I always hated the situations where people are sacked early on... I have seen some of the best guys get sacked in a few months just cause management changes their minds and or actually realizes what top notch people cost in salary.
Could it be because of your design.. probably not.
However the...
Obviously if "the operator is knocking off the DC supply to the coil and the contacts are breaking with load" arcing can occur.
Have you checked the voltage going to the coils of the contactors?
With low coil voltage they will be more prone to contact bounce, which won't help. For example I've...
I agree with sreid however, if cost is your main priority AND if the relay would not be operated very much over your intended life cycle... Then a relay still could work.
As for you question "Is a snubber necessary to protect the relay's 300V maximum contact voltage?" when switching inductive...
