a circuit configuration for the routine testing of the clock supply of a large number of digital tdm telecommunication units operated with the same clock , wherein a clock supply with stand - by generator through a corresponding arrangement of several monitor circuits and arrangements of on / alter switches , the operatability of the monitor circuits themselves can be tested so that on routine testing independent of the switching position of a change - over switching device effective upon switching a generator from stand - by operation to active operation respectively the same generator always acts upon the partial distribution system .