TN system - Principle

Principle

The automatic disconnection for TN system is achieved by overcurrent protective devices or RCD’s

In this system all exposed and extraneous-conductive-parts of the installation are connected directly to the earthed point of the power supply by protective conductors.
As noted in Chapter E Sub-clause 1.2, the way in which this direct connection is carried out depends on whether the TN-C, TN-S, or TN-C-S method of implementing the TN principle is used. In figure F12 the method TN-C is shown, in which the neutral conductor acts as both the Protective-Earth and Neutral (PEN) conductor. In all TN systems, any insulation fault to earth results in a phase to neutral short-circuit. High fault current levels allow to use overcurrent protection but can give rise to touch voltages exceeding 50% of the phase to neutral voltage at the fault position during the short disconnection time.
In practice for utility distribution network, earth electrodes are normally installed at regular intervals along the protective conductor (PE or PEN) of the network, while the consumer is often required to install an earth electrode at the service entrance.
On large installations additional earth electrodes dispersed around the premises are often provided, in order to reduce the touch voltage as much as possible. In high-rise apartment blocks, all extraneous conductive parts are connected to the protective conductor at each level.
In order to ensure adequate protection, the earth-fault current [math]\displaystyle{ {Id}=\frac{Uo}{Zs} }[/math] or [math]\displaystyle{ 0.8\frac{Uo}{Zc} }[/math] must be higher or equal to Ia, where:

• Uo = nominal phase to neutral voltage
• Id = the fault current
• Ia = current equal to the value required to operate the protective device in the time specified
• Zs = earth-fault current loop impedance, equal to the sum of the impedances of the source, the live phase conductors to the fault position, the protective conductors from the fault position back to the source
• Zc = the faulty-circuit loop impedance (see “conventional method” Sub-clause 6.2)

Note: The path through earth electrodes back to the source will have (generally) much higher impedance values than those listed above, and need not be considered.
Example (see Fig. F12)

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 Fig. F12: Automatic disconnection in TN system

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The fault voltage [math]\displaystyle{ Uf=\frac{230}{2}=115\ V }[/math] and is hazardous;
The fault loop impedance

Z_S = Z_AB + Z_BC + Z_DE + Z_EN + Z_NA

If Z_BC and Z_DE are predominant, then: [math]\displaystyle{ Zs=2\rho\frac{L}{S}=64.3\ m\Omega }[/math], so that
[math]\displaystyle{ Id=\frac{230}{64.3\times{10^{-3}}}=3,576 A }[/math] (≈ 22 In based on a NS X 160 circuit-breaker).

The “instantaneous” magnetic trip unit adjustment of the circuit-breaker is many time less than this short-circuit value, so that positive operation in the shortest possible time is assured.
Note: Some authorities base such calculations on the assumption that a voltage drop of 20% occurs in the part of the impedance loop BANE.
This method, which is recommended, is explained in chapter F “conventional method” and in this example will give an estimated fault current of  [math]\displaystyle{ \frac{230\times{0.8}\times{10^3}}{64.3}= 2,816 }[/math] (≈ 18 In)

Specified maximum disconnection time

The IEC 60364-4-41 specifies the maximum operating time of protective devices used in TN system for the protection against indirect contact:

• For all final circuits with a rated current not exceeding 32 A, the maximum disconnecting time will not exceed the values indicated in Figure F13
• For all other circuits, the maximum disconnecting time is fixed to 5s. This limit enables discrimination between protective devices installed on distribution circuits

Note: The use of RCDs may be necessary on TN-earthed systems. Use of RCDs on TN-C-S systems means that the protective conductor and the neutral conductor must (evidently) be separated upstream of the RCD. This separation is commonly made at the service entrance.

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Uo(1) (V)	T (s)
50 < Uo ≤ 120	0.8
120 < Uo ≤ 230	0.4
230 < Uo ≤ 400	0.2
Uo > 400	0.1

(1) Uo is the nominal phase to earth voltage

Fig. F13: Maximum disconnecting time for AC final circuits not exceeding 32 A

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