KYORITSU 6050 - Page 16

― 14 ―
InaccordancewiththeinternationalstandardIEC60364foraTTsystemthe
followingconditionshallbefulfilledforeachcircuit.
RAmustbe
<

50/la
where;
RAisthesumoftheresistancesofthelocalearthsystemRandthe
protectiveconductorconnectingittotheexposedconductorpart.
50Visthemaximumvoltagelimit(itmaybe25Vincertaincircumstances).
laisthevalueofcurrentthatcausesautomaticdisconnectionofthe
protectivedevicewithin5seconds.
Whentheprotectivedeviceisaresidualcurrentdevice(RCD),Iaistherated
residualoperatingcurrentI⊿n.
ForexampleinaTTsystemprotectedbyanRCDthemaximumRAvalues
areasfollows;
Fig.10
RatedresidualoperatingcurrentI⊿nmA
10
5000
2500
30
1667
833
100
500
250
300
167
83
500
100
50
1000
50
25
RA(at50V)Ω
RA(at25V)Ω