IT system - Fault protection

In this type of system:

• The installation is isolated from earth, or the neutral point of its power-supply source is connected to earth through a high impedance
• All exposed and extraneous-conductive-parts are earthed via an installation earth electrode.

First fault situation

In IT system the first fault to earth should not cause any disconnection

On the occurrence of a true fault to earth, referred to as a “first fault”, the fault current is very low, such that the rule Id x R_A ≤ 50 V (see Automatic disconnection for TT system) is fulfilled and no dangerous fault voltages can occur.

In practice the current Id is low, a condition that is neither dangerous to personnel, nor harmful to the installation.

However, in this system:

• A permanent monitoring of the insulation to earth must be provided, coupled with an alarm signal (audio and/or flashing lights, etc.) operating in the event of a first earth fault (see Fig. F16)

Fig. F16 – Phases to earth insulation monitoring device obligatory in IT system

• The location and repair of a first fault is imperative if the full benefits of the IT system are to be realised. Continuity of service is the great advantage afforded by the system. As continuity of service is provided, it is not mandatory to repair the fault immediately avoiding to operate under stress and urgency.

For a network formed from 1 km of new conductors, the leakage (capacitive) impedance to earth Zf is of the order of 3,500 Ω per phase. In normal operation, the capacitive current to earth is therefore:

[math]\displaystyle{ \frac{Uo}{Zf}= \frac{230}{3,500}=66\ mA }[/math] per phase.

During a phase to earth fault, as indicated in Figure F17,the current passing through the electrode resistance RnA is the vector sum of the capacitive currents in the two healthy phases. The voltages of the healthy phases have (because of the fault) increased to [math]\displaystyle{ \sqrt3 }[/math] the normal phase voltage, so that the capacitive currents increase by the same amount. These currents are displaced, one from the other by 60°, so that when added vectorially, this amounts to 3 x 66 mA = 198 mA, in the present example.

The fault voltage Uf is therefore equal to 198 x 5 x 10^-3 = 0.99 V, which is obviously harmless.

The current through the short-circuit to earth is given by the vector sum of the neutral-resistor current Id1 (=153 mA) and the capacitive current Id2 (198 mA).

Fig. F17 – Fault current path for a first fault in IT system

Since the exposed-conductive-parts of the installation are connected directly to earth, the neutral impedance Zct plays practically no part in the production of touch voltages to earth.

Second fault situation

The simultaneous existence of two earth faults (if not both on the same phase) is dangerous, and rapid clearance by fuses or automatic circuit-breaker tripping depends on the type of earth-bonding scheme, and whether separate earthing electrodes are used or not, in the installation concerned

On the appearance of a second fault, on a different phase, or on a neutral conductor, a rapid disconnection becomes imperative. Fault clearance is carried out differently in each of the following cases:

1^st case

It concerns an installation in which all exposed conductive parts are bonded to a common PE conductor, as shown in Figure F18.

Fig. F18 – Circuit breaker tripping on double fault situation when exposed-conductive-parts are connected to a common protective conductor

In this case no earth electrodes are included in the fault current path, so that a high level of fault current is assured, and conventional overcurrent protective devices are used, i.e. circuit-breakers and fuses.

The first fault could occur at the end of a circuit in a remote part of the installation, while the second fault could feasibly be located at the opposite end of the installation.

For this reason, it is conventional to double the loop impedance of a circuit, when calculating the anticipated fault setting level for its overcurrent protective device(s).

Where the system includes a neutral conductor in addition to the 3 phase conductors, the lowest short-circuit fault currents will occur if one of the (two) faults is from the neutral conductor to earth (all four conductors are insulated from earth in an IT scheme). In four-wire IT installations, therefore, the phase-to-neutral voltage must be used to calculate short-circuit protective levels i.e

[math]\displaystyle{ 0.8\frac{Uo}{2Zc}\ge Ia }[/math]

where

Uo = phase to neutral voltage
Zc = impedance of the circuit fault-current loop (see Automatic disconnection for TN systems)
Ia = current level for trip setting

If no neutral conductor is distributed, then the voltage to use for the fault-current calculation is the phase-to-phase value, i.e.

