English to Russian Tech/Engineering Construction / Civil Engineering

Добрый день! Прошу помочь с переводом термина "unspared":

There are two or three unspared steam turbine driven compressors in each of the ammonia units and one in both of the urea units. The loss of any one of these compressors, would result in the shutdown of the unit concerned. The risk of machinery breakdown is considered to be ‘HIGH’