The high penetration of electronically coupled distributed energy resources (DERs) rapidly changes the landscape of the emerging distribution systems. The fault currents of these DERs are limited to no more than two times their rated values, after a short sub-transient period. However, during the sub-transient period, these currents are not limited and can reach up to five times their rated values. This phenomenon has been neglected in the recently proposed relay protection methods, which can cause erroneous setting of the protective devices and consequently endanger the safety of the entire system. This paper discusses challenges to relay protection setting caused by the neglection of the sub-transient fault currents of the electronically coupled DERs, and proposes directions for future research to overcome these urgent challenges.