Few Tips For Correct Protection Against Indirect Contact In TT System

Earth fault in a TT system

An earth fault in a TT system originates the circuit represented in Figure 1. The fault current flows through the secondary winding of the transformer, the line conductor, the fault resistance, the protective conductor, and the earth electrode resistances (R_A, of the user’s plant, and R_B, of the neutral).

According to IEC 60364-4 prescriptions, the protective devices MUST be coordinated with the earthing arrangement in order to rapidly disconnect the supply if the touch voltage reaches harmful values for the human body.

In TT systems the neutral and the exposed conductive parts are connected to earth electrodes electrically independent (Figure 1). Therefore the earth fault current returns to the power supply node through the soil.

Before describing such prescriptions, it is useful to know the different circuit types described in the above mentioned Standard. In particular, in a plant, the circuits can be divided into:

1. Final circuit
   It is a circuit which usually supplies equipment (for example an aspirator, a bridge crane, etc.)
2. Distribution circuit
   It is a circuit which supplies a distribution board to which other final circuits are connected.

In a TT system, to achieve a correct protection against indirect contact through the automatic disconnection of the circuit, it is necessary to respect one of the following conditions (in compliance with IEC 60364-4):

1. Protection by means of residual current devices (RCD)
   • Operating principle of residual current devices
2. Protection by means of overcurrent protective devices
3. Protection against indirect contact by means of circuit breakers equipped with electronic releases

At the end of this technical article, we’ll see the example of the LV circuit breaker settings (Tmax 250A equipped with an electronic release) and also a few words for final conclusions.

• Example with CB settings
• Conclusions

1. Protection by means of residual current devices (RCD)

By assuming 50V as limit voltage (standard environments), to achieve protection against indirect contact by means of residual current devices it is necessary to satisfy the following condition:

R_A · I_∆n ≤ 50V then: R_A ≤ 50V / I_∆n

where:

• R_A is the total resistance (in ohm) of the earth electrode and of the protective conductor of the exposed conductive parts;
• I_∆n is the rated residual operating current of the residual current circuit-breaker.

Table 1: Maximum disconnection times for final circuits not exceeding 32A

U_o is the nominal a.c. or d.c. line to earth voltage.
Where in TT systems the disconnection is achivied by an overcurrent protective device and the protective equipotential bonding is connected with all extraneous-conductive-parts within the installation, the maximum disconnection times applicable to TN systems may be used.

• NOTE 1 // Disconnection may be required for reasons other than protection against electric shock.
• NOTE 2 // Where compliance with the above mentioned requirement is provided by an RCD, the disconnecting times in accordance with the table above relate to prospective residual currents significantly higher than the rated residual operating current of the RCD (typically 5·I_∆n).

In fact, by using a residual current device with 30mA sensitivity, an earthing resistance value lower than

R_A ≤ 50 / 0.03 = 1666.6 Ω

shall be obtained, whereas with a less sensitive residual current device (for example with 300mA sensitivity) an earthing resistance value lower than:

R_A ≤ 50 / 0.3 = 166.6 Ω

shall be obtained.

As shown in the example, thanks to a more sensitive residual current device, from a practical point of view it will be easier to realize an earthing system coordinated with the characteristics of the device itself. The Table 2 shows the maximum values of earth resistance which can be obtained with residual current devices and making reference to a common environment (50V):

Operating principle of residual current devices

The operating principle of residual current devices consists in the detection of an earth fault current by means of a toroidal transformer which encloses all the live conductors, included the neutral, if distributed. In absence of an earth fault the vectorial sum of the currents I_∆ is equal to zero.

In addition to the coordination with the earthing arrangement, to select the rated operating residual current I_∆n, also the total leakage current of the installation under normal operating conditions shall be taken into consideration and, in order to avoid unwanted trips, such current shall not exceed 0.5 × I_∆n.

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2. Protection by means of overcurrent protective devices

The choice of the automatic device for the protection against phase-to-earth faults and indirect contact shall be carried out by coordinating properly the disconnection times with the impedance of the fault loop. As a consequence, it is necessary to fulfill the following condition:

Z_s · I_a ≤ U₀

where:

• Z_s is the impedance (in ohms) of the fault loop comprising
  – the source;
  – the line conductor up to the point of the fault;
  – the protective conductor of the exposed-conductive-parts;
  – the earthing conductor;
  – the earth electrode of the installation;
  – the earth electrode of the source;
• I_a is the disconnection current in the times shown in Table 1 for final circuits with currents not exceeding 32A or within 1 second for distribution circuits and for final circuits with currents exceeding 32A;
• U₀ is the nominal a.c. r.m.s. voltage to earth (V).

The choice of the automatic device shall be made by coordinating properly the disconnection times with the impedance of the fault loop.

The relationship Z_s · I_a ≤ U₀ may be expressed as :

I_a ≤ U₀ / Z_s = I_{kL-to earth}

where I_{kL-to earth} is the phase-to-earth fault current. Therefore, it is possible to state that the protection against indirect contact is veri ed when the tripping current I_a of the protective device (within the times shown in Table 1 or within 1s) is lower than the phase-to-earth fault current I_{kL-to earth} at the exposed-conductive-part to be protected.

Besides, in such circumstances, it could be very difficult to calculate the impedance of the fault loop (Z_s), because the earthing resistance of the neutral cannot be considered negligible (in fact it could reach values of the same quantity of the earth resistance).

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3. Protection against indirect contact by means of circuit breakers equipped with electronic releases

As previously indicated, the earth fault currents in TT systems have low values and consequently providing this protection by using thermomagnetic/electronic releases with phase protections within the times required by the Standard might be difficult or even impossible.

In such cases it is possible to use advanced electronic releases providing protection function G, which improves the protection conditions with earth fault currents not particularly high.

If this current is higher than the value set for protection G, the circuit breaker shall trip in the time set on the electronic release.

By using function G, the condition to be fulfilled to obtain protection against indirect contact becomes:

Z_s · I₄ ≤ U₀

where I4 is the value in amperes of the setting of the protection function against earth fault. Since this value can be set from 0.2 to 1 for In, it is easy to realize how, by using function G, it is possible to provide protection against indirect contact for high impedance values of the fault loop and therefore low earth fault currents.

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Example with CB settings

The following example (Figure 2) shows the possible settings of the LV circuit breaker Tmax T4N250 In250A equipped with an electronic release type PR222DS/P LSIG.

An example:
By using protection function G set at 0.20×In (see Figure 2), the current value over which tripping is obtained within 1s is 60A (value including the upper tolerance). The value of the phase-to-earth fault current (100A) results to be higher than the trip value within 1 second. Therefore protection against indirect current is achieved.

If protection function G were not used, the phase protections would not sense the current of 100A, since the setting of these functions is too high in comparison with the fault currents under consideration.

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Reference // Distribution systems and protection against indirect contact and earth fault by ABB