In Reply to: Signal ground v.s. chasis earth posted by toysl on 05/30/00 at 8:06 PM:
There is a big difference between signal and earth ground especially in analog circuits and high speed digital circuits as well.
The earth (sometimes referred to as third wire ground) and signal grounds can be separate or common depending on the circuit's needs. In most all medical product designs the circuit ground is isloated from earth ground to protect the patient from potentially harmful third wire (earth) currents. In high end test equipment and professional/broadcast audio and video equipment grounds are isolated to prevent ground loops from occurring.
As to your question as to why MOV's and/tranzorbs are taken to earth ground, the MOV is placed between the AC line and earth ground to cause high peak currents that occur when the line voltage surges above the rating of the MOV to take the shortest path back to the source. You wouldn't want these high currents sharing a retun path with low level circuit currents as they would cause noise and possible circuit destruction. If properly utilized 3 MOV's are placed across the line itself and to earth ground to provide maximum protection. In some homes and buildings the AC neutral line is not tied to earth ground in close proximity to the outlet and this is where the 3 MOV topology pays off.
Tranzorbs are deployed in low voltage Ac and DC circuits and are located between circuit and circuit ground for "localized" protection. You want to use these where auxilliary devices may be plugged in by a user of the product to prevent static discharge from destroying circuitry. From headphone jacks to EEG input jacks tranzorbs are totally transparent to the circuit until their threshold is exceeded by a transient voltage then they instantly clamp to their rated voltage which should be within the supply rails of the protected circuit.
Phew ! too much coffee. Good luck