The paper presents functional safety problems of technical systems. It aims to verify the safety integrity level for a selected integrated safety system. The paper provides a comparative analysis of the safety integrity level with the application of the Monte Carlo simulation and Reliability Block Diagram (RBD) methods. Additionally, a solution is proposed to section off, from the logical structure, so called voting zones to significantly reduce the risk of causing false alarms. The calculations were preceded with a review of existing studies of functional safety in extensive technical systems. The verification was done following the requirements of IEC 61508, which is a commonly accepted standard in the field of functional safety of programmable electronic and control engineering systems. (C) 2016 Elsevier Ltd. All rights reserved.