TT system - Principle

Principle

Automatic disconnection for TT system is achieved by RCD having a sensitivity of [math]\displaystyle{ \definecolor{bggrey}{RGB}{234,234,234}\pagecolor{bggrey}I_{\vartriangle n}\le\frac{50}{R_A} }[/math] where R_A is the resistance of the
installation earth electrode

In this system all exposed-conductive-parts and extraneous-conductive-parts of the installation must be connected to a common earth electrode. The neutral point of the supply system is normally earthed at a pint outside the influence area of the installation earth electrode, but need not be so. The impedance of the earth-fault loop therefore consists mainly in the two earth electrodes (i.e. the source and installation electrodes) in series, so that the magnitude of the earth fault current is generally too small to operate overcurrent relay or fuses, and the use of a residual current operated device is essential.
This principle of protection is also valid if one common earth electrode only is used, notably in the case of a consumer-type substation within the installation area, where space limitation may impose the adoption of a TN system earthing, but where all other conditions required by the TN system cannot be fulfilled.
Protection by automatic disconnection of the supply used in TT system is by RCD of sensitivity: [math]\displaystyle{ I_{\vartriangle n}\le\frac{50}{R_A} }[/math]
where
R_A is the resistance of the earth electrode for the installation
I_Δn is the rated residual operating current of the RCD
For temporary supplies (to work sites, …) and agricultural and horticultural premises, the value of 50 V is replaced by 25 V.
Example (see Fig. F9)

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Fig. F9: Automatic disconnection of supply for TT system

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• The resistance of the earth electrode of substation neutral R_n is 10 Ω.
• The resistance of the earth electrode of the installation R_A is 20 Ω.
• The earth-fault loop current I_d = 7.7 A.
• The fault voltage U_f = I_d x R_A = 154 V and therefore dangerous, but

I_Δn = 50/20 = 2.5 A so that a standard 300 mA RCD will operate in about 30 ms without intentional time delay and will clear the fault where a fault voltage exceeding appears on an exposed-conductive-part. 

Specified maximum disconnection time

The tripping times of RCDs are generally lower than those required in the majority of national standards; this feature facilitates their use and allows the adoption of an effective discriminative protection.
The IEC 60364-4-41 specifies the maximum operating time of protective devices used in TT 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 F10

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Uo(1) (V)	T (s)
50 < Uo ≤ 120	0.3
120 < Uo ≤ 230	0.2
230 < Uo ≤ 400	0.07
Uo > 400	0.04

(1) Uo is the nominal phase to earth voltage

Fig. F10: Maximum disconnecting time for AC final circuits not exceeding 32 A

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• For all other circuits, the maximum disconnecting time is fixed to 1s. This limit enables discrimination between RCDs when installed on distribution circuits.

RCD is a general term for all devices operating on the residual-current principle. RCCB (Residual Current Circuit-Breaker) as defined in IEC 61008 series is a specific class of RCD.
Type G (general) and type S (Selective) of IEC 61008 have a tripping time/current characteristics as shown in Figure F11. These characteristics allow a certain degree of selective tripping between the several combination of ratings and types, as shown later in sub-clause 4.3. Industrial type RCD according to IEC 60947-2 provide more possibilities of discrimination due to their flexibility of time-delaying.

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X IΔn		1	2	5	> 5
Domestic	Instantaneous	0.3	0.15	0.04	0.04
	Type S	0.5	0.2	0.15	0.15
Industrial	Instantaneous Time-delay (0.06)	0.3 0.5	0.15 0.2	0.04 0.15	0.04 0.15
	Time-delay (other)	According to manufacturer

Fig. F11: Maximum operating time of RCD’s (in seconds)

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