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The sparkover gap isolates the three varistors electrically
from the PE so that leakage currents through the varistors
cannot cause voltage increases at the PE. The residual-
current circuit-breaker behind the 3+1 circuit then provides
reliable protection against hazardous touch voltages in all
situations.
In terms of its relevance for safety aspects, the 3+1 circuit
described in VDE 0100 part 534 (section 534.2.2) can
therefore be regarded as a solution for surge protection in
TT systems.
Note: Although the “four-arrester circuit”, i.e. with one
varistor each between earth potential and L1, L2, L3 and
N, is prescribed in VDE 0100 part 534 (section 524.2.1)
for consumer installations in TN-S systems, the 3+1 circuit
would also be possible here without increasing the risk.
In ÖVE/ÖNORM E 8001-1/A2:2003-11-01, the 3+1 circuit
is already expressly listed for use in TN-S and TT systems.
3+1 circuit: universal solution
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The basics of lightning and surge protection
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