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@Hobbes's answer calls attention to Emily Lakdawalla's book The Design and Engineering of Curiosity : How the Mars Rover Performs Its Job which led me to reading excerpts in several Planetary Society blogposts including Book Excerpt: The Design and Engineering of Curiosity: How the radioisotope power system works. This section caught my eye:

Question: How exactly does Curiosity's floating bus work, and how does it continue to operate when voltage drops from 11 to only 4 volts?

There is a general, generic answer here about possible ways a floating bus might work, with a diagram of a electrical → light → electrical converter. Is that what's happening on Curiosity? There's a 100 W "light bulb" in there somewhere?

4.2.3 Anomalies

On sol 456 (November 17, 2013), the rover experienced a partially conductive “soft short” in the MMRTG, apparently caused by a part of the electrical power circuit touching the aluminum housing.(7) The Cassini spacecraft had MMRTGs of the same design, and experienced similar shorts. As a result of the short, the voltage difference between the rover’s power bus and chassis changed (from 11 volts to 4 volts on that particular sol). The rover’s power system is robust to such changes in voltage, having been designed with a floating bus. The mission halted activity for 6 sols to investigate the problem, which had spontaneously disappeared by sol 461.(8) It occurred again on sols 816, 1084, and 1158, and has been happening more frequently since. The soft short is annoying because it halts operations, but it does not threaten the health of the rover.(9) Table 4.1 lists all the soft shorts to sol 1582.

enter image description here

Information courtesy Steven Lee.

Table 4.1. Dates and Effects of Curiosity MMRTG Soft Shorts to sol 1582.

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    $\begingroup$ I find this sentence a bit disingenuous: "The soft short is annoying because it halts operations, but it does not threaten the health of the rover." If it shorted permanently, halting operations forever, the health of the rover would be somewhat irrelevant. $\endgroup$
    – Organic Marble
    Jan 12, 2019 at 23:58
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    $\begingroup$ Hahah, a 2004 Cassini risk assessment doc gives the probability of a "soft short" fault as "negligible, 6.3e-12 probability of occurrence". trs.jpl.nasa.gov/bitstream/handle/2014/41231/… $\endgroup$
    – Russell Borogove
    Jan 13, 2019 at 0:05
  • $\begingroup$ @Hobbes the related answer is already linked and discussed in my question, it's not a duplicate, and "what is a floating bus" doesn't answer How exactly does Curiosity's floating bus work, and how does it continue to operate when voltage drops from 11 to only 4 volts? $\endgroup$
    – uhoh
    Jan 13, 2019 at 8:21
  • $\begingroup$ @OrganicMarble I would imagine that the mission isn't threatened unless you get two shorts, of differing potential, to the chassis. So one short represents a reduction in redundancy, but is not in itself an immediate hazard. I'm guessing that a single short triggers a safe mode, so they they can investigate and make sure it's just a single short, before continuing operations. That's all just my guess, though. $\endgroup$
    – Wayne Conrad
    Jan 14, 2019 at 15:13

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The term "floating bus" means the power supply is not connected to the rover's chassis.

In for example a car, the body is used as part of the electrical circuit: the negative terminal of the battery is connected directly to the body. This means every electrical component needs only one long wire to the positive terminal of the battery, and a short wire to the nearest part of the body.

In some cases, electrical components are designed with a metal housing that acts as the ground connection, so there's no ground wire at all.

This all works well, with one exception: when a positive wire contacts the body, you get a short that can damage the electrical system. Although most circuits are protected by fuses, there is at least one high-current circuit that isn't protected: the starter motor.

If you wanted to convert your car to use floating circuits, you'd have to replace all these ground connections with wires to the negative terminal. And you'd have to insulate all components from the chassis.

As a result, when either a positive or a negative wire contacts the chassis, nothing happens.

I'm not convinced the answer here is correct. The circuit shown there is the correct way to connect a non-floating circuit to a floating circuit. But in Curiosity's case, the entire electrical system may be floating, so you don't need this conversion. I haven't found a source that specifies this though.

Either way, the voltage they measured was between the power bus and the chassis. This indicates there's contact between the bus and chassis.

The power bus voltage remained nominal, I presume, or they would have mentioned it. A short between the positive and negative sides of the power bus would have been a much bigger problem, as this would have reduced the amount of power available to run the rover.

The short did not threaten the mission:

"The vehicle is safe and stable, fully capable of operating in its present condition, but we are taking the precaution of investigating what may be a soft short," said Mars Science Laboratory Project Manager Jim Erickson at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

A "soft" short is a leak through something that's partially conductive of electricity, rather than a hard short such as one electrical wire contacting another.

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  • $\begingroup$ I think the graphic is a bit misleading - it shows a data signal, but in Curiosities case we're talking about the power bus, which is a substantially different thing, an optocoupler is not actually helpful there. $\endgroup$
    – asdfex
    Jan 13, 2019 at 11:16
  • $\begingroup$ Optocouplers often use phototransistors instead of photodiodes. $\endgroup$
    – Uwe
    Jan 13, 2019 at 11:45
  • $\begingroup$ So, if the power and ground are both separate buses/wires, then why does a short result if the power contacts the chassis? e.g. "a part of the electrical power circuit touching the aluminum housing." Isn't that the exact reason for doing this floating bus thing in the first place? $\endgroup$
    – Organic Marble
    Jan 13, 2019 at 14:14
  • $\begingroup$ Maybe it's just a detection mechanism? if there's a short->contact where there shouldn't be any->mechanical damage somewhere. The book isn't more specific than what uhoh quoted in the question, nor are the other sources I found. $\endgroup$
    – Hobbes
    Jan 13, 2019 at 16:34
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    $\begingroup$ @OrganicMarble Yes and no. A switching mode power supply (like all compact mobile chargers today) can be operated on DC as well. It produces the AC for the transformer internally - at a much higher frequency than mains and this allows the compact design. $\endgroup$
    – asdfex
    Jan 14, 2019 at 12:04

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