My rapsberry pi 3 b+ constantly showed the under-voltage warning, so I connected my PSU to th RPI and setted the output voltage to be around 5.5v. I measured a voltage of around 5.2v when the RPI was working. It worked well until I changed to a slightly better cable with lower impedance. Then my RPI just broke down, the red LED is still on, but the 3.3v power rail is down. The RPI now draws only about 70 mA. I checked the impedance between the 3.3v pin and the ground pin, the multimeter showed nearly 0. I cannot figure out what exactly the problem is. The voltage is not that high as far as I am concerned, the power input circuit should be able to handle that. Can anybody help me figure out what happened?
Supplying 5.5V to a Pi should not cause damage, although it is inadvisable. See https://raspberrypi.stackexchange.com/a/99653/8697
Test the voltage on Pin 1 (3.3V) - if this is absent the Pi is dead. NOTE be careful to avoid accidentally shorting 3.3V to any other pin - this is invariably fatal!
PS Measuring anything on a Pi with a multimeter on the resistance range is at best meaningless and potentially dangerous.