The standard IEC 60617 symbol for an on-delay timer coil is a rectangle containing an "X" or two opposing diagonal arrows, while the NEMA standard uses a simple box labeled "TD" (Time Delay) or "DO" (Delay On). When the coil is energized, the associated contacts delay their state change by the preset time. If you are reading a European or global schematic, look for the IEC arrows pointing toward the contact line; if you are in North America, look for the NEMA "TD" box with standard normally open (NO) or normally closed (NC) contact symbols attached.
Complete On-Delay Timer Symbol & Contact Reference
Before wiring a control panel, you must correctly identify both the coil and the specific contact configuration. The table below maps the exact schematic symbols to their functional behavior in a live circuit.
| Component | IEC 60617 Symbol Description | NEMA / ANSI Symbol Description | Functional Behavior (On-Delay) | Common Designator |
|---|---|---|---|---|
| Timer Coil | Rectangle with "X" or opposing arrows inside | Rectangle with "TD" or "DO" text | Starts timing immediately upon receiving voltage. | K1-T, TR1, T1 |
| NOTC Contact | NO symbol with arrow pointing toward the line | NO symbol with "TD" or "DO" attached | Normally Open, Timed Closed. Closes only after delay expires. | 13-14, 5-6 |
| NCTO Contact | NC symbol with arrow pointing toward the line | NC symbol with "TD" or "DO" attached | Normally Closed, Timed Open. Opens only after delay expires. | 11-12, 7-8 |
| Instantaneous Contact | Standard NO/NC (no delay arrow) | Standard NO/NC (no TD label) | Changes state immediately when coil is energized, ignores delay. | 15-16, 18 |
What this means in practice: The NOTC (Normally Open, Timed Closed) contact is the workhorse of motor star-delta starters and soft-start bypass circuits. It keeps the delta contactor open during the initial high-inrush star phase, only closing to engage the delta run winding once the timer expires and the motor reaches near-rated RPM.
Regional Standards and the "Rows People Get Wrong"
Schematic interpretation changes drastically depending on the region and the age of the prints. Here is how the standards break down:
- IEC 60617 (Global/Europe/Modern): Uses geometric shapes and directional arrows. The critical detail is the arrow direction on the contact. For an on-delay timer, the arrow points toward the horizontal contact line, symbolizing that the contact is "delayed" from reaching its final state.
- NEMA / ANSI (North America): Relies heavily on text abbreviations inside boxes (TD, DO, TO). It is less visual but highly standardized in US industrial panels.
- Old UK (BS 3939): Largely obsolete but still found in legacy plant retrofits. Uses a semicircle with a flat top for the coil, which is easily confused with a standard relay if the internal "T" marking has faded.
The most common bench mistake is misreading the IEC contact arrow. If the arrow points away from the contact line, it is an off-delay (DTOE) contact, meaning it changes state instantly on energization but delays returning to normal when power is removed. Wiring an off-delay contact into a star-delta transition circuit will cause a dead short across the line contactors. Always verify the arrow points toward the line for on-delay applications.
Translating Schematics to Physical 8-Pin and 14-Pin Modules
Symbols on paper must eventually map to physical pins on a DIN-rail socket. The two most common form factors in industrial control are the 8-pin octal and the 14-pin plug-in bases. Below is the exact pin mapping for benchmark modules like the Omron H3Y (14-pin) and standard 8-pins (like the Omron H3CR-A8 or Schneider Zelio RE22).
| Function | 8-Pin Octal Base (Standard) | 14-Pin Base (e.g., Omron H3Y) |
|---|---|---|
| Coil Power (+ / L1) | Pin 2 | Pin 13 (or 14 depending on AC/DC variant) |
| Coil Power (- / L2) | Pin 7 | Pin 2 (Common for AC/DC coil return) |
| COM 1 (Pole 1 Common) | Pin 1 | Pin 9 |
| NC 1 (Timed Open) | Pin 4 | Pin 1 |
| NO 1 (Timed Closed) | Pin 6 | Pin 3 |
| COM 2 (Pole 2 Common) | Pin 8 | Pin 10 |
| NC 2 (Timed Open) | Pin 5 | Pin 6 |
| NO 2 (Timed Closed) | Pin 3 | Pin 8 |
Note: Always cross-reference the socket diagram printed on the side of the relay base. Pinouts on 14-pin bases can shift between AC coil and DC coil variants, particularly regarding pins 13 and 14.
