Timer Relay (TON, TOF, Flasher) & Phase Failure / PTC Relay Wiring Diagram
IEC 61812-1 on-delay (TON), off-delay (TOF Y1 trigger), and flasher timer relay wiring (A1-A2 / 15-16-18), plus IEC 60255 phase sequence, voltage asymmetry, and PTC thermistor motor protection.
Timer relay and phase-failure / motor protection relay wiring governs sequential automation timing (IEC 61812-1) and three-phase supply fault protection (IEC 60255) inside industrial control panels. Neither device switches heavy motor current directly; instead, its `15-16-18` SPDT dry contact switches the power contactor's `A1` coil circuit.
> Related Category: Select timing ranges and protection thresholds in Timer Relays, Multi-Function Timer Relays, Phase Sequence Protection Relays, and Phase and Motor Protection Relays.
1. Timer relay terminals (`A1-A2`, `Y1/B1`, `15-16-18`) and timing modes
- `A1` & `A2`: Supply terminals (e.g., universal `24–240 V AC/DC` or dedicated `230 V AC`).
- `Y1` (or `B1` / `S`): External control/trigger input referenced to `A1`.
- `15` (`COM`), `16` (`NC`), `18` (`NO`): Changeover output contact (`5–8 A` AC-1 pilot duty) wired to contactor coil `A1`.
| Timing function | Supply & trigger rule | Output (`15-18`) sequence | Typical panel application |
|---|---|---|---|
| On-Delay (`TON`) | Timing starts when `A1-A2` energizes | `15-18` closes after preset delay `t`; resets immediately on power-off | Staggered pump/compressor startup, pre-start horn delay |
| Off-Delay (`TOF`) | Keep `A1-A2` permanently powered; closing `Y1` closes `15-18` immediately, opening `Y1` starts countdown `t` | `15-18` stays closed for `t` after `Y1` opens, then drops out | Post-shutdown spindle cooling fan, lubrication overrun |
| Flasher (Cyclic) | Cycles continuously while `A1-A2` is powered | Alternates `t_on` / `t_off` | Beacon pulsing, intermittent chemical dosing, agitators |
2. Phase sequence, asymmetry, and PTC thermistor relay wiring
Single-phasing (losing one supply phase) and accidental phase reversal during utility work are leading causes of 3-phase motor burnout. Wire phase and motor protection relays and voltage protection relays as follows:
- Voltage sensing (`L1, L2, L3` + `N` if 4-wire): Connect fused `1.5 mm²` sensing leads downstream of the main isolator. When `L1-L2-L3` rotation is clockwise and phase-to-phase voltages lie within under/overvoltage (`e.g., 340–440 V`) and asymmetry (`5–15%`) windows, `15-18` closes.
- Series coil interlock (`15-18`): Wire `15-18` in series with the emergency-stop chain, thermal overload `95-96` NC contact, and contactor coil `A1`.
- Why asymmetry/phase-angle detection beats plain undervoltage relays: A spinning 3-phase motor that loses one fuse regenerates 75–85% back-EMF onto the dead phase wire! Phase-angle and asymmetry relays detect the angular/unbalance shift immediately even when regenerated voltage looks high.
- PTC thermistor input (`T1-T2` / `PTC`): Connects to winding-embedded PTC sensors (tripping above `~3.6 kΩ`). If the motor has no PTC leads, install a wire jumper across the `PTC` terminals so the open-circuit fault monitor does not lock out the relay.
Frequently asked questions
Why does my Off-Delay (TOF) timer drop out immediately when I switch off power?
Standard electronic TOF timers require continuous `A1-A2` auxiliary supply to run their internal microcontroller during the off-delay period, using `Y1/B1` as the switched trigger. Only supercapacitor "True Off-Delay" relays time out after `A1-A2` loses power.
What should I swap when the Phase Sequence fault LED lights up?
Isolate incoming power completely and swap any two incoming phases (e.g., `L1` and `L2`) at the main incoming terminals so both the protection relay and the downstream pumps/compressors receive correct forward rotation.
Why won't a new PTC motor protection relay energize on the bench?
The PTC circuit monitors wire breakage as well as overheating (`R > 3.6 kΩ = trip`). Leaving `T1–T2` open-circuit mimics an overheated winding; fit a short link across `T1–T2` during bench testing or on motors without PTC thermistors.
Can a timer relay contact switch a 2.2 kW motor directly?
No. Timer contacts are rated `5–8 A` resistive (`AC-1`); motor starting inrush (`6–8 × In`) welds miniature relay contacts immediately. Always drive an AC-3 power contactor coil through the timer contact.