For calibration of the environmental monitoring field meters, a reference standard of electric field strength based on parallel plates is established at the Faculty of Electrical Engineering and Computing, Primary Electromagnetic Laboratory (FER-PEL), Zagreb, Croatia. The plate system incorporates guard ring electrodes with a set potential distribution to improve field uniformity within the center calibration volume. In addition, the concept of the Kelvin guard-ring electrode applied to one of the plates allows the measurement of the displacement current through the guarded plate system. The reference electric field strength is derived from the nondimensional field distribution factor, calculated from the ratios of the system’s quantities, and the measured voltage and capacitance of the plates. The elaborate uncertainty budget confirms expanded uncertainty of 1.2% for generating low-frequency reference fields up to 20 kV/m, with exclusively electrical traceability.