## 1 Introduction  
Traditionally, applications in industrial environments are based on wired communication solutions [1]. However, recently, the industry has shown interest in moving part of the communication infrastructure from a wired to a wireless environment, in order to reduce costs related with installation, maintenance and scalability of the applications. In this context, Wireless Sensor Networks (WSN) actually represent the best candidate to be adopted as the communication solution for the last mile connection in process monitoring and control applications in industrial environments [2]. Among many advantages, the absence of a wired infrastructure enables WSN to extract information in a simpler way than traditional monitoring and instrumentation techniques [3]. Industrial applications have usually stringent dependability (reliability and availability) requirements, since faults may lead to system failures which can result in economic losses, environmental damage or hurting people [4; 5]. In this context, we can classify faults as transient or permanent [6]. Transient faults usually affect communication links between devices and are caused by noise or electromagnetic interferences. Permanent faults affect network devices and have their origin in hardware malfunctions. After a permanent fault a device is considered (permanently) failed, and to become operational again a repair activity is necessary. In this paper we focus on permanent faults that affect network devices leading to its failure (note: the failure modes of a failed element become the fault types for the elements interacting with it [6]). Permanent faults have, typically, a major impact on the system operation [7]. Their immediate consequence is that communications with the affected device are no longer possible. However, in worst case situations various network devices can become isolated, as when a network device that acts as a router fails. As a result, the control algorithm is disturbed which may lead to a system failure with serious consequences. The use of a methodology to evaluate the dependability requirements of a WSN can anticipate decisions regarding the topology, criticality of the devices, levels of redundancy and network robustness, that can be used to take decisions during the system life-cycle, and particulary, on early planning and design phases. For example, depending on the topology, alternative paths to the sink can be created improving the overall reliability of the network. In the same way, if a sensitivity analysis is supported, critical devices can be identified and decisions about different redundancy approaches can be taken. The main contribution of this paper is to propose a methodology to evaluate the reliability and availability of Wireless Sensor Networks in industrial environments that are subject to permanent faults on network devices. The approach is based on Fault Tree Analysis (FTA), which is a technique used to obtain the probability of occurrence of an undesired state or event [8]. In the addressed case, the undesired event is related to the failure of a specific device or group of devices. A device is considered to be faulty if it suffers a permanent failure or if there is not any route to sink that includes the device. The proposal addresses several aspects, being very flexible and able to be easily adapted to different kinds of scenarios.