To purchase or generate power for mine requirements is determined on basis of minimum power charge against coal production. Alternating-current transmission results in better efficiencies and lower costs. The choice of coal hoisting systems is based on reliability with cost secondary. Skip hoisting for large mines gives lower costs of installation and operation, with greater reliability. Man-and-material hoists arrange to take care of every possible contingency against shut-down is advisable. Converting stations is the logical method of supplying direct current to underground equipment.