The symbol for an off-delay timer (TOFF) relies on a standard relay coil paired with a specific modifier: in IEC 60617 schematics, it is an "X" placed on the trailing side of the contact arm (or a left-pointing arrow), while NEMA/JIC standards use a downward-pointing arrow inside the coil or a "TOFF" designation next to a standard timed-contact symbol. Off-delay timers—also called delay-on-release or dropout timers—keep their contacts in the actuated state for a preset time after the coil is de-energized.
The Complete Reference Table: Off-Delay Timer Symbols
The following table maps the exact schematic symbols for off-delay timers across the three dominant global standards. Use this as your bench reference when reading prints or designing control panels.
| Component | IEC 60617 (Global / EU) | NEMA ICS 19 (US Legacy) | JIC / Modern US |
|---|---|---|---|
| Timer Coil | Rectangle with a downward-pointing arrow inside, or labeled "TOFF". | Circle with "TD" (Time Delay) and a downward-pointing arrow. | Rectangle labeled "TR" or "1TR" with "TOFF" subscript. |
| NO Contact (Off-Delay) | Standard NO symbol with an "X" on the right side of the moving arm slash. | Labeled NOT (Normally Open, Timed) or NOTDO (Normally Open, Timed Delay Open). | Standard NO symbol with a small downward arrow next to the contact gap. |
| NC Contact (Off-Delay) | Standard NC symbol with an "X" on the right side of the moving arm slash. | Labeled NCTO (Normally Closed, Timed Open). | Standard NC symbol with a small downward arrow next to the contact gap. |
| Timing Mechanism | Not explicitly symbolized; assumed solid-state or pneumatic based on part list. | Sometimes a small bellows symbol is drawn next to the circle (pneumatic legacy). | Not symbolized; relies on BOM (Bill of Materials) for digital vs. analog. |
Regional Standards and the 'Rows People Get Wrong'
While modern digital timers like the Siemens SIRIUS 3RP25 or Schneider Zelio RE22 use internal microcontrollers to achieve precise millisecond drop-out delays, the schematic symbols remain tied to legacy standards. Misinterpreting these symbols is a leading cause of control logic faults in retrofitted panels.
Never assume a timed contact is instantaneous. If an off-delay NC contact (NCTO) is used in an E-stop or safety interlock circuit, the machine will remain energized for the duration of the timer's drop-out delay after the stop button is pressed. Always verify the timing modifier on the print before locking out a panel.
Rows People Get Wrong
- The IEC 60617 "X" Placement: This is the most common error on the bench. In IEC schematics, time-delay contacts use an "X" on the moving arm. If the "X" is on the left side of the slash (the side where the contact closes), it is an on-delay. If the "X" is on the right side (the side where the contact opens), it is an off-delay. Technicians frequently swap these when redlining prints.
- NEMA Acronym Soup (NOT vs. NCTO): NEMA standards rely heavily on text acronyms rather than graphical modifiers. "NOT" means Normally Open, Timed (which implies it closes on a delay, but opens instantly—an on-delay NO contact). Conversely, "NCTO" means Normally Closed, Timed Open (an off-delay NC contact). Readers often see the "NC" and assume instantaneous operation, missing the "TO" modifier.
- Old UK BS 3939 Prints: If you are troubleshooting legacy equipment in the UK or Commonwealth nations, you may encounter BS 3939 symbols. These use a standard relay symbol but add a small "delay" text box or a specific hatched rectangle on the coil. BS 3939 was largely superseded by IEC 60617, but legacy industrial sites still rely on these older prints.
Safe Interpretation When Markings Are Faded or Missing
In harsh industrial environments—paper mills, wastewater treatment plants, or marine applications—schematic prints inside the panel door often suffer from oil saturation, UV fading, or physical tearing. When the graphical modifier (the "X", the arrow, or the NCTO text) is illegible, you must safely deduce the timer's function without relying on the drawing.
Step 1: Trace the Terminal Designations
Modern DIN-rail timers follow strict IEC terminal numbering. The coil will be A1 (+) and A2 (-). The contacts will follow standard numbering: 15/16 or 25/26 for NC, and 17/18 or 25/28 for NO. If the device is a dedicated off-delay timer (like an Omron H3Y configured for TOFF), all contacts will behave as off-delay. If it is a multi-function timer, check the DIP switch or rotary dial on the faceplate for the "TOFF" or "D" (Delay) setting.
Step 2: The Multimeter Drop-Out Test
If the part number is obscured and the faceplate is missing, use a digital multimeter (like a Fluke 87V) in continuity mode to bench-test the relay.
- Apply the rated coil voltage (e.g., 24V DC to A1/A2).
- Verify the contacts change state immediately (or after a delay, which would indicate an on-delay function).
- The critical test: Remove power from A1/A2. If the contacts immediately return to their normal state, it is an instantaneous relay or an on-delay timer. If the contacts remain in their actuated state for a measurable duration (e.g., 0.5 to 30 seconds) before snapping back, you have confirmed an off-delay timer.
Step 3: Check for Pneumatic Dashpots
On older motor control centers (MCCs), off-delay was often achieved mechanically using a pneumatic dashpot attached to a standard control relay (like a NEMA Size 1 starter). If you see a small brass cylinder with a knurled adjustment screw and an air bleed valve mounted directly on top of the relay armature, it is a pneumatic off-delay. Adjusting the screw changes the air bleed rate, altering the drop-out time.
Frequently Asked Questions
What is the difference between an on-delay and off-delay timer symbol?
The core difference lies in the graphical modifier applied to the contact arm or coil. In IEC 60617, an on-delay contact has an "X" on the left side of the moving arm slash (indicating delayed closure), while an off-delay contact has the "X" on the right side (indicating delayed opening). For coils, an on-delay symbol typically features an upward-pointing arrow or an "X" on the left side of the coil rectangle, whereas an off-delay coil uses a downward-pointing arrow or an "X" on the right side, signifying that the timing action occurs upon power removal (drop-out).
How do I wire an off-delay timer if the schematic symbol is missing?
Wiring an off-delay timer requires a continuous power source to the coil, switched by a control signal. For a standard 24V DC off-delay timer, wire your 24V+ to terminal A1 and 0V to A2. To trigger the off-delay sequence, place a momentary pushbutton or limit switch in series with A1. When the switch is closed, the timer energizes and the contacts change state instantly. When the switch opens (removing power from A1), the internal RC circuit or microcontroller maintains the coil's magnetic field or solid-state switch for the preset duration before the contacts revert. Always consult the manufacturer's datasheet for specific terminal mappings, as NEMA and IEC terminal numbering conventions can differ.
Does the off-delay timer symbol indicate the specific timing range?
No. Schematic symbols (whether IEC, NEMA, or JIC) only indicate the logic function (delay on release) and the contact state (NO or NC). They do not convey the timing range (e.g., 0.1s–10s or 10s–100h) or the specific time setting. The exact timing value must be documented in the Bill of Materials (BOM), the timing schedule on the print's title block, or indicated as a note next to the coil symbol (e.g., "1TR TOFF - 5.0s"). If the print lacks this note, you must verify the physical dial setting on the timer module installed in the panel.
