This microcontroller based device was designed to measure the true effective value (TRMS) of current and voltage in all three phases and the frequency of the system. The multimeter is able to store minimum and maxi­mum demands for both current and voltage, furthermore is capable  to show these values to the user when desired. The DTT-3 type also has programmable under and over voltage limits, under and over current limits and delay time before producing contact output. The DTT-2 type is similar with DTT-3 type but without relay output. The DTT-3 type has two different contact outputs for current and voltage fault. The connection for power supply and metering is available trough pluggable terminations on the back side of device. The multimeter displays the momentary value of the current in all three phases and the frequency of the net. The line or phase voltage can be selected with front panel pushbuttons and these momentary values are readable from the display. A LED lamp marks the selected phase. The current transformer (CT) ratio is also selectable with front panel pushbuttons.