The off delay timer symbol indicates a control relay that keeps its output contacts in their actuated state for a preset duration after the input control voltage is removed. In modern schematics, this is universally recognized by a standard relay coil paired with contacts featuring a downward-pointing arrow (or a slash with a downward arrow), signifying a delay on de-energization (release). Whether you are troubleshooting a Siemens SIRIUS 3RP25 or wiring a Schneider Zelio RE22, reading this symbol correctly prevents catastrophic sequencing errors in motor controls and HVAC systems.
Off Delay Timer Symbol Reference Table
Timer relay symbols vary depending on the drafting standard used by the original equipment manufacturer (OEM). Below is the complete reference matrix for identifying off-delay (release delay) coils and contacts across the three major standards you will encounter in the field.
| Standard / Region | Coil Designation | NO Off-Delay Contact | NC Off-Delay Contact | Key Visual Identifier |
|---|---|---|---|---|
| IEC 60617 (Global / Modern US / EU) |
Rectangle (Designator: K, KT, or specific alphanumeric like K4) | Normally Open (NO) contact with a solid downward-pointing arrow or diagonal slash with downward arrow. | Normally Closed (NC) contact with a solid downward-pointing arrow. | Arrow points down toward the horizontal rest-state line. |
| NEMA ICS 18 / JIC (US Industrial / Legacy) |
Circle containing 'TR', 'TRD', or 'OFF' (Designator: CR or TR) | NO contact with a downward-pointing 'bellows' or dashed-line dashpot symbol. | NC contact with a downward-pointing 'bellows' or dashed-line dashpot symbol. | Dashpot/bellows arrow points down (toward de-energized state). |
| Legacy UK (BS 3939) (Obsolete / Old Panels) |
Circle or Rectangle (Designator: T or TR) | NO contact with a delayed-break slash (often without a distinct arrow, relying on context). | NC contact with a delayed-make slash. | Relies on textual notes or specific slash angles; highly ambiguous. |
Regional Standards and the 'Rows People Get Wrong'
While the IEC 60617 standard has largely conquered the global market—and is the default for modern PLCs and VFDs from manufacturers like ABB, Schneider Electric, and Omron—older industrial facilities in the US still rely heavily on NEMA ICS 18 (and the related JIC) symbology. According to the NEMA standards for electrical control diagrams, the 'bellows' or dashpot symbol represents the pneumatic or electronic timing mechanism resisting the contact's movement.
The Rows and Symbols People Get Wrong
- The Up vs. Down Arrow Trap: This is the most common error among junior technicians. An upward arrow on an IEC contact means the contact delays when moving into the actuated state (On-Delay / Pickup). A downward arrow means it delays when returning to the normal, de-energized state (Off-Delay / Release). If you confuse the two, your cooling fans might shut off the second a compressor stops, rather than running for a 5-minute cool-down period.
- NOTC vs. NCTO Terminology: The acronyms trip up even experienced builders. 'Normally Open, Timed Closed' (NOTC) is an On-Delay function (it is NO, and times before it closes). For an off-delay timer, you are looking for NOTO (Normally Open, Timed Open—it closes instantly on power, but times before opening when power is removed) or NCTC (Normally Closed, Timed Closed). Always map the acronym to the arrow direction: if the delay happens on the return stroke, it's an off-delay.
- The 'Instantaneous' Contact Confusion: Many multi-function timers (like the Macromatic TR-6 series) feature both timed and instantaneous contacts on the same relay block. The schematic will show one contact with the downward off-delay arrow, and an adjacent contact on the same coil with no arrow. The unmarked contact acts instantly; do not assume the entire relay block is delayed.
Safe Interpretation When Markings Are Faded or Missing
In harsh environments—like wastewater treatment plants or mining conveyors—the printed schematic on the side of a relay (or the physical label on the component) often degrades. If you pull a 14-pin octal timer relay or a DIN-rail module with unreadable symbols, you must bench-test it to confirm whether it is a true off-delay, an on-delay, or a star-delta transition timer. For deeper symbol identification and standard mapping, resources like the Electrical Engineering Portal's symbol database are invaluable for cross-referencing obscure legacy markings.
Here is the exact bench-test procedure to identify an off-delay timer using a 24VDC power supply and a digital multimeter (DMM) like a Fluke 87V:
- Identify the Coil Pins: For standard IEC DIN-rail relays, the coil is almost always A1 (+)A2 (-)13 and 14.
- Set Up the DMM: Switch your multimeter to Continuity mode (the diode/sound icon). Place probes across the suspected NO output pins (usually 15 and 18 on IEC blocks, or 1 and 3 on octals).
- Energize the Coil: Apply 24VDC to A1/A2. If it's an off-delay timer, the NO contacts will close instantly (your DMM will beep immediately). If it's an on-delay timer, the DMM will remain silent until the dial time expires.
- The Drop Test (Crucial Step): While monitoring the DMM, remove the 24VDC power from the coil.
- If the DMM continues to beep for the duration set on the timer's potentiometer, and then clicks open, you have confirmed a True Off-Delay (Release Delay).
- If the contacts snap open the millisecond power is removed, it is either an On-Delay timer, an instantaneous relay, or the off-delay potentiometer is dialed to zero.
Frequently Asked Questions
What does the downward arrow mean on an off delay timer symbol?
The downward-pointing arrow on an IEC schematic indicates that the timing delay occurs when the contact is returning to its normal, de-energized state. Physically, this means the contact shifts immediately when the coil is powered, but 'holds' its actuated position for the preset time after the coil loses power. The arrow points down toward the horizontal line, which represents the unpowered rest state.
How is the off delay timer symbol different from an on delay timer symbol?
The difference is entirely in the arrow direction. An on-delay timer symbol features an upward-pointing arrow, indicating the contact delays its movement when the coil is first energized (pickup delay). An off-delay timer symbol features a downward-pointing arrow, indicating the delay happens when the coil is de-energized (release delay). In NEMA symbology, the 'bellows' arrow points up for on-delay and down for off-delay.
Can I use an off-delay timer for a motor dynamic braking circuit?
Yes, this is one of the most common applications for off-delay timers. When a stop button is pressed, the main drive contactor drops out instantly, but the off-delay timer's coil (wired in parallel with the main contactor) loses power and begins its countdown. During this countdown, its NO contact remains closed, keeping the dynamic braking contactor engaged to inject DC current into the motor windings. Once the timer expires, the braking contactor drops out, preventing the motor from overheating.
What is the IEC standard number for timer relay symbols?
The governing standard for graphical symbols in industrial schematics, including timer relays, is IEC 60617 (specifically Part 7 for switchgear and controlgear). In the US, the IEEE 315 standard largely harmonizes with IEC 60617 for these specific control symbols, though legacy NEMA ICS 18 drawings are still legally valid and widely used in older American manufacturing facilities.
