IT system - Practical aspects
Modern monitoring systems greatly facilitate first-fault location and repair
First-fault condition
The earth-fault current which flows under a first-fault condition is measured in milli-amps.
The fault voltage with respect to earth is the product of this current and the resistance of the installation earth electrode and PE conductor (from the faulted component to the electrode). This value of voltage is clearly harmless and could amount to several volts only in the worst case (1,000 Ω earthing resistor will pass 230 mA[1] and a poor installation earth-electrode of 50 ohms, would give 11.5 V, for example).
An alarm is given by the permanent insulation monitoring device.
Principle of earth-fault monitoring
A generator of very low frequency a.c. current, or of d.c. current, (to reduce the effects of cable capacitance to negligible levels) applies a voltage between the neutral point of the supply transformer and earth. This voltage causes a small current to flow according to the insulation resistance to earth of the whole installation, plus that of any connected appliance.
Low-frequency instruments can be used on a.c. systems which generate transient d.c. components under fault conditions. Certain versions can distinguish between resistive and capacitive components of the leakage current.
Modern equipment allow the measurement of leakage-current evolution, so that prevention of a first fault can be achieved.
Examples of equipment
Fault-location systems comply with IEC 61157-9 standard
Manual fault-location
(see Fig. F53)
The generator may be fixed (example: IM400) or portable (example: XGR permitting the checking of dead circuits) and the receiver, together with the magnetic clamp-type pick-up sensor, are portable.
Fixed automatic fault location
(see Fig. F54)
The PIM IM400, together with the fixed detectors XD301 or XD312 (each connected to a toroidal CT embracing the conductors of the circuit concerned) provide a system of automatic fault location on a live installation.
Moreover, the level of insulation is indicated for each monitored circuit, and two levels are checked: the first level warns of unusually low insulation resistance so that preventive measures may be taken, while the second level indicates a fault condition and gives an alarm.
Upstream supervision can centralize insulation & capacitance levels thanks to the IM400 embedded modbus communication.
Automatic monitoring, logging, and fault location
(see Fig. F55)
With Vigilohm system connected to a supervision system though Modbus RS485 communication, it is possible for a centralized supervision system to monitor insulation level and status at global level as well as for every feeder.
The central monitor XM300, together with the localization detectors XL308 and XL316, associated with toroidal CTs from several circuits, as shown below in Fig. F55, provide the means for this automatic exploitation.
Implementation of permanent insulation-monitoring (PIM) devices
Connection
The PIM device is normally connected between the neutral (or articificial neutral) point of the power-supply transformer and its earth electrode.
Supply
Power supply to the PIM device should be taken from a highly reliable source. In practice, this is generally directly from the installation being monitored, through overcurrent protective devices of suitable short-circuit current rating.
Level settings
Certain national standards recommend a first setting at 20% below the insulation level of the new installation. This value allows the detection of a reduction of the insulation quality, necessitating preventive maintenance measures in a situation of incipient failure.
The detection level for earth-fault alarm will be set at a much lower level.
By way of an example, the two levels might be:
- New installation insulation level: 100 kΩ
- Leakage current without danger: 500 mA (fire risk at > 500 mA)
- Indication levels set by the consumer:
- Threshold for preventive maintenance: 0.8 x 100 = 80 kΩ
- Threshold for short-circuit alarm: 500 Ω
Notes:
- Following a long period of shutdown, during which the whole, or part of the installation remains de-energized, humidity can reduce the general level of insulation resistance.
- This situation, which is mainly due to leakage current over the damp surface of healthy insulation, does not constitute a fault condition, and will improve rapidly as the normal temperature rise of current-carrying conductors reduces the surface humidity.
- Some PIM device (IM20, IM400 & XM300) can measure separately the resistive and the capacitive current components of the leakage current to earth, thereby deriving the true insulation resistance from the total permanent leakage current.
The case of a second fault
A second earth fault on an IT system (unless occurring on the same conductor as the first fault) constitutes a phase-phase or phase-to-neutral fault, and whether occurring on the same circuit as the first fault, or on a different circuit, overcurrent protective devices (fuses or circuit-breakers) would normally operate an automatic fault clearance.
The settings of overcurrent tripping relays and the ratings of fuses are the basic parameters that decide the maximum practical length of circuit that can be satisfactorily protected, as discussed in TN system - Protection against indirect contact
Note: In normal circumstances, the fault current path is through common PE conductors, bonding all exposed conductive parts of an installation, and so the fault loop impedance is sufficiently low to ensure an adequate level of fault current.
