This article evaluates the performance of a signal similarity index (SSI) based relaying scheme for fault detection in lines connected to inverter-based resources (IBRs). During fault events, the IBRs respond differently than conventional synchronous machine based sources. Due to various control schemes and power electronic protection implemented for IBRs, fault current is limited leading to failure in fault detection and isolation by traditional protection methods. In this article, six different SSIs are calculated using time-domain current signals at both the ends of the protected line and the performance of SSI-based line protection philosophy is evaluated for various operating conditions. The relaying schemes are extensively tested on two different test systems. The test results are utilized to perform a comparative assessment of the SSI-based protection scheme. The tradeoff between the performance of the relaying scheme based on processing time, fault resistance, threshold selection, presence of noise and presence of missing data are demonstrated thoroughly on three different test systems. © 2007-2012 IEEE.