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Over the last decade, there has been a notable rise in the number of forced ventilated cable tunnel schemes in the U.K., with new construction at transmission and distribution levels. The ability to accurately calculate continuous and emergency circuit ratings for these installations is vital in ensuring that their full operational benefit can be realized. While the Electra 143 calculation method in present use is fast and easy to use, it relies on several simplifying assumptions which make it unsuitable for application to tunnels with multiple independent cable circuits. This paper details a series of modifications to the present method which allow the direct calculation of ratings for tunnels containing multiple independent cable circuits. Significant benefits can be obtained from using this approach to calculate emergency ratings in these circumstances, as demonstrated by the example calculations provided. Implementation of an axially varying ac resistance also improves the accuracy of loss calculations. A number of key tunnel design considerations are illustrated through the results of the example calculations.
Date of Publication: Oct. 2010