Normal Mode And Common Mode - Advantech ADAM-5510/P31 Manual

Have you ever tried to measure the voltage between 'Hot' and
concrete floor, or measure the voltage between 'Neutral' and concrete
floor? You will get nonsense value with above testing. 'Hot' and
'Neutral' were just a relational signal, so you will get the AC110V or
AC220V by measure those two signal. Normal mode and common
mode just show you that the 'frame ground' is the most important
reference signal for all the systems and equipments.