TN systems have one point connected directly to earth. All exposed conductive parts must be connected to that point by protective conductors.
Depending on the way the neutral and protective conductors are fed, there are three types of TN systems:
TN-S system: A separate protective conductor is used in the system.
TN-C-S system: the neutral and protective conductors are combined to one single conductor in a part of the system and separated into two conductors in another part of the system.
TN-C system: the neutral and protective conductors are combined to one single conductor in the whole system.
Reference to IEC/EN 60364-4-41 4220.127.116.11
All exposed conductive parts of the installation must be connected to the earthed point of the power system by protective conductors which must be earthed at or near to each relevant transformer or generator.
Exposed conductive parts that are accessible at the same time must be connected to the same earthing system, either individually, in groups or collectively.
Normally the earthed point of the power system is the neutral point. If a neutral point is not available or accessible, a phase conductor must be earthed. The phase conductor must not serve as a PEN conductor.
In fixed installations a single conductor may serve both as a protective conductor and a neutral conductor (PEN conductor).
Reference to IEC/EN 60364-5-54, §543.4.3
If, from any point of the installation, the neutral and protective functions are provided by separate conductors, it is not permitted to connect the neutral conductor to any other earthed part of the installation (e.g. protective conductor from the PEN conductor). However, it is permitted to form more than one neutral conductor (N) and more than one protective conductor (PE) from the PEN conductor. Separate terminals or bars may be provided for the protective and neutral conductors. In this case, the PEN conductor shall be connected to the terminal or bar intended for the protective conductor (PE).
UPS Systems are to be Considered as a Generating Set
IEC/EN60364-5-55 Clause 551.1.2.
Generating sets with the following electrical characteristics are considered:
mains excited and separately excited synchronous generators;
mains excited and self-excited asynchronous generators;
mains-commutated and self-commutated static inverters with or without by-pass facilities.
Additional Requirements when the Generating Set (the UPS) Provides a Switched Alternative to the Public Supply (IEC/EN 60364-5-55 518.104.22.168)
Protection by automatic disconnection of supply must not rely on the connection to the earthed points of the public supply system when the generator is operating as a switched alternative. A suitable earth electrode must be provided.
Protective Devices in TN Systems
The following protective devices are recognized in TN systems:
Overcurrent protective devices
Residual current protective devices (not to be used in TN-C systems)
When a residual current protective device is used in a TN-C-S system, a PEN conductor must not be used on the load side. The connection of the protective conductor to the PEN conductor must be made on the source side of the residual current protective device (see below illustration):
Due to the required installation of the functional earth, upstream protection by Residual Current Protective Devices will not be possible, as they will malfunction due to the leakage current from the UPS.
The characteristics of protective devices and the circuit impedances shall be such that, if a fault of negligible impedance occurs anywhere in the installation between a phase conductor and a protective conductor or exposed conductive part, automatic disconnection of the supply will occur within the specified time (five seconds - valid for distribution circuits, 422.214.171.124). The following condition fulfilling this requirement: IEC/EN 60364-4-41, §4126.96.36.199.
Zs x Ia ≤ U0
In the condition:
Zs is the impedance of the fault loop comprising the source, the live conductor up to the point of the fault, and the protective conductor between the point of the fault and the source
Ia is the current causing the automatic operation of the disconnecting protective device within a conventional time not exceeding five seconds
U0 is the nominal AC RMS voltage to earth
If a fault occurs directly on the output of the UPS but before the power distribution, while the UPS system is in Battery Operation and Bypass is unavailable, the available power is unable to activate the protective device. In this situation the Inverter will shut down in five seconds. (Demand in IEC 60364-4-41 4188.8.131.52). If a residual protective device is used, this device will of course disconnect the supply.