# Preventing the voltage between two isolated grounds from exceeding max rating

Consider the low-side current sensing application shown in Figure 1. Grounds 1 and 2 are galvanically isolated. Assume, for example, ground 1 is the NEUTRAL line on mains power ($$\V_g\$$), and ground 2 comes from an isolated DC power supply whose power +/- terminals are both floating relative to local EARTH ground.

The current sensor IC's data sheet gives an absolute maximum rating for the working voltage $$\V_{WORKING}\$$ between the IP+/- pins and "all other pins" on the IC. (n.b. To keep things simple, the IC's power ground pin (connected to ground 2) is the only "other pin" that's shown.)

I'm wondering what techniques I could/should use, if anything, to ensure the working voltage between ground 1 and floating ground 2 ($$\|V_{12}|\$$) never exceeds $$\V_{WORKING}\$$? (And note that in other applications, grounds 1 and 2 might both be floating relative to local EARTH ground.)

Figure 1.

• You could put a TVS diode between the two grounds, perhaps. Or anything else with the proper nonlinear I-V characteristic. – Hearth Oct 24 '18 at 23:44
• connect 2 to 1 and isolate the output. – Jasen Oct 25 '18 at 0:29
• Regarding "connect 2 to 1 and isolate the output", that solution merely "kicks the can down the road". I now need to deal with the potential difference (voltage) between the combined 1|2 ground and galvanically isolated ground #3 on the other side of the isolation barrier. – Jim Fischer Oct 25 '18 at 3:26