Neutrals earthing

Neutral earthing arrangements, e.g. solid, with a neutral earthing resistance (NER), with a common busbar, switches or circuit breakers for isolation. 

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.)... 

R. Ball and G. W. Stephens, Neutral earthing of marine electrical power systems. Institnte of Marine Engineers UK. Trans. I Mar E, 1982, Vol. 95, Paper No.32. 

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... 

The extension of this technique to the distribution board or feeder pillar where four or five core cables are concerned is relatively simple. Traditionally this termination was made using the iron box filled with bitumen. The cable box was replaced by a preformed plastics glove which is pulled down over the prepared cores so that the lower end overlaps the cable sheath and is then shrunk into position. The individual paper-insulated cores are protected by shrinkable tubing and the terminating lug is sweated or compressed on to the end of the conductor. In a conventional termination the cable box itself forms the direct connection between the cable sheath and the neutral earth bar. 

With heat-shrinkable terminations it is necessary to make special arrangements to bond the lead sheath to the neutral earth bar. 

As the public supply system employs neutral earthing, the most common method of compliance with this Regulation is to connect together any earth exposed and extraneous conductive parts by means of low impedance protective and bonding conductors. This is the EEBADS technique outlined in Chapter 3. The technique creates an equipotential zone so that when an earth fault occurs these conductive parts are raised to substantially the same potential with respect to the ground (earth). This condition persists until the protective device, be it a fuse or a circuit breaker, interrupts the circuit and clears the fault. Anyone in simultaneous contact with more than one of the conductive parts should not experience a shock because the parts are at about the same potential. 

As an example, in TN-C-S supplies to domestic premises a break in the combined neutral/earth (CNE) conductor of an overhead service line would cause the metalwork in the premises to become live at or about the supply voltage if any apparatus, such as a dishwasher, were to be connected and switched on. To prevent this type of hazard, the integrity of the CNE conductor, throughout the system, has to be maintained, so all joints have to be properly made and reliable, and fuses and solid state devices are prohibited in these conductors. 

Electrode boilers are covered by subsection 554-03. As some earth leakage is inevitable, earthing is important, so the boiler shell has to be connected to the proteetive eonductor and to the metallie armour and sheath, if any, of the incoming supply cable and also to the supply neutral. Earthing the neutral will affeet the operation of an RCD as the earth leakage current will be shared by the neutral and protective conduetors. Regulation 554-03-04 refers to a three-phase electrode boiler supplied direetly from an HV supply. In this case there is no neutral connection, but an RCD is a requirement. The neutral does not have to be connected to the boiler shell for boilers not piped to the water supply see section 554-03-07. 

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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