A Low-Cost ESP32-Based Device for Real-Time Voltage Monitoring and Automatic Load Protection in Residential Electrical Installations

Residential electrical installations that deviate from wiring standards or lack preventive maintenance are a frequent source of equipment damage and fire, and appliances are especially vulnerable to out-of-band voltage when the supply is restored after an outage, while existing low-cost monitors emphasize metering or cloud dashboards rather than fast local disconnection tied to a recognized voltage band. This study presents an affordable device, installed at the main distribution board, that measures per-phase voltage and current, disconnects loads when the voltage leaves a safe band, and alerts the user remotely. Mains signals were conditioned for an ESP32 microcontroller using a resistive divider with a DC offset for voltage and a shunt resistor for current, two relays interrupt the R and S phases, and the dual-core firmware separates metering and load control from telemetry over Wi-Fi and Telegram. The safe band was derived from NBR 5410 and PRODIST Module 8, and reliability was assessed against an INMETRO-traceable calibration bench across three ranges with twenty readings each. Voltage and current mean errors stayed below 3%, falling under 1% at higher amplitudes, whereas the power-factor error rose to 12% near 0.5, limiting energy metering but not protection. The device disconnected out-of-band voltage and reconnected only after sixty consecutive valid readings, with a response time near 1.2 s, meeting the NBR 5410 switching requirement, and a field installation confirmed stable monitoring, connectivity, and alerting. Standard-referenced protection and accessible telemetry were thus integrated into a single low-cost unit, validated against a traceable reference and deployed under real conditions.