shown below in the diagram, a resistor is placed between the return wire and chassis to limit fault currents in the scenario where there is a high side to chassis short. Does the addition of the fault isolation resistor increase the chances of common mode noise on the power bus?

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  • \$\begingroup\$ What is the common mode equivalent circuit? What are the noise sources and sinks? AC and DC equivalent circuits can be very different. \$\endgroup\$
    – Tim Williams
    Commented Jun 30, 2023 at 1:52

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It's easier to see things with superfluous detail removed. I've also added a source voltage (blue) along with a common mode interfering voltage (red): -

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Given that the thick black horizontal connection could be regarded as a zero ohm link, you should be able to see that the load will receive the source voltage (minus the volt drop in the source impedance) irrespective of what the common-mode voltage is.

However, a problem arises if the thick black horizontal connection has resistance or impedance like this: -

enter image description here

So, here's a simulation with the undesirable resistance set to zero ohms: -

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As would be expected 9 volts is developed across the load resistor but, if the undesirable impedance was non-zero: -

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We would naturally get a reduction in the voltage across the load resistor but, is this reduction also modified by the common mode voltage? We can set that common mode voltage (currently at 1 volt) to 2 volts: -

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Notice how the load voltage has changed from 8.632 volts to 8.673 volts i.e. we have made the load voltage a little bit susceptible to common-mode voltages.

Does the addition of the fault isolation resistor increase the chances of common mode noise on the power bus?

Let's repeat the above with the fault isolation resistor reduced to zero ohms: -

enter image description here

Now, the susceptibility to common mode voltages is far greater. Hence, the addition (and value) of the fault isolation resistor is beneficial.

However, it's only beneficial because we have recognized that the thick black horizontal interconnection cannot be zero ohm. If it could be zero ohm then, the fault isolation resistor's value won't affect things. Nevertheless, it's important to restrict ground/earth loop currents and, for that reason, it's good to have some form of impedance.

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  • \$\begingroup\$ Interesting. I am a little confused as to why Section 4.2.2 in NASA grounding document says adding impedance between return line and chassis induces common mode noise. \$\endgroup\$
    – flamezdude
    Commented Jun 30, 2023 at 13:30
  • \$\begingroup\$ @flamezdude simulators are good for testing scenarios therefore I would encourage you to figure out what particular scenarios NASA might be considering. In my answer I've tested a few scenarios and, the fault isolation resistor appears beneficial but, there may be scenarios where it isn't. On that basis, I think we're done here. If you do think of another scenario, you can test it in a simulator and, if problematic, you can always come back here and raise a new question. What should I do when someone answers my question? \$\endgroup\$
    – Andy aka
    Commented Jun 30, 2023 at 13:54

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