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.
Methods of determining levels of short-circuit current
Three methods of calculation are commonly used:
- The method of impedances, based on the trigonometric addition of the system resistances and inductive reactances
- The method of composition
- The conventional method, based on an assumed voltage drop and the use of prepared tables
A reasonably accurate assessment of short-circuit current levels must be carried out at the design stage of a project.
A rigorous analysis is not necessary, since current magnitudes only are important for the protective devices concerned (i.e. phase angles need not be determined) so that simplified conservatively approximate methods are normally used. Three practical methods are:
- The method of impedances, based on the vectorial summation of all the (positive-phase-sequence) impedances around a fault-current loop
- The method of composition, which is an approximate estimation of short-circuit current at the remote end of a loop, when the level of short-circuit current at the near end of the loop is known. Complex impedances are combined arithmetically in this method
- The conventional method, in which the minimum value of voltage at the origin of a faulty circuit is assumed to be 80% of the nominal circuit voltage, and tables are used based on this assumption, to give direct readings of circuit lengths.
These methods are reliable only for the cases in which wiring and cables which make up the fault-current loop are in close proximity (to each other) and are not separated by ferro-magnetic materials.
Method of impedances
The software Ecodial is based on the “method of impedance”
This method is identical for both the IT and TN systems of earthing. It is described in TN system - Protection against indirect contact - Method of impedances
Method of composition
This method is identical for both the IT and TN systems of earthing. It is described in TN system - Protection against indirect contact - Method of composition
Conventional method
(see Fig. F56)
The maximum length of an IT earthed circuit is:
- For a 3-phase 3-wire scheme [math]\displaystyle{ \definecolor{bggrey}{RGB}{234,234,234}\pagecolor{bggrey}Lmax=\frac{0.8\ Uo\ \sqrt{3}\ Sph}{2\rho Ia\left ( 1+m \right )} }[/math]
- For a 3-phase 4-wire scheme [math]\displaystyle{ \definecolor{bggrey}{RGB}{234,234,234}\pagecolor{bggrey}Lmax=\frac{0.8\ Uo\ S1}{2\rho Ia\left ( 1+m \right )} }[/math]
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 in Automatic disconnection on a second fault in an IT system. 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.
- 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{ \sqrt 3 Uo }[/math].
The maximum circuit length is given by:
[math]\displaystyle{ Lmax=\frac{0.8\ Uo\ \sqrt{3}\ Sph}{2\rho Ia\left ( 1+m \right )} }[/math] metres
- 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, Uo is the value to use for computing the maximum cable length, and
[math]\displaystyle{ Lmax=\frac{0.8\ Uo\ S1}{2\rho Ia\left ( 1+m \right )} }[/math] metres
i.e. 50% only of the length permitted for a TN scheme[2]
In the preceding formulae:
Lmax = longest circuit in metres
Uo = phase-to-neutral voltage (230 V on a 230/400 V system)
ρ = resistivity at normal operating temperature (23.7 x 10-3 ohms-mm2/m for copper, 37.6 x 10-3 ohms-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
[math]\displaystyle{ m=\frac{Sph}{SPE} }[/math]
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
Tables
The following tables[3] give the length of circuit which must not be exceeded, in order that persons be protected against indirect contact hazards by protective devices
The following tables have been established according to the “conventional method” described above.
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 take into account:
- 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: Figure F57 indicates the correction factor to apply to the lengths given in tables Fig. F41 to Fig. F44, when considering an IT system
| 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 | |
Example
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 aluminum. 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?
Figure F42 indicates 603 metres, to which must be applied a correction factor of 0.36 (m = 2 for aluminium cable).
The maximum length is therefore 217 metres.
Notes
- ^ On a 230/400 V 3-phase system.
- ^ Reminder: There is no length limit for earth-fault protection on a TT scheme, since protection is provided by RCDs of high sensitivity.
- ^ The tables are those shown in (Figure F41 to Figure F44). However, the table of correction factors (Figure F57) which takes into account the ratio Sph/SPE, and of the type of circuit (3-ph 3-wire; 3-ph 4-wire; 1-ph 2-wire) as well as conductor material, is specific to the IT system, and differs from that for TN.
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