Last edit: 29/06/2023
TN Systems: the basics
In TN Systems the neutral point of the power supply system is earthed. In USA and Canada this grounding system is called “Solidly Grounded Wye”.
If a neutral point or midpoint is not available or not accessible, a line conductor shall be earthed; this is what the North Americans call a Corner grounded Delta; it is rarely used in Europe. The grounding of the Neutral is the first characteristic of a TN system. The second is that Exposed-conductive-parts of the installation shall be connected by a protective conductor to the main earthing terminal of the installation which shall be connected to the earthed point of the power supply system.
In essence: the neutral point is grounded (or earthed) and all Exposed Conductive Parts are connected directly to the neutral point. The reason why all exposed conductive parts are bonded to the neutral point is to create a fault loop having a high value of fault current.
TN Systems: how safety is guaranteed
The creation of a fault loop is not enough to protect people from electric shock. The reason of the creation of the fault loop is to make sure, in case of a fault, there is a high value current circulation in the fault loop.
The high value current has a “mission”: to make a protection open on the feeder side and de-energise the circuit before the fault can be dangerous for a person.
Table 41.1 in 411.3.2.2 prescribes the maximum disconnecting time.
[IEC 60364-4-41] 411.3.2.2 The maximum disconnection time stated in Table 41.1 shall be applied to final circuits with a rated current not exceeding:
- 63 A with one or more socket-outlets, and
- 32 A supplying only fixed connected current-using equipment.
|
|
50 Vac < U0 ≤ 120 Vac |
120 Vac < U0 ≤ 230 Vac |
230 Vac < U0 ≤ 400 Vac |
UO > 400 Vac |
|
TN |
0,8 s |
0,4 s |
0,2 s |
0,1 s |
In case of a system voltage of 230 Vac phase to neutral, the reason why a time of 0,4 seconds is specified is because 0,4 seconds is the maximum time a person can be subject to 92 Vac. That is the normative touching voltage in a TN system operating at 230 / 400 Vac.
The reasoning behind the value of 92 Vac is detailed in IEC TR 61200-413: 1996 – part 413.1.3.5 Distribution circuits, whereby the touch voltages in TN systems, in case the PE and the phase cables have the same sizes, is Ut=c*U0/2. “c” is a conventional factor that can be taken = 0,8.
Important is to notice that:
[IEC 60364-4-41] 411.3.2.3 In TN systems, a disconnection time not exceeding 5 s is permitted for distribution circuits, and for circuits not covered by 411.3.2.2.
TN systems are the common low voltage neutral grounding system all over the world. In Europe both TN and TT systems are allowed. In USA and Canada TT systems are forbidden. In Norway we noticed the use of IT distribution systems.