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Programmable Logic Controllers (PLCs) drive industrial processes critical to society, e.g., water treatment and distribution, electricity and fuel networks. Search engines (e.g., Shodan) have highlighted that Programmable Logic Controllers (PLCs) are often left exposed to the Internet, one of the main reasons being the misconfigurations of security settings. This leads to the question -- why do these misconfigurations occur and, specifically, whether usability of security controls plays a part? To date, the usability of configuring PLC security mechanisms has not been studied. We present the first investigation through a task-based study and subsequent semi-structured interviews (N=19). We explore the usability of PLC connection configurations and two key security mechanisms (i.e., access levels and user administration). We find that the use of unfamiliar labels, layouts and misleading terminology exacerbates an already complex process of configuring security mechanisms. Our results uncover various (mis-) perceptions about the security controls and how design constraints, e.g., safety and lack of regular updates (due to long term nature of such systems), provide significant challenges to realization of modern HCI and usability principles. Based on these findings, we provide design recommendations to bring usable security in industrial settings at par with its IT counterpart.