Reliability of mining equipment is influenced by different operational and environmental risk factors. However, in practice, including all relevant factors in reliability analysis is not possible. Ignoring the risk factors leads to unobserved heterogeneity and biased estimation of reliability characteristics. This research proposes a semi-parametric mixed-effects model to assess the simultaneous effects of observed and unobserved risk factors on reliability. Furthermore, the model can be used for investigating inter-cluster dependency and variability. To illustrate the model, a comprehensive case study is presented using data from three mines in Iran. The results show that the model can address the effect of unobserved risk factors. Moreover, it is found that the hazard of dump trucks is significantly associated with operator skill, season and the elevation difference between dumping and loading points.