Protective relays are designed to protect zones of the power system. When applying protective relaying, the power system is divided into sections so that the protective relays provide “zones of protection.”
Sometimes it is common for zones of protection to overlap so that multiple layers of protection are afforded to each piece of equipment. This points to the idea of primary and secondary (backup) protection.
Overlapping and backup protection is implemented to avoid the possibility of unprotected areas, especially for critical equipment. This is accomplished by the strategic placement of the instrument transformers (current transformers or potential transformers). Otherwise, simple redundancy of the protective relay scheme provides backup protection.