The is an followed up to my queries on "devices for circuit protection."
I often see over-voltage protection devices (for e.g. MOV, Transzorb, TVS, Zener) being connected between the signal lines and chasis earth, or signal line and signal ground. Just wondering what is the different between the two? For e.g., if we have a RS485 line running from one building to another, should be protect the RS485 port by having the OV protection between signal line and chasis earth or signal ground? What is their different? What if the two buildings are connected to two different power phase?
Furthermore, we know that voltage at the signal line is always measured with respect to the signal ground, and not the chasis earth. As such, if we connect over-voltage protection devices between signal line and chasis ground, how could we make sure that the over-voltage protection devices is not falsely turn on by the potential difference between signal line and chasis earth. On the other hand, if the potential different between signal line and chasis earth is too small, can the same over-voltage protection device be used?