23. For the purposes of paragraph 4, the main components of a generating station shall only be regarded as having been renewed since 31st December 1989 where–
(a)in the case of a hydro generating station the following parts have been installed in the generating station after 31st December 1989 and were not used for the purpose of electricity generation prior to that date–
(i)either all the turbine runners or all the turbine blades or the propeller; and
(ii)either all the inlet guide vanes or all the inlet guide nozzles; or
(b)in the case of any other generating station all the boilers and turbines (driven by any means including wind, water, steam or gas) have been installed in the generating station after 31st December 1989 and were not used for the purpose of electricity generation prior to that date.