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that the RCD interprets this discharge process as residual Therefore, type 1 and type 2 SPDs are arranged between L
current due to the impulse current discharged to PE and and N in TT systems. This arrangement is supposed to en-
the RCD interrupts the circuit as already described in sec- sure that, in the event of a faulty protective device in the TT
tion 8.1.3. If type 1 SPDs are used, it must also be assumed system, a short-circuit current is formed and trips the next
that the dynamics of the discharged partial lightning cur- upstream overcurrent protective device. Since, however,
rent causes mechanical damage to the RCD when the type
1 SPDs operate as is the case with TN systems. This would
damage the protective device for “protection against elec-
tric shock under fault conditions” and make the protection L1 U 0 = 230 V a.c.
measure ineffective. Such a state, which can result in life L2 Phase conductor to
L3
hazard, must be avoided. Therefore, in TT systems type 1 N neutral conductor:
and type 2 SPDs must always be installed upstream of the PE U c ≥ 1.1 x 230 V = 253 V a.c.
residual current protective device and must be arranged in Neutral conductor to PE:
such a way that the conditions for the use of overcurrent 1.1 U 0 U c ≥ 230 V a.c.
protective devices for “protection against electric shock un- 3 x arrester with U c ≥ 253 V a.c.
der fault conditions” are met. 1 x arrester with U c ≥ 230 V a.c.
In the event of a fault, namely an SPD fault, short-circuit cur- U 0 The values of U 0 between the
rents must flow to initiate an automatic disconnection of the neutral conductor and PE al-
overcurrent protective devices within 5 s. If the arresters in ready refer to the most un-
favourable operating con-
the TT system were arranged as shown in Figures 8.1.3.4 ditions. Therefore, a tolerance
and 8.1.3.5 for a TN system, only earth fault currents would of 10 % is not considered.
occur in the event of a fault instead of short-circuit currents. R A
In certain cases, however, these earth fault currents do not U 0 = Phase-to-earth nominal a.c. voltage
trip an upstream overcurrent protective device within the
required time. Figure 8.1.4.1 TT system (230/400 V); “3+1” circuit

external LPS lightning current arrester sub-distribution board surge arrester terminal devices
main distribution board

lightning and surge protection as per IEC 62305 (EN 62305)

F1 surge protection as per IEC 60364-4-44 (HD 60364-4-443)
L1
L2
Wh RCD L3
N
SEB PE
F2
F3

MEB local EBB

Figure 8.1.4.2 SPDs used in a TT system

228 LIGHTNING PROTECTION GUIDE www.dehn-international.com