In smart grid, the scale of pole devices that monitor the health of power line is very large. Moreover, with the upgrade of smart grid, the number of these resource-constrained (in terms of memory and computation) devices is further increasing. These devices are easy targets to security attacks as they are accessible via wireless network, and use weak passwords for authentication and transferring telemetric data to the pole maintenance personnel. In this paper, we present a SCalable and Automated PAssword- CHanging protocol, SCAPACH, for unique authentication of human personnel (operator) and secure collection of telemetric data from a large number of pole devices. SCAPACH employs physical per- operator, per-pole-device information as well as changeable secret salts to generate new unique passwords and secret keys every time a pole device is accessed. Our experiments confirm that the password-changing protocol authenticates and transmits pole device data securely and in real-time under varying maintenance scenarios.