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Why is there a voltage present at the terminals of the open contact of an IHP digital time clock.?

The new generation programmable time switches (CCT15720, CCT15721, CCT15722, CTT15723, CCT15850, CCT15851,CCT15852, CCT15853, CCT16650 and CCT15652) integrate the O-crossing technique. This is an electronic procedure enabling the circuit to be opened at zero current, which enables the electromechanical contact to be operated off-load.

This is used to avoid the inrush currents of certain loads, particularly lighting.

This solution involves a low leakage current traveling through the electronics which generates residual voltage at the contact terminals, even when open. This can cause problems when very low loads (small relays, LEDs, etc.) are operated, as the receiver may be "held on" by this spurious voltage.

Note:
For this type of application (low load), a conventional electromechanical programmable time switch should be used.