Tn Network; It Network - Chauvin Arnoux C.A 6710 Notice De Fonctionnement

3.2

TN network

Reminder:
This type of system is used, for example, in tertiary electrical installations powered from a private transformer station.
In a TN system, the neutral of the private transformer is connected to the ground and the earths of the installation are
connected to the neutral of this transformer station.
If the neutral conductor is combined with the PE protective conductor, it is a TN-C system.
If these 2 conductors are separate, it is a TN-S system.
Many installations use a TN-C system upstream and a TN-S system downstream (the opposite is prohibited), so that leakage to
earth at a specific point can be monitored, for example, because with TN-S it is possible to insert differential devices.
Connections (see figure 2) :
Connect the neutral and earth terminals at the level of A (if TN-C system)
The 1000 Ω resistance between terminals B and C must be shunted
Terminal D must be linked to the earth T
Earths E or ES can also be connected to the earth bar F of the building to ensure that the potential of the conductor serving as
PE and N (upstream of A) will remain close to that of the earth despite the neutral current which may pass through it (if a load
is connected).
In reality, this causes problems above all when the PE/N conductor is very long, with a resistance significantly different from
zero.
Principle:
A fault current is closed by the neutral conductor and becomes a phase-neutral short circuit current (with a very high value
because the phase-neutral loop has a low impedance).
A protective system against excess currents (circuit breakers or fuses) cuts off the installation and protects it.
3.3

IT network

Reminder:
This type of system is used, for example, in industrial electrical installations powered from a private transformer station and
operated by qualified personnel.
In an IT system, the neutral of the private transformer is connected with impedance or insulation from the ground and the earths
of the installation are connected to the ground.
This is the only system which ensures operating continuity when there is an initial fault, as the installation is only cut off with the
second fault.
This is particularly useful in certain sensitive buildings or industries in which the process cannot tolerate interruptions: hospitals,
chemicals, glass, oil, metallurgy...
Connections (see figure 2) :
Do not connect the neutral and the earth at the level of A
The 1000 Ω resistance between terminals B and C must not be shunted. In the field, it is sometimes set up in parallel with a
capacity.
Terminal D must be connected to earth T. In certain practical cases, this connection is not necessary. The transformer is then
totally insulated from the earth.
Earth E must be connected to the earth bar of the building F
Theoretical principle:
The initial fault current is low because it is closed off by the earth of the transformer, which has a high impedance (often 1000 Ω
or infinite). The resulting fault voltage on the chassis earth is therefore low and not dangerous.
In the event of a second fault, the current loops via the two faulty loads and protection is provided in the same way as in the TN
system: high current cut off by fuses or circuit breakers.
In the field, a Permanent Insulation Controller indicates the occurrence of an initial insulation fault (buzzer + visual indication).
This informs the personnel responsible for operation that the first fault has to be repaired before the second cuts off the
installation.
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