Neutral earthing Resistor

The thermal withstand time for the neutral earthing resistors are usually specified as 10 seconds for the duration of the fault current. This allows adequate time for main and back-up protection relays to operate and clear the fault. The standby earth fault relay (51 G) time-current characteristic must be chosen so that its Pt curve is lower than that for the neutral earth resistor and the connecting cables. (The curve is derived directly from the I-t data, and not by integrating the curve.)... 

The high impedance is usually a resistance chosen to limit the earth current to about 20 mA. A current detector is used in conjunction with the resistance to raise an alarm if a line-to-earth fault occurs. A zig-zag transformer, or reactor, is sometimes used with three-wire supplies such as used in drilling rigs and emergency systems. It is specially designed and internally connected to create a very low zero sequence impedance to earth currents. Therefore, the current is limited only by the resistance of the neutral earthing resistor. Some special purpose earth current alarm systems are available that inject a small DC current into the three-phase system, which is used to identify the actual location of the fault. 

Figure 13.3 Earthing a high voltage system by using a neutral earthing resistor.

In view of the difficulties found in providing a satisfactory solution, it would be advisable to include the ladder network referred to in H.l and re-calculate the results. If this does not improve the situation significantly then two main improvements should be considered. Firstly use a neutral earth resistor at the source to restrict the earth return current to between 50 and 100 amps. This will directly reduce Em to values below Etouchso- Secondly reduce the fault clearing time L from 0.5 to... 

Loop impedance tests are carried out to determine the loop impedance between the power source(s) and the point in the installation where the test is done. The device employed measures the current which passes through a resistor and displays the result in ohms. It is used to determine the loop impedance between phases, phase to neutral or any phase to earth. Some instruments incorporate a transformer to enable the neutral/earth loop impedance to be measured. From Ohm s law these readings can then be expressed in prospective short circuit fault currents. 

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