BS EN 61703 pdf free download

BS EN 61703-2016 pdf free download.Mathematical expressions for reliability, availability, maintainability and maintenance support terms.
NOTE In text books on the subject of reliability, the acronym MTBF is often used for the mean time between failures. According to IEC 60050-192, this acronym is used for the mean “operating time between failures. Therefore to avoid any confusion, the acronym METBF (see 3.3) is used in this standard for the mean time between failures.
5.2 Introduction to models and assumptions
In order to derive correct mathematical expressions for selected dependability measures defined in lEG 60050-192, distinction is made between items considered as a whole (individual items) and items made of several individual items (systems). The following classes of items are considered separately in this standard:
• Systems;
• Individual items:
— non-repairable items [192-01-12];
— repairable items [192-01-11]:
i) with zero time to restoration;
ii) with non-zero time to restoration.
NOTE 1 The term item” is used in this standard when it applies to systems and individual items.
BS EN 61703:2016
—20— IEC 61703:2016 © IEC 2016
NOTE 2 The term “non-repairable” item encompasses items which are either not repairable or are repairable but are not repaired when failed. The term “repairabletm item encompasses only those items which are repairable and actually repaired when failed.
In order to keep the standard self-contained and the mathematical formulae as simple as possible, the following basic mathematical models are used to quantify dependability measures:
— state-transition models for systems:
— renewal processes for individual items.
The general way to model an item is to identify its various states and to analyse how it goes from state to state when the time is elapsing: this can be done by using state-transition models. Such models are useful in illustrating the development of the mathematical expressions for various dependability measures. When no particular assumptions are made regarding the probabilistic distributions, these models can be handled only in particular and simple cases using analytical calculations. Otherwise Monte Carlo simulation needs to be used. This is why, for systems, as well as individual items, the hypothesis of constant transition rates, is often made in order to use Markovian models which are well known and for which powerful analytical algorithms are available.
For individual items, under the assumption that they have only two states, the general formulae with non-constant transition rates can be derived to some extent. This is shown in 6.2, 6.3 and 6.4:
— non-repairable individual items: this is the simplest mathematical model as only one random variable applies: the time to failure of the item [1 92-05-01]. It can be characterized by its reliability R(t), its instantaneous failure rate, 2(i) [192-05-06], also referred to as the hazard rate associated with the time to failure distribution (i.e. the unreliability function) or its mean time to failure MTTF [192-05-1 1] which is also its mean time to first failure MTTFF [1 92-05-12].
— repairable individual items: the basic model is a simple (ordinary) renewal process, when the time to restoration of the item may be neglected, or a simple (ordinary) alternating renewal process in which the time to restoration of the item is non-zero. In the latter case, the item alternates between an up state and a down state, and, in addition to the common dependability measures, a widely used measure of reliability of the item is the failure intensity [1 92-05-08], which, in this case, is equal to the renewal density.
To avoid improper use of these mathematical expressions, which could yield erroneous results, the specific assumptions detailed in 5.4 and 5.5 should be observed.
In order to allow for easy comprehension, some definitions have been repeated in different parts of this standard, as needed.
5.3 State-transition approach
A simple way to introduce the dependability concepts is to analyse the behaviour of an item (i.e. an individual item or a system) during its evolution between up and down states.
This can be done by using a state-transition graph such as the one presented in the right hand side of Figure 4. It is related to the system represented by the reliability block diagram (see IEC 61078 [25]) on the left hand side of the figure. This system is made of three similar blocks (A,) organized in a two out of three architecture and of one block B in standby position which immediately takes over the function of the first of the blocks A. going to its down state. The switching between A, and B is considered to be perfect (i.e. instantaneous and without failure).BS EN 61703 pld download.