A live zero is a loop signal where the zero value is some number higher than zero. 4-20 mADC, 1-5 VDC, 10-50mV, etc., are all examples of live zero. A dead zero is where the zero value of the signal is itself the zero value of the loop. 0-20 mADC, 0-100 mv, 0-5 vdc...etc.
The significant advantage of a live zero, which is why 4-20 mADC became the international analog signal standard, is that you can easily detect a broken wire or a failed power supply by seeing that the loop reading is zero.
https://control.com/forums/threads/what-is-live-zero-and-dea...The well established 4–20 mA current range is used to differentiate live zero and dead zero. A live zero is indicated by 4 mA and a dead zero by 0 mA. If the control signal is itself zero, it is live zero (4mA). Under fault conditions, the electronic signal would fall to zero (dead zero).
https://www.quora.com/Why-is-4to20mA-called-life-zero-and-de...