Protect your RV?s major appliances from potential shock hazards due to improper polarity and ground wiring with the most advanced power source monitor and portable diagnostics tool on the market! Constantly monitors line RMS voltage and checks frequency. Plugs into any standard 3-pronged, 15 amp power outlet. Automatically starts, alternating between displaying RMS voltage and frequency on the LCDscreen. Features three different display modes -- alternating RMS voltage and frequency (default), RMS voltage only and frequency only. Small, rugged construction allows for convenient placement in an electrical outlet or use as a portable service tool.