Residual current transformers are used in combination with evaluation units for monitoring and protecting circuits. DCT series transformers form a modular residual current device (MRCD) with the DMRCD evaluation unit or a residual current monitoring device (RCM) when combined with the DRCM evaluation unit. Type A residual current transformers detect sinusoidal alternating and pulsating direct residual currents. Properties: suitable for detecting residual and fault currents of types A and AC, monitored frequency range 50 Hz - 60 Hz (type A), detection of nominal response differential and fault current 30, 100, 300, 1000 and 3000 mA, available versions with inner diameters of 20, 35, 70 and 105 mm, rated voltage of the circuit to be monitored up to 690 V, for rated currents up to 400 A, compact, robust plastic housing, easy installation, type of installation: The device is attached to load-bearing surfaces or a mounting plate using the existing screw points. Areas of application: DCT A series transformers are used in conjunction with DRCM type A series residual current monitors and DMRCD type A series modular residual current devices. Notes: The residual current transformers may only be operated as push-through transformers with insulated cable and line systems. The inner diameter of the transformer must be at least 1.5 times larger than the outer diameter of the cable(s) being passed through.