N conductor in 3-phase IT systems
IEC 60364-4-43:2008-08 subclause 431.2.2 includes a comment to the effect that it advisable not to route an N conductor in IT systems. This should be taken into account whenever single-phase loads are also connected to a 3-phase/N IT system. If an insulation fault occurs in L1, the voltage on conductors L2/L3 to earth is increased to the phase-to-phase voltage, e.g., 400 V. This could damage suppression capacitors that are switched to earth. Ensure that the voltage offset only affects the voltage to earth. No voltage offset between active conductors arises. Single-phase equipment must be configured appropriately, i.e., they must be suitable for operation in 3-phase /N systems. In practice, two separate IT systems are frequently set up, one for single-phase loads and one for 3-phase loads.
At this point, a comment on the generally applicable comment from IEC 60364-4-43:2008-08 sub-clause 433.3.3, that overload protective devices can be omitted if an unexpected disconnection of the circuit constitutes a source of risk. In such cases, an overload alarm should be considered.
Summary
IT systems are always of greatest benefit when they protect against the disconnection of the power supply in the event of a first fault. The fundamental basis for fault-free and safe operation comprises setting the system up in accordance with the standards and the correct choice of protective and monitoring devices.
Recommended further reading:
Hofheinz, Wolfgang - Protection technology with insulation monitoring, VDE-Verlag GmbH, Berlin
Standards referred to in this article
DIN VDE 0100-100 VDE 0100-100:2009-06
IEC 60364-1:2005-11
Low-voltage electrical installations
Part 1: Fundamental principles, assessment of general characteristics, definitions
IEC 60364-4-41:2017-03
Low-voltage electrical installations
Part 4-41: Protective measures - Protection against electrical shock
IEC 60364-4-42:2010/AMD1 2014
Low-voltage electrical installations
Part 4-42: Protective measures - Protection against thermal effects
IEC 60364-4-43:2008-08
Low-voltage electrical installations
Part 4-43: Protective measures - Protection against overcurrent
IEC 60364-7-710:2002-11
Electrical installations of buildings
Part 7-710: Requirements for special installations or locations - Medical locations
DIN EN 61557-8:2014-12
Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. – Equipment for testing, measuring or monitoring of protective measures
Part 8: Insulation monitoring devices for IT systems
IEC 61557-9: 2014-12
Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c. – Equipment for testing, measuring or monitoring of protective measures
Part 9: Equipment for insulation fault location in IT systems
DIN VDE 0100-530 (VDE 0100-530):2018-06
Errichten von Niederspannungsanlagen
Teil 530: Auswahl und Errichtung elektrischer Betriebsmittel – Schalt- und Steuergeräte
Standards can be obtained from VDE-Verlag [publishers] or Beuth.