Lack of details promotes guessing, so I'll guess that it is a mechanically latched lighting contactor.
wst01,
This means that your contactor would have TWO coils. You would pulse one to pull the contacts in, then pulse the other to drop them out. This is done on a lot of lighting circuits because they don't want the buzzing sound of an electrically held contactor that is held closed for hours, days or even months at a time. One quick way to tell would be if your old switch had a center position, you would flip the switch up to turn the lights on, then down to turn them off, but it would spring return to a neutral center position; either that or it was a set of momentary push buttons, push-on / push-off or something else non-ordinary.
If it is not a latching contactor, then another possibility is that when installing the necessary circuit for the time clock control, someone routed the control circuit wires in the same conduit or raceway as the power wires for the lighting. When you energize the lighting power circuit it creates a voltage on the control wires through inductive coupling from the magnetic fields surrounding the power wires. This voltage could be enough to not allow the coil to drop out when you open the circuit coming from the time clock. Typically it is difficult to induce enough voltage to do this, but if you have a very long run of control wire back through the same conduits, as is sometimes the case with lighting, it is remotely possible.
Go look at the contactor and start there. If you aren't sure, post the part number of it, someone here will be able to tell.
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376