[math]\displaystyle{ 0.8\frac{\sqrt{3}Uo}{2Zc}\ge Ia }[/math]

Maximum tripping times

Disconnecting times for IT system depends on how the different installation and substation earth electrodes are interconnected.

For final circuits supplying electrical equipment with a rated current not exceeding 32 A and having their exposed-conductive-parts bonded with the substation earth electrode, the maximum tripping time is given in table F8. For the other circuits within the same group of interconnected exposed-conductive-parts, the maximum disconnecting time is 5s. This is due to the fact that any double fault situation within this group will result in a short-circuit current as in TN system.

For final circuits supplying electrical equipment with a rated current not exceeding 32 A and having their exposed-conductive-parts connected to an independent earth electrode electrically separated from the substation earth electrode, the maximum tripping time is given in Figure F13. For the other circuits within the same group of non interconnected exposed-conductive-parts, the maximum disconnecting time is 1s. This is due to the fact that any double fault situation resulting from one insulation fault within this group and another insulation fault from another group will generate a fault current limited by the different earth electrode resistances as in TT system.

Protection by circuit-breaker

In the case shown in Figure F18, the adjustments of instantaneous and short-time delay overcurrent trip unit must be decided. The times recommended here above can be readily complied with. The short-circuit protection provided by the NSX160 circuit-breaker is suitable to clear a phase to phase short-circuit occurring at the load ends of the circuits concerned.

Reminder: In an IT system, the two circuits involved in a phase to phase short-circuit are assumed to be of equal length, with the same cross sectional area conductors, the PE conductors being the same cross sectional area as the phase conductors. In such a case, the impedance of the circuit loop when using the Conventional method will be twice that calculated for one of the circuits in the TN case, shown in Automatic disconnection for TN systems

The resistance of circuit loop [math]\displaystyle{ FGHJ = 2R_{JH}= 2\rho\frac{L}{a} }[/math]

where:
ρ = resistance of copper rod 1 meter long of cross sectional area 1 mm², in mΩ
L = length of the circuit in meters
a = cross sectional area of the conductor in mm²

FGHJ = 2 x 23.7 x 50/35 = 67.7 mΩ and the loop resistance B, C, D, E, F, G, H, J will be 2 x 67.7 = 135 mΩ.

The fault current will therefore be [math]\displaystyle{ 0.8\times \sqrt 3 \times 230 \times 10^3/135 = 2361 A }[/math].

Protection by fuses

The current I_a for which fuse operation must be assured in a time specified according to here above can be found from fuse operating curves, as described in Figure F15.

The current indicated should be significantly lower than the fault currents calculated for the circuit concerned.

• Protection by Residual current circuit-breakers (RCCBs)

For low values of short-circuit current, RCCBs are necessary. Protection against indirect contact hazards can be achieved then by using one RCCB for each circuit.

2^nd case

It concerns exposed conductive parts which are earthed either individually (each part having its own earth electrode) or in separate groups (one electrode for each group). If all exposed conductive parts are not bonded to a common electrode system, then it is possible for the second earth fault to occur in a different group or in a separately earthed individual apparatus. Additional protection to that described above for case 1, is required, and consists of a RCD placed at the circuit-breaker controlling each group and each individually-earthed apparatus.

The reason for this requirement is that the separate-group electrodes are “bonded” through the earth so that the phase to phase short-circuit current will generally be limited when passing through the earth bond by the electrode contact resistances with the earth, thereby making protection by overcurrent devices unreliable.

The more sensitive RCDs are therefore necessary, but the operating current of the RCDs must evidently exceed that which occurs for a first fault (see Fig. F19).

Leakage capacitance (µF)	First fault current (A)
1	0.07
5	0.36
30	2.17

Note: 1 µF is the 1 km typical leakage capacitance for 4-conductor cable.

Fig. F19 – Correspondence between the earth leakage capacitance and the first fault current

For a second fault occurring within a group having a common earth-electrode system, the overcurrent protection operates, as described above for case 1.

Note 1: See also Protection of the neutral conductor.

Note 2: In 3-phase 4-wire installations, protection against overcurrent in the neutral conductor is sometimes more conveniently achieved by using a ring-type current transformer over the single-core neutral conductor (see Fig. F20).

Fig. F20 – Application of RCDs when exposed-conductive-parts are earthed individually or by group on IT system