Quality in Design - Medical Device Design: Innovation from Concept to Market - page 144

Table 7.15: Standard Fault Tree Symbols
Basic event:
An independent fault event.
Combined event:
A combination of events leads to this.
AND logic:
All inputs to this are required for there to be a failure leading to a combination event.
OR logic:
Any of the inputs lead to a failure leading to a combination event.
However this diagram in Figure 7.11 is too simplistic for a detailed analysis as sometimes
things compound together to cause a failure, and sometimes there are basic events that are the
root cause. To accommodate for this we have a list of symbols, as in Table 7.15 .
If we redraw Figure 7.11 to accommodate these symbols we get the diagram shown in Figure 7.12 .
We can use this analysis to develop statistics about the system. This can be horrendously complex
and reliability engineers spend a lifetime learning the intricacies. For the purposes of example, let
us examine the path laid out in gray. We need to determine the probability of there being a local
Internet interruption due to this failure branch. Assuming the individual failures are independent,
it is much like the chances of tossing a dice twice and getting two sixes, one after the other.
The chance of getting two sixes are
1st throw 1/6
2nd throw 1/6
P = 1/6 × 1/6 = 1/36
Hence there is a 1/36 chance of getting two sixes.
How many times have you used a computer only to find someone has fiddled with the Internet
connection? Let us assume this is a 1/1000 chance. Therefore the probability of this occurring is
1/1000. If this was the only fault then the overall probability of the system failing is 1/1000. But
there are another three potential reasons for loss of Internet connection. These are all based on an