IT system - Practical aspects
From Electrical Installation Guide
|Minimum functions required||Components and devices||Examples|
|Protection against overvoltages at power frequency||(1) Voltage limiter||Cardew C|
|Neutral earthing resistor (for impedance earthing variation)||(2) Resistor||Impedance Zx|
|Overall earth-fault monitor with alarm for first fault condition||(3) Permanent insulation monitor PIM with alarm feature|| Vigilohm IM10
|Automatic fault clearance on second fault and protection of the neutral conductor against overcurrent|| (4) Four-pole circuit-breakers
(if the neutral is distributed) all 4 poles trip
|Compact circuit-breaker or RCD-MS|
|Location of first fault||(5) With device for fault-location on live system, or by successive opening of circuits||Vigilohm XGR+XRM or XD312 or XL308|
Fig. F42: Essential functions in IT schemes and examples with Schneider Electric products
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
(see Figure F43)
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 Figure F44)
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 Figure F45)
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 in Figure F45, provide the means for this automatic exploitation.
Implementation of permanent insulation-monitoring (PIM) devices
The PIM device is normally connected between the neutral (or artificial neutral) point of the power-supply transformer and its earth electrode.
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.
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: 300 mA (fire risk at I>300 mA)
- Indication levels set by the consumer:
- Threshold for preventive maintenance: 0.8 x 100 = 80 kΩ
- Threshold for short-circuit alarm: 500 Ω
- 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.