The electrical symbol for a timer is not universal; it changes entirely based on the drafting standard (IEC vs. ANSI/NEMA) and the control environment (electromechanical schematics vs. PLC ladder logic). In IEC 60617 standards, a timer relay is designated by the letter code KT inside a rectangular box, while ANSI/IEEE 315 and NEMA standards use specific directional arrows on switch contacts to denote on-delay or off-delay behavior. In PLC environments, timers are represented by block instructions like TON (Timer On-Delay) or TOF (Timer Off-Delay).
Below is the definitive bench and jobsite reference for identifying, wiring, and troubleshooting timer symbols across all major standards.
Master Electrical Symbol Timer Reference Chart
This table maps the four primary time-delay functions across the three dominant schematic standards. Keep this reference handy when reading legacy panels or designing new control circuits.
| Timer Function | IEC 60617 Symbol / Designation | ANSI / NEMA Contact Symbol | PLC Ladder Logic Block | Typical Application |
|---|---|---|---|---|
| On-Delay (TON) | Rectangle with 'KT' and a diagonal arrow pointing down-right. Contacts marked with a left-pointing arrow. | Normally Open (NO) contact with an arrow pointing up (towards the contact line) indicating delay on make. | TON block with Preset (PRE) and Accumulated (ACC) values. Bit enables when rung is true. | Motor star-delta starting transitions; blower delayed start after furnace ignition. |
| Off-Delay (TOF) | Rectangle with 'KT' and a diagonal arrow pointing down-left. Contacts marked with a right-pointing arrow. | Normally Open (NO) contact with an arrow pointing down (away from the contact line) indicating delay on break. | TOF block. Timer starts accumulating when rung goes false; resets when rung goes true. | Cooling fans that run for 5 minutes after a compressor shuts down; lubrication pumps. |
| Pulse / One-Shot (TP) | Rectangle with 'KT' and a square-wave pulse graphic inside. Often labeled as a monostable multivibrator. | Not typically represented as a standalone contact in basic NEMA; usually drawn as a specialized relay coil. | OSR (One-Shot Rising) or TP block. Outputs a fixed pulse width regardless of input duration. | Debouncing mechanical switch inputs; triggering a solenoid valve for a precise 50ms glue dispense. |
| Retentive / Interval (RTO) | Rectangle with 'KT' and a memory/latch indicator (often a small box or 'R' suffix). | Standard timed contacts, but the coil symbol includes a mechanical latch indicator. | RTO block. Accumulates time across multiple true/false cycles until PRE is reached; requires manual reset. | Equipment maintenance hour meters; thermal overload accumulation tracking. |
Regional Variants and the 'Rows People Get Wrong'
When you are tracing wires in a 480V control panel, misreading a timer symbol can lead to catastrophic sequencing errors. The divide between North American (NEMA/ANSI) and International (IEC) drafting styles is the most common source of confusion.
IEC vs. ANSI/NEMA: The Core Differences
According to the All About Circuits reference on time-delay relays, the IEC standard relies heavily on alphanumeric codes. The letter 'K' designates a relay, and the suffix 'T' designates time. Therefore, KT is your IEC timer. Auxiliary relays are KA, and contactors are KM. The IEC symbol focuses on the coil designation and uses small directional arrows on the switch blade to show the delay direction.
Conversely, NEMA and IEEE 315 standards focus on the contact behavior. They use a standard switch symbol but add a detached arrow to indicate the pneumatic or electronic dashpot action.
The Rows People Get Wrong
- The NEMA Arrow Direction Trap: This is the most frequently misread symbol on the bench. In NEMA schematics, an arrow pointing towards the contact line (upwards for a top-hinged contact) means the contact is delayed in making (On-Delay). An arrow pointing away from the contact line (downwards) means it is delayed in breaking (Off-Delay). Hobbyists and junior techs often assume the arrow points in the direction of time passing, which is backwards for NEMA make/break logic.
- Confusing IEC 'KT' with 'KA': On faded schematics, the 'T' can look like an 'A'. If you treat a standard auxiliary relay (KA) as a timer (KT), you will bypass the intended time delay, causing immediate contactor engagement and potential mechanical shock to the load.
