@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?
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.
Information courtesy Steven Lee.
Table 4.1. Dates and Effects of Curiosity MMRTG Soft Shorts to sol 1582.