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5
In accordance with the international standard IEC 60364 for a
TT system the following condition shall be fulfiled for each cir-
cuit.
R
A
must be
≤
50/la where;
R
A
is the sum of the resistances of the local earth system R and
the protective conductor connecting it to the exposed conduc-
tor part. 50V is the maximum voltage limit (it may be 25V in
certain circumstances). Ia is the value of current that causes
automatic disconnection of the protective device within 5 sec-
onds.
When the protective device is a residual device (RCD), Ia is the
rated residual operating current I n. For example in a TT sys-
tem protected by an RCD the maximum R
A
values are as fol-
lows:
Rated residual
operating cur- 10
30
100
300
300
1000
rent I n mA
Ra (at 50V)
Ω
5000
1667
500
167
100
50
Ra (at 25V)
Ω
2500
833
250
83
50
25