B 1003
Issue 1, Oct 1970


1. General
A Class C system is one involving all types of clocks which are complete in themselves and need no connections to external power supplies. The only Class C clocks available at the moment are mechanical clocks. Investigations are in hand to find a transistorised battery clock of sufficient accuracy for PO use. Until these investigations are complete and a standard type available, battery clocks should not be used, since the majority of battery clocks on the market are not good enough for use in the PO.

In the Class C system, since the individual clocks are not synchronised one with another, the accuracy of the system is not of a high degree. All clocks should be adjusted to within one minute a week, although it is possible to adjust them to within one minute a month.

2. Provision
Because the clocks need winding once a week, and correction nearly as often, Class C systems should not be installed where it can possibly be avoided. Their use should be limited to temporary accommodation (e.g. extra sorting offices for the Christmas Pressure Period) and accommodation, such as some leased buildings, where it is not possible to wire a Class A or Class B system.

3 Clocks
All PO Clocks with an odd number or the suffix C are suitable for use in this system. The clocks are detailed in B1101 and described individually in the B1100 series.

EI Miscellaneous, Time B3101 - Installation of mechanical clocks.

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Last revised: July 06, 2023