How do I troubleshoot a problem with my landscape floodlights( not low voltage)? All of a sudden the timer is tripping the circuitbreaker when plugged in. Is this a bad timer or how do I determine if there is a short in the lights? Can the lights cause the timer to trip the circuit breaker? What can I do before I call the professional electrican?

Thanks,
Dennie

It is highly unlikely that it is the timer itself that is causing the problem. It is more likely that it is the lights or the wiring to the lights. However, you can check by disconnecting the wires at the load terminals of the timer. If breaker still trips it is the timer. If breaker does not trip it is the lights.

Inspect the lights and wiring for damage, cuts or loose wiring. Presumably this wiring is underground and probably in conduit. Check conduit for water.

Turn power off. If you know or you think you know, the sequence of the light wiring, disconnect the wiring at the middle or center light. Cap all wires with wire nut. Turn power on. If breaker trips, the fault is in the first half of the lights. If breaker does not trip, fault is in second half.

If I were to venture a guess, it would be that the wiring is underground, in EMT conduit and filled with water. EMT conduit is not water proof.

The Above is exact, I would unscrew and screw EVERY bulb in circuit, I bet it will be visual.
Any recent Yard Excavation, new trees?
Some contractors have been known to cut lines accidentally and forget to tell you, or thought it was obsolete.