IEC 60364-4-41 specifies the maximum operating time of protective devices used in TN system for fault protection: 1. For all final circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only f. IEC 60364-4-41 specifies the maximum operating time of protective devices used in TN system for fault protection: 1. For all final circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment, the maximum disconnecting time will not exceed the values indicated in F. (see Fig. F18) The fault voltage displaystyle{ U_f=frac{230}{2}=115 V }[/math]and is hazardous; The fault loop impedance displaystyle{ Z_S = Z_{AB} + Z_{BC} + Z_{DE} + Z_{EN} + Z_{NA} }[/math]. If displaystyle{ Z_{BC} }[/math] and displaystyle{ Z_{DE} }[/math] are predominant, then: displaystyle{ Zs=2rhofrac{L. (see Fig. F20) The instantaneous trip unit of a circuit breaker will eliminate a fault to earth in less than 0.1 second. In consequence, automatic disconnection within the maximum allowable time will always be assured, since all types of trip unit, magnetic or electronic, instantaneous or slightly retarded, are suitable: Ia = Im. The maximum tolera. (see Fig. F21) The value of current which assures the correct operation of a fuse can be ascertained from a current/time performance graph for the fuse concerned. The fault current U0/Zs or 0.8 U0/Zc as determined above, must largely exceed the current necessary to ensure positive operation of the fuse. The condition to observe therefore is that [m. Residual Current Devices must be used where: 1. The loop impedance cannot be determined precisely (lengths difficult to estimate, presence of metallic material close to the wiring) 2. The fault current is so low that the disconnection time cannot be met by using overcurrent protective devices The rated tripping current of RCDs being in the order of.
