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发表于 2015-6-26 13:35
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来自： 新加坡
Unearthed System 1. IT system unearthed (high impedance earthed neutral)First Letter I = the neutral is unearthed at Transformer or Generator side.
Second Letter T = Frame parts of the loads are interconnected and earthed at Load Side
System characteristics- It is compulsory to install an over voltage limiter between the MV/LV transformer neutral point and earth.
- If the neutral is not accessible, the overvoltage limiter is installed between a phase and earth.
- It runs off external over voltages, transmitted by the transformer, to the earth and protects the low voltage network from a voltage increase due to flashover between the transformer’s medium voltage and low voltage windings.
Advantages- System providing the best service continuity during use.
- When an insulation fault occurs, the short-circuit current is very low.
- Higher operational safety only a capacitive current flows, which is caused by the system leakage capacitance if an earth fault occurs.
- Better accident prevention the fault current is limited by the body impedance, earthing resistance and the high impedance of the earth fault loop.
Disadvantages- Requires presence of maintenance personnel to monitor and locate the first fault during use.
- Requires a good level of network insulation (High leakage current must be supplied by insulating transformers).
- Overvoltage limiters must be installed.
- Requires all the installation’s exposed conductive parts to be Same Voltage level. If this is not possible RCDs must be installed.
- Locating faults is difficult in widespread networks.
- When an insulation fault with reference to the earth occurs, the voltage of the two healthy phases in relation to the earth take on the value of the phase-to-phase voltage So when Select Size of equipments it is need to higher insulation level of the Equipments.
- The risk of high internal over voltages making it advisable to reinforce the equipment insulation.
- The compulsory insulation monitoring, with visual and audible indication of the first fault if tripping is not triggered until the second fault occurs.
- Protection against direct and indirect contact is not guaranteed.
- 10. Short-circuit and earth fault currents may cause fires and destroy parts of the plant.
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楼主: DraftMan
