AS IEC 61078:2017 pdf free download.Reliability block diagrams.
AS IEC 61078 describes:
• the requirements to apply when reliability block diagrams (RBDs) are used in dependability analysis;
• the procedures for modelling the dependability ol a system with reliability block diagrams;
• how to use RBDs for qualitative and quantitative analysis;
• the procedures for using the RBD model to calculate availability, failure frequency and reliability measures for different types of systems with constant (or time dependent) probabilities of blocks success/failure, and for non-repaired blocks or repaired blocks;
• some theoretical aspects and limitations in performing calculations for availability, failure frequency and reliability measures;
• the relationships with fault tree analysis (see IEC 61025 111) and Markov techniques (see IEC 61165 (2J).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC 60050-192, International Electrotechnical Vocabulary — Part 192: Dependability (available at http://www.electropedia.org)
IEC 61703, Mathematical expressicrns br reliability, availability, maintainability and maintenance support terms
3 Terms and definitIons
For the purposes of this document, the terms and definitions given in IEC 60050-192 as well
as the following apply.
NOTE Some terms have been taken Irom IEC 60050-1 92 and modified tot the needs of this standard.
3.1 reliability block diagram RBD logical, graphical representation of a system showing how the success states of its sub-items (represented by blocks) and combinations thereof, affect system success state.
3.2
Boolean related model
mathematical model where the state of a system is represented by a logical function of
Boolean variables representing the states of its components
Note I to entry: A Boolean variable a only has two values and a logical function of several Boolean variables also has only two values. Those two values may be for example. (0, 1), (up, down). (true. false). (working, failed), etc. The underlying mathematics behind the logical functions is Boolean algebra.
3.3 RBD driven Markov process Markov process modelled by an RBD made of blocks modelled by individual sub-Markov models behaving independently from each other
Note 1 to entry: The underlying logic of an RBD allows to combine the individual availabilities of the blocks to obtain the system availability. When the block are modelled by small individual Markov processes (e.g. with less than 10 states) the RBD is equivalent to the Markov process related to the system which may encompass millions of states. This is the basis for most of the probabilistic calculations achieved with RBDs. Such Markov process built through the use of the RBD as guideline is called RBD driven Markov process.
Note 2 to entry: The independent Markov process is developed in (2J.
3.4 dynamic RBD DRBD reliability block diagram where the assumption of independency between the blocks is not fulfilled
Note 1 to entry: The blocks of a DRBD can have interactions w,th elements external to the RBD itself.
3.5 non-coherent RBD reliability block diagram modelling a non-monotonic logical function
5 Preliminary considerations, main assumptions, and limitations
5.1 General considerations
An RBD models a system using the logical links existing between the success state (up state) of the system (i.e., the overall RBD) and the success states (up states) of its components (i.e., the blocks of the RBD). Therefore, an RBD embeds a logical formula and this is why an RBD is not necessarily similar to the physical architecture of the system (e.g. two redundant isolation valves in series on the same pipe are represented by two blocks in parallel into the corresponding RBD).
An RBD can be firstly used for qualitative analysis purposes by identifying the combinations of the blocks in the up state allowing the system to be in an up state (success paths or tie sets) or the combinations of the blocks being in down states leading to the system down state (failure paths or cut sets).
Secondly an RBD can be used for probabilistic calculations and, as this is a static representation (i.e., independent of the time), the probabilistic rules are basically related to blocks with constant probabilities of success or failure.
This can be extended to time dependent probabilistic calculations. This may be difficult for reliability calculations but. for availability and frequency calculations and provided that the blocks behave independently from each other, there is no restriction, other than mathematical tractability, on the distribution that may be used to describe the times to failure or repair of the blocks. This allows, for example, to model the (un)availabilities of each of the blocks by individual analytical formulae whose results are combined through the logic of the RBD to obtain the system (un)availability. When those analytical formulae are obtained through individual Markov processes, the RBD is equivalent to a global Markov process modelling the whole system. Such a model is called RBD driven Markov process. This is the basis for most of the probabilistic calculations achieved with RBDs.
5.2 Pre-requisite/main assumptions
An RBD is an acyclic directed graph (i.e. no loops or retroactions are modelled in an RBD) which can be drawn by using the basic logical structures presented in Table 3. It is used to model the behaviour of a system on the basis of the following fundamental assumptions:
a) the system has only two states: working (success state. up” state) or failed (down state);
b) the blocks of an RBD model the components of a system or parts (e.g. groups of components) of a system. Each of them has only two states: working (success state, up” state) or failed (down state):
c) the RBD represents the logic linking the success state of the system to the success states of its components (blocks);
d) each block behaves independently from the others at all times.
The above assumptions have to be generally fulfilled to apply the analytical calculations (i.e. calculations with formulae) developed in this standard. When they are not fulfilled, the analytical calculations can be replaced by Monte Carlo simulation or other techniques like Markov analysis [2J or Petri nets [31 or the dynamic RBDs described in 12.2 and Annex E.AS IEC 61078 pdf free download.