# Safety Issues Referred to Induced Sheath Voltages in High-Voltage Power Cables--Case Study
## 5 Conclusions  
Most of the high-voltage power cable lines are significantly loaded in practice, so in normal operating conditions the induced sheath voltages or power losses may be high. In order to exclude the risk of the electric shock and the negative effect of the power losses in the long cable lines, during such load conditions, cross bonding of the cable sheaths is most often indicated as the best solution. However, if the cable line carries a relatively low load, as in the analyzed case, the choice of the best type of the bonding system is not obvious. For the analyzed cable line, a comparison of the possible bonding and earthing solutions is presented in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Criterion** & **Single-Point Bonding** & **Both-Ends Bonding** & **Cross Bonding** \\ \hline Induced voltages in normal & (0) 1 & (0) & (0) \\ operating condition & & (+) & (+) \\ Induced voltages in the case of & (-) & (+) & (+) \\ the short circuit & & with earthing in p.6 & with earthing in p.6 \\ Power losses & & (0) & (0) & (+) \\ in the cable system & & (+) & (+) \\ Simplicity of the solution and & & (+) & (+) \\ economic aspects & (0) with ECC & & (+) & (0) \\ \hline \hline \end{tabular}  
* Markings: (+) preferred solution; (0) acceptable solution; (-) excluded solution.  
\end{table} 
Table 1: A summary of the properties of the bonding systems in the analyzed cable line. Figure 13: Power losses in the analyzed three-phase cable system: (**a**) for the transferred power 1 MW, tg\(\phi\) = 0; (**b**) for the transferred power 80 MW, tg\(\phi\) = 0; s-p: single-point bonding; b-e: both-ends bonding; c-b: cross bonding. When the risk of electric shock in normal operating conditions is considered, the values of the induced sheath voltages are acceptable in every case--for single point-bonding the highest value is below the permissible 80 V and for both-ends bonding or cross bonding the voltages are close to 0 V.  
In case of the line-to-earth fault, the single point-bonding is excluded in this cable line, due to very high induced voltages. For the system without ECC, the voltage reaches 22 kV. When the ECC is installed, it decreases only to 12 kV. In the light of the assumed permissible level being equal to 5 kV, these high-value voltages are dangerous for the outer non-metallic sheath of the cable. It should also be noted that, due to the induced voltages in the case of the earth fault, the both-ends bonding, as well as the cross bonding, requires an additional earthing in the middle of the power cable length (point p.6). Without this additional earthing, the voltages may exceed 7 kV but with the earthing, they are reduced to approx. 4 kV. A comparison of the power losses shows that the best solution is the cross bonding but two other solutions are acceptable as well--the total losses in the cable system are within the range 15.4-15.8 kW for the single-point bonding and both-ends bonding, whereas for the cross bonding they are equal to 13 kW. Such low and comparable power losses are the effect of the relatively low transferred power (10 MW). Considering the economic aspect and simplicity of the solution, the single-point bonding without ECC (no additional parallel cable is installed) or both-ends bonding (no equipment for transpositions of the sheaths is required) are recommended. Taking into account all the aforementioned aspects, the both-end bonding or cross bonding can be used in the analyzed cable line. For each of the two solutions, the additional earthing in p.6 is required. To achieve safe operation of relatively long high-voltage cable lines, which are loaded in a low percentage, special attention must be given to the study of short-circuit currents. The induced sheath voltages coming from high-value currents can give unacceptably high-voltage stress, not only in the case of single-point bonding, but also in the case of both-ends bonding. In the latter case, this stress applies to points located close to the centre of the cable section between the earthing arrangements.