- PLC Retentive vs. Non-Retentive: In PLC ladder logic programming, confusing a standard TON with an RTO (Retentive Timer) is a classic error. A TON resets to zero the millisecond the input drops. An RTO holds its accumulated time and requires a specific RES (Reset) instruction. Using a TON for a motor run-hour tracker will result in the timer resetting every time the motor stops.
Physical Pinouts: Translating Symbols to 8-Pin and 11-Pin Bases
Schematic symbols are abstract; physical wiring requires exact pin numbers. The vast majority of industrial plug-in timers (like the ubiquitous Omron H3Y or Macromatic time-delay relays) use standard octal (8-pin) or 11-pin bases. Here is how the schematic coil and contacts map to the physical socket.
Standard 8-Pin Timer Base (e.g., Omron H3Y-2)
Used for DPDT (Double Pole, Double Throw) timed contacts.
- Pins 2 & 7: Timer Coil (Power Supply). This is where you wire your 24VDC, 120VAC, or 240VAC control voltage. Always verify the coil voltage printed on the side of the relay before applying power; feeding 120VAC into a 24VDC coil will instantly destroy the internal switching power supply.
- Pins 1, 3, 4: First SPDT contact set. Pin 1 is Common (C), Pin 4 is Normally Open (NO), Pin 3 is Normally Closed (NC). The timer logic dictates whether this is a timed-closed or timed-open contact.
- Pins 8, 6, 5: Second SPDT contact set. Pin 8 is Common (C), Pin 5 is NO, Pin 6 is NC.
Standard 11-Pin Timer Base (e.g., Omron H3Y-4)
Used for 4PDT (Four Pole, Double Throw) timed contacts, common in complex interlocking circuits.
- Pins 2 & 10: Timer Coil (Power Supply).
- Pins 1, 3, 4: Contact Set 1 (1=Common, 3=NC, 4=NO).
- Pins 5, 6, 7: Contact Set 2 (7=Common, 6=NC, 5=NO).
- Pins 8, 9, 11: Contact Set 3 (8=Common, 9=NC, 11=NO).
- Pins 12, 13, 14: Contact Set 4 (14=Common, 13=NC, 12=NO). Note: Pin numbering on 11-pin bases skips certain physical positions to maintain keying alignment.
Safe Interpretation of Faded or Missing Panel Markings
In aging industrial facilities, the schematic taped to the inside of the panel door is often sun-faded, oil-stained, or missing entirely. If you encounter an unmarked time-delay relay in a circuit and need to determine its function and coil voltage safely, follow this bench-tested diagnostic sequence.
Step 1: Identify the Hardware Type
Look at the physical adjustment mechanism. If the timer has a small slotted potentiometer dial on the front face marked with a time scale (e.g., 0.5s to 30s), it is an electronic solid-state or microprocessor timer. If it has a large knurled knob or a physical dashpot cylinder attached to the armature, it is an older pneumatic timer. Electronic timers require clean DC or stable AC; pneumatic timers are highly tolerant of dirty power but suffer from ambient temperature and altitude drift.
Step 2: Determine Coil Voltage Without Energizing
Do not guess the coil voltage by looking at the surrounding components. Use your multimeter set to the Ohms (Ω) range to measure across the suspected coil pins (Pins 2 & 7 on an 8-pin base).
- Reading 100Ω to 500Ω: Likely a low-voltage DC coil (12VDC or 24VDC).
- Reading 2kΩ to 10kΩ: Likely an AC mains coil (120VAC or 240VAC). AC coils have higher resistance and rely on inductive reactance to limit current.
- Reading Infinite (OL): The internal coil is open (burned out), or you are probing the wrong pins. Check for a built-in fuse on the timer module.
- Reading 0.0Ω (Dead Short): The internal varistor or power supply has shorted. Do not apply power; replace the unit.
Step 3: Bench-Test the Delay Function
Once you have identified a safe voltage, remove the timer from the panel and test it on the bench using a current-limited DC power supply or an isolated control transformer. Apply power and use your multimeter in continuity mode across the Common and NO pins. Use a stopwatch to measure the exact delay. If the schematic symbol is missing, this physical test is the only 100% reliable way to confirm whether the unit is wired as an On-Delay (contacts close after the timeout) or an Off-Delay (contacts open after power is removed and the timeout expires). Always label the physical component with a piece of heat-shrink or a Brady marker once verified, saving the next technician from repeating the process.
