I've been doing some testing, with light CPU load, just web browsing, reading/answering emails, etc.
Running TPFanControl in mode 2.
CPU temp rarely goes above 40C, fan never comes on. However, in maybe 10-15 mins of use the underside of the laptop, especially immediately below the trackpad, gets quite warm. If I then put TPFanControl in BIOS mode, the fan comes on immediately at ~2500RPM (faster and louder than usual), even though the CPU temp was below 40C. I might be wrong but I think the fan comes on not for the CPU but for something else that's getting hot.
IDK I think it might be dangerous to run TPFanControl on a T520 because TPFanControl only detect the CPU temp and nothing else. It appears, to me anyway, that the fan is always on in these machines not to cool the CPU but to cool some other component.
I wonder why TPFanControl only detects CPU temps on the T520? IIRC on my T60 it detected a whole bunch of system temps.
tpfancontrol does only sees the CPU temp on T410/T410s/T510/W510 as well. Maybe the program is due for an update.