1. R1 picked-up & held by its self-holding contact. 2. R2 is picked-up via C1 & Fwd-biased D & held by its self-holding contact C2 3. If now C1 is opened only R1 will open but R2 will remain held by its self-holding contact C2. 4. Now D is reverse-biased so R1 cannot pick-up via C2 & D. 5. Now if D fails it is not the hi-current as no fwd-current is flowing thru it, but it will be the Reverse-Break-Down of D as it is not rated to block the suppliy voltage. 6. Instead of replacing it with a higher-current rated diode but with a higher-voltage PIV rated diode. 7. You should know that diodes for almost 50v to 1200v are made in the same package size, like 1N-4000 series comes 4001 <100v piv> to 4012 <1200v piv>
