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.
therefore thatT thhee ffaauulltt ccuurrrreenntt
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 shortas
determined above, must largely exceed that
to ensure positive necessary to ensure positive operation of the fuse. The condition to observe
therefore is that
Ia <
Uo
Zs
or 0.8
Uo
Zc
as ainsd iincdaitceadt eind Finig Fuirgeu rFe1 F5.15.
Protection by means of circuit-breaker (see Fig. F14)The instantaneous trip unit of a circuit-breaker will eliminate a short-circuit to earth inless than 0.1 second.In consequence, automatic disconnection within the maximum allowable time willalways be assured, since all types of trip unit, magnetic or electronic, instantaneousor slightly retarded, are suitable: Ia = Im. The maximum tolerance authorisedby the relevant standard, however, must always be taken into consideration. It issufficient tthheerreefforre tthatt tthe ffault currentUoZsor 0.8UoZcdetermined by calculation (or estimatedon site) be greater than the instantaneous trip-setting current, or than the very shortdeterminedby calculation(or estimated on site) be greater than the instantaneous trip-setting current, or thanthe very short-time tripping threshold level, to be sure of tripping within the permittedtime limit.Protection by means of fuses (see Fig. F15)The value of current which assures the correct operation of a fuse can beascertained from a current/time performance graph for the fuse concerned.therefore thatT thhee ffaauulltt ccuurrrreennttUoZsor 0.8UoZcdetermined by calculation (or estimatedon site) be greater than the instantaneous trip-setting current, or than the very shortasdetermined above, must largely exceed thatto ensure positive necessary to ensure positive operation of the fuse. The condition to observetherefore is thatIa <UoZsor 0.8UoZcas ainsd iincdaitceadt eind Finig Fuirgeu rFe1 F5.15.
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