Yeah each diode rated 1000vAre all the diodes rated to withstand at least 400V?
how DC operated pcb's are grounded? This circuit found one of the pcb i have seen.What is the circuit for?
It makes very little sense without an explanation.
I thought that too, although the capacitive coupling to ground seems odd to me. I would've expected D2 and D3 to have direct ground connections, perhaps also with a capacitor in parallel, but not in series.In a practical circuit D2 and D3 could be TVS or Zener diodes, to suppress transients by clamping voltages.