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| {{Menu_Protection_against_electric_shocks}} | | {{Menu_Protection_against_electric_shocks}} |
| | {{Section-TOC}} |
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| 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”.
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| 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:
| | [[fr:Protection contre les chocs et incendies électriques]] |
| *Either isolated from earth
| | [[de:Schutz gegen elektrischen Schlag]] |
| *Or connected to earth through a high resistance (commonly 1,000 ohms or more)
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| 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.
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| In practice, the system earthing requires certain specific measures for its satisfactory exploitation:
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| *Permanent monitoring of the insulation with respect to earth, which must signal (audibly or visually) the occurrence of the first fault
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| *A device for limiting the voltage which the neutral point of the supply transformer can reach with respect to earth
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| *A “first-fault” location routine by an efficient maintenance staff. Fault location is greatly facilitated by automatic devices which are currently available
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| *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){{fn|1}}.
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| The second fault results in a short-circuit through the earth and/or through PE bonding conductors.
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| {{Manual_TOC |
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| * [[IT system - Preliminary conditions]]
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| * [[IT system - Protection against indirect contact]]
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| * [[IT system - High-sensitivity RCDs]]
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| * [[IT system - Protection in high fire-risk locations]]
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| * [[IT system - When the fault current-loop impedance is particularly high]]}}
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| {{footnotes}}
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| <references>
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| {{fn-detail|1|On systems where the neutral is distributed, as shown in Figure F56.}}
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| </references>
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| [[ru:Реализация системы IT]]
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| [[zh:IT 系统的应用]] | |
