<![CDATA[This paper presents the design, implementation, and experimental validation of a low-cost Internet-of-Things (IoT) gateway for real-time power quality (PQ) monitoring of industrial distribution panels. The proposed system acquires electrical parameters—phase voltages, line current, active power, and energy consumption—from commercially available powermeters (Schneider Electric PM5560 and Siemens SMART 7KT) via the Modbus RTU protocol over RS-485 serial links. An ESP32 DevKit V4 microcontroller serves as the gateway node, forwarding sensor data to the Google Firebase Realtime Database over Wi-Fi using the HTTPS protocol. A modular PHP-based web dashboard visualises the live and historical PQ data, generates automated daily email reports through a server-side cron job, and issues alarm notifications within 8 s of threshold exceedance. The gateway is positioned as a retrofit enabler for medium-voltage (MV) feeders that supply sustainable transportation infrastructure such as electric-vehicle chargers and urban-rail substations. Experimental validation on a live 20 kV MV distribution panel confirms a measurement accuracy of 99% relative to the Class 0.5S powermeter reference across all voltage channels. The dashboard data refresh latency is 5 s, and the Firebase Realtime Database synchronisation rate reaches 114 allowed transactions per monitoring session without data loss. The complete gateway hardware cost is under USD 4, making the approach economically viable for retrofitting legacy industrial monitoring installations]]>
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