Grounding Problems

We are implementing a voltage divider circuit to measure the three phase line voltages. This circuit will require a common ground.

We have to measure the voltage across the resistor using an ADC.

Is it advisable to use this circuit connecting the AC ground line
with the digital ground?

If not can anyone suggest a better circuit? We don't want to use transformers, its too costly as a solution.

How can we handle line transients at this point?

I have heard something about class x1/y1/y2 suppression capacitors.
Need more info on this. Please recommend a site to read also.

Thanks.

giaracam said:

Is it advisable to use this circuit connecting the AC ground line
with the digital ground?

If not can anyone suggest a better circuit? We don't want to use transformers, its too costly as a solution.

Click to expand...

No, it is not wise. I wonder if some do it for totally plastic cases and buttons, with no outputs but an LCD with no exposed metal frame, but it's still not the greatest idea. The first suggestion is transformers, which you're ruling out. So, the only way to do this is going to be to attach the ADC's digital ground to the AC ground or another phase, and use optocouplers to isolate the digital signal going to the main system. The ADC will need its own 5V power supply too.

Why not just measure voltage magnitude of one phase?
Then you know that the other phases are offset by 120 degrees.

What voltage magnitude do you want to measure?
If it is 120 V, then you can obtain a very cheap transformer, and calculate the primary voltage by using the transformer ratio.

Like Oznog said, try to isolate as much as possible. This will be safer for you and your TTL circuits.