Automation intelligence Enlighten your automation mind..! Issue 01

Friends, It brings me immense pleasure to introduce “Automation intelligence” a knowledge series about automation, innovation, solutions & Technology. This series will discuss about PLC, DCS, Drives, SCADA, HMI, Historian, solutions and everything about Automation. Today in this first issue of Automation intelligence we will discuss about commonly used terminology of “AVAILABILITY”.

Most of the time we read about system availability of 99.999%, but do we all know what exactly it stands for? Let’s explore it then.

Availability In simple words is "Produce in spite of everything.” The term High Availability is often used when discussing fault Tolerant Systems.

Did You Know? A Fault Tolerant System usually refers to a system that can operate even though a hardware component becomes inoperative. The Redundancy principle is often used to implement a Fault Tolerant Systems, because an alternate component takes over the task transparently.

For example, your telephone line is supposed to offer you a high level of availability: the service you are paying for has to be effectively accessible and dependable. Your line availability is compared related to the continuity of the service which you are provided. As an example, assume you are living in a remote area with occasional violent storms. Because of your location and the damage these storms can cause, long delays are required to fix your line once it is out of order. In these conditions, if on average your line appears to be usable only 50% of the time, you have poor availability. By contrast, if on average each of your attempts is 100% satisfied, then your line has high availability. This example demonstrates that Availability is the key metric to measuring a system’s tolerance level, that it is typically expressed in percent (for example 99.999%), and that it belongs to the domain of probability.

Automation intelligence Enlighten your automation mind..! Issue 01

Some Basics about Availability:MUT: Mean Up Time MUT qualifies the average duration of the system being in operational state.

MDT: Mean Down Time MDT qualifies the average duration of the system not being in operational state. It comprises the different portions of time required to subsequently detect the error, fix it, and restore the system to its operational state.

MTBF: Mean Time between Failures MTBF is average time interval (normally specified in hours) between two consecutive occurrences of inoperative states.

MTTF (or MTTFF): Mean Time to First Failure MTTF is the mean time before the occurrence of the first failure.

MTTF (and MTBF by extension) is often confused with Useful Life, even though these two concepts are not related in any way. For example, a battery may have a Useful Life of 4 hours and have a MTTF of 100,000 hours. These figures indicate that for a population of 100,000 batteries, there will be approximately one battery failure every hour (defective batteries being replaced).

Considering a repairable system with an exponential distribution Reliability and a constant Failure Rate (λ), MTTF = 1 / (λ) Mean down Time is usually very low compared to mean up Time. This equivalence is extended to MTBF, and assimilated to MTTF, resulting in the following relationship: MTBF = 1 / (λ)

Automation intelligence Enlighten your automation mind..! Issue 01

This relationship is widely used in additional calculations. Example: Given the MTBF of a communication adapter, 618,191 hours, what is the probability for that module to operate without failure for 5 years? Calculate the module Reliability over a 5-year time period:

a) Divide by 5 years that is 8,760 * 5 = 43,800 hours, by the given MTBF: 43,800 / 618,191 = 0.07085 b) Then raise e to the power of the negative value of that number:

Thus there is a 93.16% probability that the communication module will not fail on a 5year period.

FIT: Failures in Time Typically used as the Failure Rate measurement for non- repairable electronic components, FIT is the number of failures in one billion hours.

Did You Know? In one year we have 8760 hours. Simply 365 days X 24 Hours= 8760

Automation intelligence Enlighten your automation mind..! Issue 01

Various calculations of Availability Instantaneous Availability -is the probability that this device will be in the functional state for which it was designed, under given conditions and at a given time (t), with the assumption that the required external conditions are met. Asymptotic (or Steady State) Availability: is the limit of the instantaneous availability function as time approaches infinity,

Availability Intrinsic (or Inherent) Availability: Ai Intrinsic Availability does not include administrative time and logistic time, and usually does not include preventive maintenance time. This is primarily a function of the basic equipment/system design.

Operational Availability: corresponds to the probability that an item will operate satisfactorily at a given point in time when used in an actual or realistic operating and support environment.

Operational Availability includes logistics time, ready time, and waiting or administrative downtime, and both preventive and corrective maintenance downtime. This is the availability that the user actually experiences. It is essentially the a posteriori availability based on actual events that happened to the system.

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Automation intelligence Enlighten your automation mind..! Issue 01

Classification A quite common may used to classify a system in terms of Availability consists of listing the number of 9s of its availability figure. The following table defines types of availability:

For example, a system that has a five-nine availability rating means that the system is 99.999 % available; with a system downtime of approximately 5.26 minutes per year.

So, we learned about some basic of System availability and how to calculate the same. In next issue of Automation Intelligence we will bring you some basics on “Safety”.

For giving any valuable inputs please feel free to contact Pradeep Ghumre Contact No. 9820332250 Email ID:- [email protected] Schneider Electric India Private Limited, th th 1141&1142, 4 Floor, 11 Building, Solitaire corporate park, Chakala, Mumbai 400059. Office Contact:- 022-3940 4000