Overhead Earthing conductor

The overhead earthing conductor will divert some of the L-G fault current from entering the ground at the foot of the pole. The extent of diversion will be in proportion to the impedance of the overhead line compared with that of the earth resistance path back to the source. The calculations required for determining the fault current and its diverted amounts are shown in Appendix H by way of an example, and Figure 13.12. 

Let the overhead earthing conductor divert some of the fault current. 

Cable sheath. Additional overhead line conductor Protective multiple earthing fP.M.E.) Buried Strip/rod/plate Earth-leakage circuit breaker, Voltage-operated Earth-leakage circuit breaker, Current-operated... 

Table H.la shows the value of // for different values of Rep. It also shows the division of current between the overhead line earthing conductor and the footings of the pole.

Regulation 542-01-02 refers to TN-S systems where the supply company runs a protective conductor from the distribution transformer earth into the consumer s premises and makes it available for connection to the consumer s main earth terminal. This protective conductor may consist of the metallic sheath/armour of the underground service cable or an earthing conductor connected to the protective conductor of an overhead line. 

Ametani, A., Y. Miyamoto, and J. Mahseredjian. 2014. Derivation of earth-return impedance of an overhead multi-conductor considering displacement currents. lEE Japan Trans. PE, 134 936-940. 

This is the type of supply more often found when the installation is fed from overhead cables. The supply authorities do not provide an earth terminal and the installation s circuit protective conductors (CPCs) must be connected to earth via an earth electrode provided by the consumer. lET Regulation 542.2.3 lists the type of earth rod, earth plate or earth tapes recognized by BS 7671. An effective earth connection is sometimes difficult to obtain and in most cases a residual current device (RCD) is provided when this type of supply is used. The arrangement is shown in Fig. 3.36. 

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. 

The outer media of an overhead conductor are the air and the earth since the conductor is isolated by the air from the earth, which is a conducting medium. Therefore, the outer-media impedance Zq of the overhead conductor is composed of the following component ... 

Using the penetration depth for the earth, the outer-media impedance of an overhead conductor is readily obtained based on image theory. Figure 1.2 illustrates a single overhead conductor and its image ... 

Pollaczek derived a general formula that can deal with earth-return impedances of overhead conductors, underground cables, and multiconductor systems composed of overhead and underground conductors in the following form [7,13] ... 

Each element of the impedance matrix is composed of the cable internal impedance and the cable outer media (earth-return) impedance, as explained in Chapter 1 of this volume. In the overhead line case, the conductor internal impedance is composed of only one impedance (i.e., the outer surface impedance of a conductor). The cable internal impedance consists of the following six components [1] ... 

Ametani, A., T. Yoneda, Y. Baba, and N. Nagaoka. 2009. An investigation of earth-return impedance between overhead and underground conductors and its approximation. IEEE Trans. EMC 51(3) 860-867. 

Earth is stratified, as is well-known, and its resistivity varies significantly at the top layer depending on the weather and climate. The earth-return impedance of an overhead conductor above the stratified earth was derived in Reference 16, and the stratified-earth effect was investigated in Reference 17. The stratified-earth effect may be far more significant than the accurate evaluation of the homogenous earth-return impedance of Pollaczek and Carson, and this requires further investigation. 

The second mechanism is one of resistive coupling, whereby ac currents are directly transmitted to earth during transmission line faults. Causes of such faults include grounding of an overhead conductor, lightning strikes, and major load imbalances in the conductors. Usually such faults are of very short duration, but due to the high currents involved, substantial physical damage to coated structures is possible. Ancillary equipment such as motorized valves, sensors, and... 

---