Chapter A General rules of electrical installation design

F - Protection against electric shock
(1) On systems where the neutral is distributed, as shown in
Figure F56.
7 Implementation of the IT system
The basic feature of the IT earthing system is that, in the event of a short-circuit to
earth fault, the system can continue to operate without interruption. Such a fault is
referred to as a “first fault”.
In this system, all exposed conductive parts of an installation are connected via
PE conductors to an earth electrode at the installation, while the neutral point of the
supply transformer is:
b Either isolated from earth
b Or connected to earth through a high resistance (commonly 1,000 ohms or more)
This means that the current through an earth fault will be measured in milli-amps,
which will not cause serious damage at the fault position, or give rise to dangerous
touch voltages, or present a fire hazard. The system may therefore be allowed to
operate normally until it is convenient to isolate the faulty section for repair work. This
enhances continuity of service.
In practice, the system earthing requires certain specific measures for its satisfactory
exploitation:
b Permanent monitoring of the insulation with respect to earth, which must signal
(audibly or visually) the occurrence of the first fault
b A device for limiting the voltage which the neutral point of the supply transformer
can reach with respect to earth
b A “first-fault” location routine by an efficient maintenance staff. Fault location is
greatly facilitated by automatic devices which are currently available
b Automatic high-speed tripping of appropriate circuit-breakers must take place in
the event of a “second fault” occurring before the first fault is repaired. The second
fault (by definition) is an earth fault affecting a different live conductor than that of the
first fault (can be a phase or neutral conductor) (1) .
The second fault results in a short-circuit through the earth and/or through
PE bonding conductors.
7.1 Preliminary conditions (see Fig. F51 and Fig. F52)
Minimum functions required Components and devices Examples
Protection against overvoltages (1) Voltage limiter Cardew C
at power frequency
Neutral earthing resistor (2) Resistor Impedance Zx
(for impedance earthing variation)
Overall earth-fault monitor (3) Permanent insulation Vigilohm TR22A
with alarm for first fault condition monitor PIM with alarm feature or XM 200
Automatic fault clearance (4) Four-pole circuit-breakers Compact circuit-breaker
on second fault and (if the neutral is distributed) or RCD-MS
protection of the neutral all 4 poles trip
conductor against overcurrent
Location of first fault (5) With device for fault-location Vigilohm system
on live system, or by successive
opening of circuits
Fig. F51 : Essential functions in IT schemes and examples with Merlin Gerin products
HV/LV
2 1 3
Fig. F52 : Positions of essential functions in 3-phase 3-wire IT-earthed system
Schneider Electric - Electrical installation guide 2008
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