KYORITSU 6050 - Page 17

― 15 ―

Uo(Volts)
120
230
400
>400
T(seconds)
0.8
0.4
0.2
0.1
Fig.11
PracticalexampleofverificationoftheprotectionisaTTsystemaccordingto
theinternationalStandardIEC60364.
Forthisexamplethemaximumvalueis1667Ω,thelooptesterreads12.74
ΩandconsequentlytheconditionRAis
<

50/laismet.
ItisfundamentalforthisexampletotestalsotheRCDtoensurethat
operationtakesplacequicklyenoughtorespectthesafetyrequirement.
AccordingtotheinternationalStandardIEC60364forTNsystemthe
followingconditionshallbefulfilledforeachcircuit:
Zs
<

Uo/la
Where;
Zsistheearthfaultloopimpedance.
Uoisthenominalvoltagebetweenphaseandearth.
la is the current that causes the automatic disconnection of the protective
devicewithinthetimestatedinthefollowingtable.