IT system - Implementation of protections

From Electrical Installation Guide

Protection by circuit breaker

In the case shown in Fig. F37, the adjustments of instantaneous and short-time delay overcurrent trip unit must be decided. 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 be twice that calculated for one of the circuits in the TN case.

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


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.

Therefore the fault current will be

[math]\displaystyle{ 0.8 \times \sqrt {3} \times 230 \times 103/135 = 2361 A }[/math].

Protection by fuses

The current Ia 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 Fig. F21.

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

Protection by Residual Current Devices (RCDs)

Where circuit lengths are unavoidably long, and especially if the appliances of a circuit are earthed separately (so that the fault current passes through two earth electrodes), reliable tripping on overcurrent may not be possible.

In this case, an RCD is recommended on each circuit of the installation.

Where an IT system is resistance earthed, however, care must be taken to ensure that the RCD is not too sensitive, or a first fault may cause an unwanted trip-out.

Tripping of residual current devices which satisfy IEC standards may occur at values of 0.5 ΙΔn to ΙΔn, where ΙΔn is the nominal residual-current setting level.

Maximum circuit length

The principle is the same for an IT system as that described for a TN system: the calculation of maximum circuit lengths which should not be exceeded downstream of a circuit breaker or fuses, to ensure protection by overcurrent devices.

It is clearly impossible to check circuit lengths for every feasible combination of two concurrent faults.

All cases are covered, however, if the overcurrent trip setting is based on the assumption that a first fault occurs at the remote end of the circuit concerned, while the second fault occurs at the remote end of an identical circuit, as already mentioned. This may result, in general, in one trip-out only occurring (on the circuit with the lower trip-setting level), thereby leaving the system in a first-fault situation, but with one faulty circuit switched out of service.

Fig. F39 – Calculation of Lmax. for an IT-eathed system, showing fault-current path for a double-fault condition
  • For the case of a 3-phase 3-wire installation the second fault can only cause a phase/phase short-circuit, so that the voltage to use in the formula for maximum circuit length is [math]\displaystyle{ \sqrt3 U_0 }[/math].
The maximum circuit length is given by:

[math]\displaystyle{ Lmax=\frac{0.8\,\ U_0\,\sqrt{3}\,S_{ph}}{2\rho\,I_a\left ( 1+m \right )} }[/math] meters

  • For the case of a 3-phase 4-wire installation the lowest value of fault current will occur if one of the faults is on a neutral conductor. In this case, U0 is the value to use for computing the maximum cable length, and

[math]\displaystyle{ Lmax=\frac{0.8\,U_0\,S_1}{2\,\rho\,I_a\left ( 1+m \right )} }[/math] meters

i.e. 50% only of the length permitted for a TN scheme[1]

In the preceding formulae:

Lmax = longest circuit in metres
U0 = phase-to-neutral voltage (230 V on a 230/400 V system)
ρ = resistivity at normal operating temperature (23.7 x 10-3 Ω-mm²/m for copper, 37.6 x 10-3 Ω-mm²/m for aluminium)
Ia = overcurrent trip-setting level in amps, or: Ia = current in amps required to clear the fuse in the specified time
m = Sph / SPE
Sph = cross-sectional area of the phase conductors of the circuit concerned in mm²
SPE = cross-sectional area of PE conductor in mm²
S1 = S neutral if the circuit includes a neutral conductor
S1 = Sph if the circuit does not include a neutral conductor


The tables shown in Fig. F25 to Fig. F28 have been established according to the “conventional method”.

The tables give maximum circuit lengths, beyond which the ohmic resistance of the conductors will limit the magnitude of the short-circuit current to a level below that required to trip the circuit breaker (or to blow the fuse) protecting the circuit, with sufficient rapidity to ensure safety against indirect contact. The tables consider:

  • The type of protection: circuit breakers or fuses, operating-current settings
  • Cross-sectional area of phase conductors and protective conductors
  • Type of earthing scheme
  • Correction factor: Fig. F40 indicates the correction factor to apply to the lengths given in tables Fig. F25 to Fig. F28, when considering an IT system
Fig. F40 – Correction factor to apply to the lengths given in tables Fig. F25 to Fig. F28 for IT systems
Circuit Conductor material m = Sph/SPE (or PEN)
m = 1 m = 2 m = 3 m = 4
3 phases Copper 0.86 0.57 0.43 0.34
Aluminium 0.54 0.36 0.27 0.21
3ph + N or 1ph + N Copper 0.50 0.33 0.25 0.20
Aluminium 0.31 0.21 0.16 0.12


A 3-phase 3-wire 230/400 V installation is IT-earthed.

One of its circuits is protected by a circuit breaker rated at 63 A, and consists of an aluminium-cored cable with 50 mm² phase conductors. The 25 mm² PE conductor is also aluminium. What is the maximum length of circuit, below which protection of persons against indirect-contact hazards is assured by the instantaneous magnetic tripping relay of the circuit breaker?

Fig. F26 indicates 603 meters, to which must be applied a correction factor of 0.36 (m = 2 for aluminium cable).

The maximum length is therefore 217 meters.

Maximum tripping times

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

For final circuits (= circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment), the maximum tripping time is the same as in TN system (see Fig. F19).

For the other circuits within the same group of interconnected exposed-conductive parts, the maximum disconnecting time is 5s. This is because any double fault situation within this group will result in a short-circuit current as in TN system.

For final circuits (= circuits with a rated current not exceeding 63 A with one or more socket-outlets, and 32 A supplying only fixed connected current-using equipment) 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 Fig. F13. For the other circuits within the same group of non-interconnected exposed-conductive-parts, the maximum disconnecting time is 1s. This is because 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.


  1. ^ Reminder: There is no length limit for earth-fault protection on a TT scheme, since protection is provided by RCDs of high sensitivity.