System integrity protection schemes (SIPS) are a widely used solution to the challenges in operating electrical power transmission systems during the last few decades. Since these protection schemes have become an integral part of the system, it must be ensured that their performance satisfies the reliability requirements of electrical utilities, when expressed in terms of dependability and security. This study proposes a method based on Markov modelling and fault tree analysis for assessing the reliability of a generic SIPS, but it is illustrated using the Dinorwig intertrip scheme, located in North Wales and operated by National Grid (Great Britain system operator). In addition, two reliability indices, widely used in the process control industry, are suggested for quantifying the reliability level of SIPS: (i) safety integrity level and (ii) spurious trip level. Many operators tend to have SIPS permanently in service; this reduces the probability of a 'failure to operate' because of a problem in the arming software or an error by a human operator that prevented the scheme being armed when required. Therefore, the impact of having SIPS always armed on SIPS reliability is compared with the impact of switching IN the scheme only when the arming conditions are fulfilled.