Description
This Czech-language technical publication explains the conditions for using overcurrent protective devices for protection by automatic disconnection of the supply. Since 1 February 1996, the ČSN 34 1010 standard has been fully replaced by the ČSN 33 2000 series, making the tabular overviews of maximum permissible loop impedance values previously prepared by many electrical and inspection technicians unusable under ČSN 33 2000-4-41. The ESČ 33.01.96 recommendation sets out tripping currents Ia for various types and characteristics of protective devices, including B, C, D, L and U, together with the corresponding loop impedances Zs, protective-conductor impedances ZPE and earthing resistances RA. These values are given for different disconnection times and for TN, TT and IT systems, so that the conditions for automatic disconnection of the supply are met. The publication also reflects the newer approach to determining loop impedance, which takes account of conductor heating and the impedance of the supply network. It provides informative values for cables, conductors and 22/0.4 kV transformers, allowing electrical designers to assess whether the requirements of ČSN 33 2000-4-41 are fulfilled or whether other protective measures, such as a residual-current device, are needed. An appendix contains tabulated Zs values for currents Ia from 1 to 2,500 A in 230 V TN systems. The new regulation is intended as a practical aid for inspection technicians, operating electricians and electrical designers.