A further power accumulator effectively connected with the position member and maintained ineffective by shackling (26) during normal operation, with a direction of effect opposite to that of the first power accumulator, has an accumulated power greater than that of the first power accumulator. Joining devices (27) irreversibly release the first shackling when a predetermined limit temperature is exceeded for the purpose of transferring the position member - drive rod (18) into a second safety position. The cylinder cover-shaped shackling (26) between abutments (32,34) engaging in one another is filled with solder (27) with a predetermined melting point.
