Hello m.shoaib khatana;
A few years ago we started a thread on the same subject, and after reading ot again it looks like the information is still actual:
fail safe & fault tolerant

You could start a search yourself on the forum (look for "fail safe fault tolerant". for example, or "H FH") and locate quite a few discussions that will provide good answers to your query.
Hope this helps,
Daniel Chartier