Maximum tolerable failure rate

SET MAXIMUM TOLERABLE FAILURE RATES by carrying out a quantified risk assessment based on a maximum tolerable probability of death or injury, arising from the event in question. This is dealt with in the next Chapter and takes into account how many simultaneous risks to which one is exposed in the same place, the number of fatalities and so on. 

The question arises of how long an individual is exposed to a risk. Earlier practice has been to factor the maximum tolerable failure rate by the proportion of time it offers the risk (for example, an enclosure which is only visited 2 hrs per week). However, that approach would only be valid if persons (on-site) suffered no other risk outside that 2 hrs of his/her week. In case of off-site, the argument might be different in that persons may well only be at risk for a proportion of the time. Thus, for on-site personnel, the proportion of employee exposure time should be taken as the total working proportion of the week. 

The maximum tolerable failure rate is then targeted by taking the Maximum Tolerable Risk and factoring it according to the items assessed. Thus, for the examples given in Table 2.4 (assuming a 10 pa involuntary risk) ... 

Meeting lEC 61508 Part 1 29 Table 2.4 Factors leading to the maximum tolerable failure rate. 

From Table 2.2, the Maximum Tolerable Risk is 10 pa. Thus, the maximum tolerable failure rate (leading to the event) is calculated as ... 

Thus, 2.4 X 10 pa, being the more stringent of the two, is taken as the maximum tolerable failure rate target. 

As a simple example of selecting an appropriate SIL, assume that the maximum tolerable frequency for an involuntary risk scenario (e.g., customer killed by explosion) is 10 pa (A) (see Table 2.1). Assume that 10 (B) of the hazardous events in question lead to fatality. Thus the maximum tolerable failure rate for the hazardous event will be C = A/B = 10 pa. Assume that a fault tree analysis predicts that the unprotected process is only likely to achieve a failure rate of 2 x 10 pa (D) (i.e., 1/5 years). The FAILURE ON DEMAND of the safety system would need to be E = C/D =10 column of Table 1.1, SIL 2 is applicable. 

It follows that the target maximum tolerable failure rate for the hazardous event can be calculated as 10 x 400 = 4 x 10 pa (i.e., 1/250 years). This is 4.6 x 10 per hour when expressed in units of per hour for the purpose of Table 1.1. 

A maximum tolerable failure rate of 5.3 x 10 pa is taken as an example. Assume that the frequency of causes (i.e., gate Gl) is 10 pa. Thus the target PFD associated with gate G2 becomes ... 

It becomes necessary to regard the whole of the system as a single safety-related system. It thus becomes a high-demand system with a Maximum Tolerable Failure Rate (see Chapter 11) of 10 pa. This is at the far limit of SIL 4 and is, of course, quite unacceptable. Thus an alternative design would be called for. 

Maximum Tolerable Failure Rate Involving Alternative Propagations to Fatality... 

The maximum tolerable failure rate for the product is therefore ... 

Assuming a maximum tolerable risk of 10 pa for this involuntary publie risk, the maximum tolerable failure rate for the mitigating effeet of the Junetion Gates is ... 

Maximum tolerable failure rate leading to single fatality is 10 pa/10 = 10 pa ... 

Thus target maximum tolerable failure rate = 10 pa/5 x 10 = 2 x 10 pa ... 

Target maximum tolerable failures rates (or PFDs). 

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