Decision Path: Specifying the Right Timer for the Load
Do not default to a single timer model for every panel. Use this decision matrix to select the correct physical module and contact rating for your specific application.
| Application Scenario | Required Feature | Concrete Module Pick |
|---|---|---|
| Motor Star-Delta Transition (High inrush, needs precise 1-30s delay) | Multi-range dial, high electrical endurance on NO contacts, 8-pin footprint. | Schneider Zelio RE22R2AMR (8-pin, 8A at 250VAC, multi-function but set to 'A' for on-delay). |
| HVAC Fan Blower Delay (Needs 0.1s to 100h wide range, simple setup) | Digital display or wide analog dial, 24VAC/DC coil compatibility. | Omron H3CR-A8-AC/DC24 (8-pin, dual contact, extremely stable solid-state output). |
| Lighting/Heating Bank Sequencing (Needs to stagger multiple loads to prevent breaker trip) | Multiple independent channels or 14-pin multi-pole switching. | Omron H3Y-4-P (14-pin, 4PDT contacts to switch multiple 2A pilot circuits simultaneously). |
| High-Current Direct Switching (>10A without external contactor) | Internal heavy-duty relay, DIN rail mount, screw terminals (not plug-in). | Macromatic TD-75521 (Panel mount, 10A SPDT, handles direct load without pilot relays). |
Field Verification: Testing Faded or Unmarked Timers
When maintaining legacy panels, you will inevitably encounter timers with rubbed-off labels, missing schematic stickers, or faded dial markings. Never assume a timer's function based on its physical size or the wiring color. Follow this safe verification protocol using a digital multimeter (DMM).
- De-energize and Isolate: Turn off the control circuit breaker. Use a non-contact voltage tester (NCVT) and verify dead with your DMM across the coil pins (e.g., pins 2 and 7 on an 8-pin base). Lock out and tag out (LOTO) the panel if required by site safety policy.
- Identify the Coil: Set your DMM to resistance (Ohms). Probe the base pins. The coil will typically read between 50Ω (for 24VDC coils) and 10kΩ (for 240VAC coils). Pins showing infinite resistance (OL) are contacts; pins showing near 0Ω are common/NC contacts.
- Verify the Delay Function (Bench Test): Remove the timer from the base. Apply the rated coil voltage (e.g., 24VDC) using a bench power supply.
- Immediately probe the NO contacts (e.g., 1 and 6). The DMM should read OL (open).
- Set the dial to the minimum time (e.g., 0.5 seconds).
- After the delay expires, you should hear an internal click (if electromechanical) or see an LED change state. The DMM should now read < 1 ohm across the NO contacts.
- Remove power. The contacts must immediately return to OL. If they remain closed and slowly open, you are holding an off-delay timer, not an on-delay timer.
If your DMM reads a high or fluctuating resistance (e.g., 500Ω to 2kΩ) across the closed NO contacts instead of < 1 ohm, you are likely testing a timer with a solid-state (triac/MOSFET) output rather than a mechanical relay. Solid-state timers require a minimum load current to fully "turn on" and will show phantom voltages or high resistance on a standard DMM. Use a 12V incandescent test lamp in series to verify solid-state contact closure.
Correctly identifying the on-delay timer symbol on a schematic is only the first step. By cross-referencing the IEC or NEMA symbols with the physical pinout tables and verifying the internal switching mechanism on the bench, you eliminate the risk of cross-wiring transition circuits and ensure your control logic operates exactly as engineered.
