Are normally open contactors virtually safe from failing closed?
EDIT - the original question referred to relays and not contactors. It might be confusing to see the question now referring to contactors with my answer suggesting the use of a contactor!

People normally use a contactor for this sort of job: -



Found here. Here's what wiki says about contactors and relays: -


  Unlike general-purpose relays, contactors are designed to be directly
  connected to high-current load devices. Relays tend to be of lower
  capacity and are usually designed for both normally closed and
  normally open applications. Devices switching more than 15 amperes or
  in circuits rated more than a few kilowatts are usually called
  contactors. Apart from optional auxiliary low current contacts,
  contactors are almost exclusively fitted with normally open ("form A")
  contacts. Unlike relays, contactors are designed with features to
  control and suppress the arc produced when interrupting heavy motor
  currents.


Here is what Rockwell say about safety contactors (you may decide on this as a preferred product): -


  Safety Contactors and 700S Safety Control Relays provide mechanically
  linked, positively-guided contacts up to 97 A which are required in
  feedback circuits for modern safety applications. 100S-D Safety
  Contactors provide safe isolation of hazardous motion loads, using
  mirror contact performance. Mirror contacts provide reliable
  indication about the open or closed status of the main power poles.


And finally, the EU machinery directive implies that hazards be assessed and appropriate safety measures taken. The impact of this is that equipment/installations can be categorized with a letter/number and appropriate equipment purchased to maintain that level of safety. I would recommend that you look into this. Here is a website related to safety relays that discusses the relevant safety categories. Incidentally you may need to use a category 3 contactor.