Ia can be determined from the fuse
performance curve. In any case, protection
cannot be achieved if the loop impedance Zs
or Zc exceeds a certain value
Fig. F14 : Disconnection by circuit-breaker for a TN system Fig. F15 : Disconnection by fuses for a TN system
3 Protection against indirect
contact
(1) Uo is the nominal phase to earth voltage
Uo(1) (V) T (s)
50 < Uo y 120 0.8
120 < Uo y 230 0.4
230 < Uo y 400 0.2
Uo > 400 0.1
Protection by means of circuit-breaker (see Fig. F14)
The instantaneous trip unit of a circuit-breaker will eliminate a short-circuit 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 tolerance authorised
by the relevant standard, however, must always be taken into consideration. It is
sufficient tthheerreefforre tthatt tthe ffault current
Uo
Zs
or 0.8
Uo
Zc
determined by calculation (or estimated
on site) be greater than the instantaneous trip-setting current, or than the very shortdetermined
by calculation
(or estimated on site) be greater than the instantaneous trip-setting current, or than
the very short-time tripping threshold level, to be sure of tripping within the permitted
time limit.
Protection by means of fuses (see Fig. F15)
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.