Cable sheath bonding

Single-core underground power cables with two-points bonding induce currents in their metallic sheaths. The sheath induced currents are undesirable and generate power losses and reduce the cable ampacity.  The values of the sheath losses in some cases could be greater than conductor losses, depending on various factors. Such these factors are type of cable layouts, cable parameters, cable spacing, sheath resistance, phase rotation, conductor current and cable armoring

SHEATH BONDING ARRANGEMENTS

Sheath bonded at two-points

In which the sheaths of three separate cables will be connected together at both ends of the 

run. For safety reasons one end of the sheaths must also be ear thed. This system 

doesn’t allow high values of the induced voltages in the metallic sheaths. In this situation, sheath 

circulating currents appear because of there is a closed circuit between the sheath and the return

path through the ground. 

Sheath bonded at one end only

In which the sheaths of three separate cables will be connected together and earthed at one 

point only along their length. At all other points, a voltage will appear from sheath to 

ground that will be a maximum at the farthest point from the ground bond. Since there is no closed 

sheath circuit current no sheath circulating current loss occurs, but sheath eddy loss will still be 

present.

Sheath cross bonded

Cross bonding of single-core cable sheaths is in use for many years. In which, each sheath 

circuit contains one section from each phase such that the total voltage in each sheath circuit sums 

to zero. If the sheaths are then bonded and earthed at the end of the run, the net voltage in the loop 

and the circulating currents will be zero and the only sheath losses will be those caused by eddy 

currents.

FACTORS AFFECTING THE SHEATH LOSSES IN SINGLE-CORE UNDERGROUND

POWER CABLES WITH TWO-POINTS BONDING

·         Cable layouts formation as Trefoil and flat formations are usually used in practice

·         Cable conductor resistivity

·         Cable spacing

·         Sheath resistance

·         Phase rotation

·         Conductor current

·         Cable armoring

CONCLUSION

1.     It must pay attention to sheath losses in single-core cables with two-points bonding as their values could be reached to more than the conductor losses.

2.     The sheath eddy losses could be neglected w.r.t the sheath circulating losses at high sheath 

3.     resistance values and high conductors spacing

4.     Sheath eddy losses are inversely proportional to sheath resistance, cable conductor resistivity 

5.     and conductors spacing, while they are proportional to conductor current.

6.     Sheath circulating losses are proportional to the conductors spacing, and conductor current and can be reduced by large increase in sheath resistance or large reduce in the sheath resistance but the later leading to high circulating current.

7.     Phase rotation plays a great role in determination of the sheath circulating losses in flat layout.

8.     Trefoil formation introduces symmetrical values of losses in its sheathes than flat formation

9.     addition to the total sheath losses in the trefoil are lower than flat layout.