For ‘protection against indirect shock hazard’ in IT systems, overcurrent protective
devices, RCDs and insulation monitoring devices have been approved. Whereas in
TN or TT systems, the ‘protection against indirect shock hazard’ in the event of the
first fault is ensured by appropriate automatic disconnection from supply through
the overcurrent protective devices or RCDs, the first fault in an IT system only
creates an alarm. An excessive shock hazard voltage cannot occur because the first
fault in the IT system simply creates an earth connection for the system. The operating
state of the IT system then becomes a TN or TT system. Hence, an IT system can be
further operated at no risk after the first fault. Thus, work or production processes
already begun (e.g. in the chemical industry) can still be completed. For the first
fault, the protective conductor adopts the potential of the faulty external conductor,
which, however, does not create a risk, because all bodies and metal components
with which persons can come into contact adopt this potential via the protective
conductor. Hence, no hazardous potential differences can be bridged either. When
the first fault occurs, however, it must be noted that the voltage of the IT system of
the intact conductors to earth corresponds to the voltage between the external conductors.
Hence, in a 230/400 V IT system, in the event of a faulty SPD there is a voltage of
400 V across the non-faulty SPD. This possible operating state must be taken into
account when choosing the SPDs with respect to their maximum continuous voltage