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what are the definitions for MTBF, MTTR, MTTF especially for Micom relays?

The Mean Time Between Failure (MTBF) is the predicted elapsed time between inherent failures of a system during operation
. The MTBF can be calculated as the average time between two failures. 

The Mean Time To Repair (MTTR) is the time needed to repair a failed hardware module. In an operational system, 
repair generally means replacing a failed hardware part.  Thus, hardware MTTR could be viewed as mean time to 
replace a failed hardware module. We do not provide statistic on MTTR. It depends on many parameters not directly 
under the control of the manufacturer (availability of spares parts at customer premises, transport lead time to the 
nearest repair center, maintenance contract). To lower the MTTR, Schneider Electric strongly recommend to customers 
to purchase spare products so that a replacement can be installed quickly. Typically, when spares are available, the 
MTTR of MiCOM products is 4 hours.

The Mean Time To Failure (MTTF) is a basic measure of reliability for non-repairable systems. It is the mean
time expected until the first failure of a piece of equipment. MTTF is a statistical value and is meant to be the mean
over a long period of time and a large number of units. 

In this reliability results, we have calculated the MTTF values of the MiCOM products and we are considering that the 
MTBF equal to the MTTF. Indeed, when a product is repaired in one of our certified repair centre by hardware 
replacement, this product is equivalent to a brand new product.
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