The TRC AECM20020 Elektra Check Digital RMS Voltage and Frequency Meter is a compact, easy to use instrument is made with advanced microelectronics and provides accurate testing and monitoring of electrical outlets and systems. Constantly monitors voltage and frequency indicating potential improper polarity and ground wiring. Its small size and rugged construction allows for convenient installation in an electrical outlet or use as a portable service tool. Voltage is measured as RMS which is much more precise than AC average. RMS metering is the only method that can accurately measure all AC power sources. The RMS voltage and frequency readings are ideal for monitoring modified sine waves such as solid-state generators, inverters and Uninterruptible Power Supply (UPS) products. The Digital Monitor display will alternate between line voltage in RMS volts and line frequency in Hertz. The LCD display shows an "F" in front of the reading to denote frequency.