The symbol for on delay timer (designated as TON in PLC ladder logic, or K/TE in hardwired schematics) represents a control relay that delays its contact state change for a preset duration after the coil receives power. Visually, the symbol combines a standard relay coil with a specific function letter and timed contacts marked with directional arrows or cross-hatching. Because global electrical standards diverge significantly on how these arrows are drawn, misreading the symbol is a leading cause of troubleshooting errors in industrial control panels.
The Complete On-Delay Timer Symbol Reference Table
Before tracing any wires, identify which drafting standard your schematic uses. The table below maps the exact visual geometry for the coil and the two most common on-delay contacts: Normally Open Timed Close (NOTC) and Normally Closed Timed Open (NCTO).
| Component Function | IEC 60617 (Global / EU / AU) | ANSI/IEEE 315 (US / Canada) | Legacy BS 3939 (Pre-1990s UK) |
|---|---|---|---|
| Timer Coil | Rectangle with letter 'T', 'TE', or 'K' and a downward arrow (t↓) inside or adjacent. | Circle with 'TD', 'TON', or a specific device number (e.g., '1T'). | Circle with 'T' and a small clock face icon or 'd' (delay) subscript. |
| NOTC Contact (Normally Open, Timed Closed) |
Standard NO contact blade with a left-pointing arrow on the moving hinge (indicating delayed movement to the left to close). | Standard NO contact blade with a downward-pointing arrow on the blade, pointing toward the hinge. | NO contact with a small 'x' on the hinge side and a delay arrow. |
| NCTO Contact (Normally Closed, Timed Open) |
Standard NC contact blade with a right-pointing arrow on the moving hinge (indicating delayed movement to the right to open). | Standard NC contact blade with an upward-pointing arrow on the blade, pointing away from the hinge. | NC contact with a small 'x' on the hinge side and a delay arrow. |
| Instantaneous Contact (If equipped) |
Standard NO/NC contact with no arrow, grouped under the same 'T' designator. | Standard NO/NC contact with no arrow, linked by a dashed mechanical line to the coil. | Standard contact with no delay markings. |
Regional Standards: Which Symbol Set Applies to Your Panel?
Assuming a single standard applies universally is a fast way to miswire a 24VDC control circuit. Here is how to determine which symbol set you are looking at based on the equipment's origin and installation date.
In modern facilities, it is common to find IEC-drawn schematics for a machine built in Germany, retrofitted with ANSI-standard Allen-Bradley PLCs in the US. Always verify the title block in the bottom right corner of the first schematic page to confirm the drafting standard before assuming the arrow directions.
IEC 60617 (International Electrotechnical Commission): This is the current standard for the EU, UK (post-1990s), Australia, and most of Asia. IEC symbols are highly abstract and rely on rectangular boxes for coils and specific arrow directions on the contact blades to denote timing. If your panel features DIN-rail components like a Schneider Electric Zelio RE7 or a Siemens SIRIUS 3RP25, the documentation will use IEC symbols.
ANSI/IEEE 315 & NEMA (North America): Used predominantly in the United States and Canada. ANSI symbols are more pictorial, using circles for coils and relying on the "arrow pointing toward the hinge" rule to indicate the direction the contact moves after the time delay expires. NEMA ladder logic diagrams heavily utilize these symbols.
Legacy BS 3939 (Old UK Standard): If you are troubleshooting machinery in an older British manufacturing plant (installed before the mid-1990s), you will encounter BS 3939. This standard used circles for coils and relied on an 'x' mark at the hinge of the contact blade to indicate a timed function. While officially superseded by IEC 60617, these prints are still actively used for maintenance on legacy equipment.
Rows People Get Wrong (And How to Read Faded Prints)
When cross-referencing the table above, there are specific rows and field conditions that consistently trip up technicians and hobbyists.
The "Arrow Direction" Confusion (NOTC vs. NCTO)
The most critical error occurs when reading the NOTC (Normally Open, Timed Closed) row. In the ANSI standard, the arrow points down toward the hinge, which intuitively makes sense: the contact "falls" closed after the delay. However, in the IEC standard, the arrow points left. Technicians used to ANSI often misinterpret the IEC left-pointing arrow as an "off-delay" or a reverse-action contact. Remember the IEC rule: the arrow always points in the direction of the delayed physical movement of the blade. For a NOTC contact, the blade must move left to bridge the gap.
Assuming the Coil Symbol Dictates All Contacts
Many readers assume that if a timer coil is labeled "TON" (On-Delay), every contact associated with that coil is a timed contact. Modern multi-function timers (like the Omron H3CR-A8, typically priced around $65-$90 USD) often feature both timed and instantaneous contacts. Only the contacts drawn with the specific delay arrows or 'x' marks are timed; any standard NO/NC contacts grouped under the same device tag operate instantly when the coil energizes.
Safe Interpretation When Markings Are Faded or Missing
Industrial environments are harsh. Thermal cycling, oil mist, and UV exposure often fade the delicate arrow markings on older blueprints or CAD printouts. If you cannot definitively read the symbol for on delay timer on the schematic, do not guess based on circuit logic. Use this physical verification protocol:
- Locate the physical timer: Find the device on the DIN rail matching the schematic tag (e.g., '2TE').
- Read the chassis schematic: Reputable manufacturers print the internal pinout directly on the side of the relay housing. On a standard 8-pin octal timer, pins 2 and 7 are the coil. Pins 1, 3, and 4 form the NO timed contact, while 5, 6, and 8 form the NC timed contact.
- Verify with a multimeter: De-energize the panel. Set your meter to continuity. Place probes on the common and NO pins. Apply 24VDC to the coil (if safe and permitted). The meter should remain open (OL) and only beep after the dial-set delay expires, confirming it is indeed an on-delay NOTC contact.
Frequently Asked Questions
What is the exact difference between an on-delay and off-delay timer symbol?
The difference lies entirely in the contact arrow direction and the coil designation. An on-delay timer (TON) delays the contact change when the coil energizes, and the contacts return instantly when power is removed. Its symbol features arrows pointing toward the "timed" state. An off-delay timer (TOF) changes contacts instantly when energized, but delays the return to the normal state when power is removed. In ANSI, the off-delay arrows point away from the hinge (indicating the delayed return stroke). In IEC, the off-delay arrows point in the opposite direction of the on-delay arrows, and the coil is often marked 't↑' or 'TE-off' to distinguish it.
How do I verify a timer function if the schematic symbol is completely faded?
If the schematic is illegible, bypass the drawing and trace the physical wires. Identify the timer model number (e.g., Schneider RE7TA11BU). Download the manufacturer's datasheet, which will provide the definitive pinout. Wire a temporary 24VDC test circuit to the coil pins and use a digital multimeter with a stopwatch function on the output pins. If continuity changes state 5 seconds after applying power, it is an on-delay. If continuity changes state 5 seconds after removing power, it is an off-delay. Always rely on the physical bench test over a degraded schematic.
Does the symbol for on delay timer differ between PLC ladder logic and hardwired schematics?
Yes, significantly. In hardwired electrical schematics (IEC/ANSI), you are looking at physical geometric shapes (rectangles, circles, lines, and arrows) representing physical copper contacts and electromagnetic coils. In PLC ladder logic (such as Rockwell Studio 5000 or Siemens TIA Portal), the symbol for on delay timer is a standardized software instruction block. It is typically drawn as a rectangular block labeled TON, with explicit inputs for Preset (PRE), Accumulated (ACC), and Enable (EN), and outputs for Done (DN), Timing (TT), and Enable (EN). While the underlying logic is identical, the visual representation shifts from physical contact geometry to digital data-flow blocks.
