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IEC 60364-4-41 (HD 60364-4-41) describes protection meas- ¨ Overcurrent protective device
ures against indirect contact with protective conductors. ¨ Residual current protective device
These protection measures include automatic disconnec- ¨ Insulation monitoring device
tion or an alarm in the event of a fault. When implementing
measures for “protection against electric shock under fault ¨ Fault-voltage-operated protective device (in special
conditions”, the protective device must be assigned accord- cases)
ing to the system configuration.
This results in the following assignments:
According to IEC 60364-4-41 (HD 60364-4-41), a low-volt- TN system
age distribution system in its entirety, from the power source ¨ Overcurrent protective device
to the last piece of equipment, is typically characterised by: ¨ Residual current protective device
¨ Earthing conditions of the power source (e.g. low-volt-
age side of the distribution transformer) and TT system
¨ Earthing conditions of the bodies of the equipment in ¨ Overcurrent protective device
electrical consumer´s installations ¨ Residual current protective device
Thus, there are three basic types of distribution systems: ¨ Fault-voltage-operated protective device (in special
TN system, TT system and IT system. cases)

The letters have the following meaning: IT system
¨ Overcurrent protective device
The first letter describes the earthing conditions of the
supplying power source: ¨ Residual current protective device
¨ Insulation monitoring device
T Direct earthing of one point of the power source
(generally the neutral point of the transformer
winding) These personal protection measures have top priority when
installing power supply systems. All other protection meas-
I Insulation of all live parts from earth or connection ures such as lightning and surge protection for electrical
of one point of the power source to earth via an systems and installations are secondary to the protection
impedance measures against indirect contact with protective conduc-
tors under consideration of the system configuration and
The second letter describes the earthing conditions of the the protective device and must not make these measures
bodies of the equipment of the electrical installation: ineffective. In this context, an SPD fault, even it is most un-
T Body of the equipment is directly earthed regardless likely, must also be taken into account. This is particularly
of whether one point of the power supply is earthed important because the surge protective devices are always
N Body of the electrical equipment is directly con- used between the live conductors and the protective con-
nected to the operational earth electrode (earthing ductor.
of the power source)
Therefore, the use of SPDs in different system configurations
Further letters describe the arrangement of the neutral will be described in the following sections. These circuit con-
and protective conductor: cepts are taken from IEC 60364-5-53 (HD 60364-5-534).
S Neutral and protective conductor are laid separately In these sample solutions, the lightning current arresters
from each other are shown in the vicinity of the service entrance box, i.e.
C Neutral and protective conductor are combined (in upstream of the meter. IEC 60364-5-53 (HD 60364-5-534)
one conductor) defines that lightning current arresters must be installed
“close to the origin of the installation”.
Thus, there are three possible TN systems: TN-S system, In Germany, the use of lightning current arresters upstream
TN-C system and TN-C-S system. of the meter is governed by the VDN (Association of German
Network Operators) guideline 2004-08: “Überspannungs-
The protective devices which can be installed in the various Schutzeinrichtungen Typ 1. Richtlinie für den Einsatz von
systems are: Überspannungs- Schutzeinrichtungen (ÜSE) Typ 1 (bisher

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