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Protective relay technologies have evolved from single-function electromechanical and static relays to modern multifunction digital relays over the past few years. Protection systems play a vital role in maintaining a desirable level of reliability in power systems. Thus, their own reliability should be evaluated and any cause of their malfunction should be fully comprehended. In this paper, a new reliability model is proposed that associates protection system failures to four main causes: 1) relay hardware, 2) relay software, 3) ancillary equipment, and 4) human error. In addition, this model can take other aspects of the protection system into consideration, such as human error during tests and repair actions, routine inspections effectiveness, level of reliance on self-checking, local backup misoperation, and stuck breakers. These considerations have a significant impact on the reliability of protection systems and have been neglected in the existing research works. Various capabilities of the model are demonstrated in the case study section.