Most of the recently occurred cascaded outages are resulted due to zone-3 mal-operations of distance relays. This paper introduces some new sensitivity factors to identify and monitor vulnerable relays functioning in the power transmission network. This information will help in enhancing the back-up protection operation of the power transmission system. To accomplish this task, indices such as line-outage-induced relay margin shift factor and generation-outage-induced relay margin shift factor are defined. An offline analysis of the power system is made to calculate the above two factors. This information helps in ranking the relays of the power system in terms of their vulnerability. Once the vulnerable relays are identified, their zone-3 operation is supervised by a new wide area information-based event detection logic. IEEE 9-bus and IEEE 39-bus test systems are used to validate the proposed scheme. The test results indicate that the proposed scheme can enhance the back-up protection operation of transmission network, which is very much essential to mitigate blackouts. © 2019 IEEE.