Time clock / programmable timer not switching its load
A time clock (lighting, heating, ventilation) isn't switching at the programmed times — the load stays on or off regardless of the schedule.
Safety first
The load can switch at the programmed time while you're working — confirm what it controls. Isolate before working on the switched load.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Wrong time / date / schedule set
Most likelyClock time, date, or the on/off program is wrong, so it switches at the wrong time or not when expected.
- 2
Manual override engaged
#2A permanent-on/off or holiday override is active, ignoring the schedule.
- 3
Backup battery flat (settings lost)
#3After a power interruption, a flat backup battery loses the time/program.
- 4
Output contact failed
#4The internal switching contact has failed and won't change the load.
- 5
Load on the wrong contact / wiring fault
Least likelyThe load is landed on the wrong terminals or the wiring is faulty.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Check the clock's current time/date and the programmed schedule against what's intended.
Correct time/date and a schedule matching the requirement.
Schedule correct — check overrides and the output.
Wrong time/date/schedule — correct it.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Time clock not switching
→ step 2 - 2decision
Are time/date and schedule correct?
Yes→ step 3No→ step 4 - 3decision
Is any manual/holiday override clear?
Yes→ step 5No→ step 6 - 4result
Correct time/date/schedule.
- 5decision
Does the output contact switch at a switch point?
Yes→ step 7No→ step 8 - 6result
Clear the override.
- 7result
Output switches — load on wrong contact/wiring fault.
- 8result
Contact won't switch — replace clock (check backup battery).
Common mistakes apprentices make
- Not noticing a manual override is engaged.
- Wrong time/date after a power cut with a flat battery.
- Watching the wrong contact set.
- Assuming the clock failed when the schedule is simply wrong.
When to stop & escalate
If the schedule requirement isn't documented, confirm the intended on/off times before changing the program. A flat backup battery that keeps losing settings means the unit needs attention.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Timer relay not switching its output
A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.
Timer completely dead (no power / no display)
The timer shows no signs of life — no display, no LED, no output activity — so nothing it controls will ever operate.
Timer behaving as the wrong type (on-delay vs off-delay)
The timer switches at the wrong point in the sequence because it's acting as the wrong function — on-delay where off-delay is needed, single-shot where cyclic is needed, and so on.