I suggest the book of "Effective Maintenance Management", it might be useful for you:

https://books.google.co.jp/books?id=39bTcOYqKFwC&pg=PA27&lpg=PA27&dq=production+lines+with+standby+equipment&source=bl&ots=VI_sQnJw7R&sig=BHKNZqXF8AqJBnNS4uo7L8ARjhY&hl=en&sa=X&ved=0ahUKEwi3hpLUqcjSAhWKbbwKHZepBxAQ6AEINzAI#v=onepage&q=production%20lines%20with%20standby%20equipment&f=false

This is a case of standby redundancy. Any reliability engineering book touches the topic and explains how to calculate key RE metrics impacting availability. You can start with these notes to get an overview of standby redundancy:

http://ieeexplore.ieee.org/abstract/document/4335434/

http://www.weibull.com/hotwire/issue21/relbasics21.htm

http://www.ni.com/white-paper/6874/en/

The fields of industrial engineering and mechanical engineering offer handbooks with sections on reliability and statistical methods to evaluate and improve equipment. Mean time between failure statistics, and failure mode analysis are some of the best known methods. In general every equipment failure is assigned a cause if possible, or otherwise assigned to a statistical method. Continuous improvement programs are popular and employ a lot of people. Standby equipment is often provided to achieve a percentage target for availability and in service operation. It can get as detailed as you wish with reliability and life cycle data from manufacturers for each component and assembly going into a calculation model.