chapteR | 43 Reliability in Instrumentation and Control 755 prevent the inputs from picking up stray noise pulses. in one recorded case (Brozendale, 1989) an unused input in an interface unit of a chemical plant was not earthed as intended and picked up an induced voltage. This caused an incorrect address to be sent to the controlling computer which gave the output commands for the wrong device. The result was that a number of valves were opened wrongly, breaking a gas line and releasing a toxic gas. Since plant safety depends critically upon correctly identifying the device which needs attention, a safe system design should include more than one means of identification. The design principle is that all informa- tion exchanges between the processor and peripheral devices should have some degree of redundancy in the interests of reliable operation. Thus in addition to checking items such as addresses, it is desirable to read back into the computer all data sent to the peripheral devices so that they can be checked. it is more likely that the system will be specified as having a particular MTBF of M hours. in this case a system comprising four similar units will require each unit to have an MTBF of 4M. Where the units have differing complexi- ties and are expected to have differing MTBFs M 1 , M 2 , M 3 , and M 4 , the relation between them must be 1 1 1 1 1 = + + + M M 1 M 2 M 3 M 4 (43.17) An initial estimate of the various MTBFs can be made using a simple parts count and refined later as the design proceeds. 43.4.2 component selection Electronic components have been developed over many years to improve their performance, consistency, and reli- ability, and consequently, the less demanding reliability targets can often be attained by using widely available com- mercial components. A modest improvement in reliability can in these cases be obtained by derating the components. There still remain, however, many applications for which high reliability is demanded and which thus need compo- nents of higher and well-established reliability. Attempts to improve the reliability of electronic equip- 43.4 building high-Reliability systems 43.4.1 Reliability budgets Few electronic systems are designed for which no reliability