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Lifetime of a wireless sensor network (WSN) is one of the most critical issues in WSNs due to limited energy resources. Earlier research results reveal that significant improvement in WSN lifetime can be achieved by making WSNs redundant by adding spare nodes. The passive (switched off) spares are ready to become active (be switched on) when any active WSN node uses up its energy. This results in extending WSN lifetime. Various strategies for managing spares optimally have been proposed. We propose the LEACH-SM protocol, which modifies the prominent LEACH protocol by enhancing it with an efficient management of spares. One of the spare management features in LEACH-SM is adding the spare selection phase to LEACH. During this phase, each node decides in parallel whether it should become an active primary node, or a passive spare node. The nodes that become spares go asleep, while the WSN as the whole maintains the required above-threshold target coverage. (The spares are awakened when the probability that any primary node exhausted its energy reaches a predefined value.) Identification of spares alone increases energy efficiency for WSNs, thus extending WSN lifetime (other WSN lifetime extension techniques from LEACH-SM are beyond the scope of this paper).