TDD1 series modular digital voltmeter can be used for voltage indication in various electrical occasions. Single-phase indication and three-phase indication are available. The voltmeter adopts modular design, which can be installed together with other modular products. The items are suitable for sine wave 50/60Hz AC power supply and conform to IEC60947-4-1. The products are forbidden to be used in an electrical environment with strong frequency conversion, high frequency, inverter and harmonic interference signals.