Latching / impulse relay stuck in one state
A latching (impulse) relay won't change state on a pulse — lighting or a load stays on or off regardless of the switch, because the relay isn't toggling.
Safety first
Latching relays hold their state with no coil power, so the load can be live even when 'off' at the switch. Isolate and prove dead before working on the 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
No pulse reaching the coil
Most likelyThe momentary switch or its wiring isn't delivering a clean pulse to toggle the relay.
- 2
Multiple switches / wiring fault confusing the pulse
#2In multi-switch impulse setups, a stuck switch or wiring fault holds the line, preventing a clean toggle.
- 3
Mechanically stuck latch
#3The relay's bistable mechanism has jammed in one position.
- 4
Failed coil
Least likelyThe set or reset coil has gone open and can no longer flip the latch.
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.
Operate the switch and check for a momentary pulse at the relay coil terminals.
A clean momentary pulse arrives on each press.
Pulse arrives but it won't toggle — suspect the latch or coil.
No clean pulse — check the switches and wiring feeding it.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Latching relay stuck
→ step 2 - 2decision
Does a clean pulse reach the coil on each press?
Yes→ step 3No→ step 4 - 3decision
Do the coils test good and the latch toggle directly?
Yes→ step 5No→ step 6 - 4decision
Is a switch stuck or the pulse line held?
Yes→ step 7No→ step 8 - 5result
Relay toggles — recheck the control pulse path.
- 6result
Stuck latch or open coil — replace the relay.
- 7result
Stuck switch / held line — repair it.
- 8result
No pulse and nothing held — trace the switch wiring.
Common mistakes apprentices make
- Forgetting a latching relay holds state without power, so the load stays live.
- Not finding a stuck momentary switch in a multi-point setup.
- Treating it like a normal relay and expecting continuous coil voltage.
- Replacing the relay before checking the pulse actually arrives.
When to stop & escalate
If a stuck switch is in an inaccessible position or part of a larger lighting control scheme, plan the work with the relevant documentation. A jammed latch means replacement.
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
Control relay coil not energising
A plug-in or interface relay isn't picking up — its indicator stays off and its contacts don't change, so whatever it controls never operates.
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.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.