What is a time-delay relay?
A time-delay relay (also called a timer relay) is a relay with a built-in, adjustable timing function that switches its output contacts only after a preset delay, or keeps them switched for a preset time, once a control signal appears or disappears. It lets you define time-dependent behaviour without a programmable controller: a motor that first runs up in star, a fan that keeps running for a while, or an alarm that only triggers when a fault persists. Time-delay relays are found in almost every control cabinet and MCC, from pumping stations to packaging lines in industrial processes.
Even in plants controlled by a PLC, the standalone time-delay relay remains popular. It works without software, can be checked at a glance and keeps working when the controller fails. Anyone who maintains or modernises OT installations therefore encounters it daily.
🧠 How does a time-delay relay work?
A time-delay relay consists of four functional parts:
- Supply and control input - many models have a universal supply, for example 24–240 V AC/DC. Some start timing as soon as the supply appears; others use a separate control input (often marked B1) while the supply stays on.
- Timing circuit - formerly a pneumatic or motor-driven delay, nowadays almost always electronic, with a microcontroller and a quartz or RC oscillator. You choose the range and value with a rotary knob or display.
- Output contacts - usually one or two changeover contacts (CO) that drive a contactor, indicator lamp or PLC input.
- Status indication - LEDs for supply and running time, so a technician can see whether the relay is timing.
Electronic DIN-rail time-delay relays are typically 17.5 to 22.5 mm wide. Multifunction versions combine ten or more functions and time ranges from fractions of a second to hundreds of hours in one device. The ABB CT range, for example, covers 0.05 s to 300 hours.
🔁 Which timing functions are there?
Names and letter codes differ between manufacturers: Schneider, for example, uses A, Ac, B and H, while other brands use their own abbreviations. Always compare the timing diagram in the datasheet rather than relying on the letter alone. In PLC programming, the standard function blocks from IEC 61131 are called TON, TOF and TP.
| Function | Common designation | Contact behaviour | Typical use |
|---|---|---|---|
| On-delay | On-delay, TON | Switches only after time t once the control signal appears; drops out immediately when it disappears | Alarm delay, start sequence |
| Off-delay | Off-delay, TOF | Switches on immediately and stays on for time t after the signal disappears | Fan or pump run-on |
| Pulse (interval) | Pulse, TP | Produces a pulse of fixed length on an edge; further changes during the pulse are ignored | Lubrication pulse, short valve actuation |
| Flasher (cyclic) | Flasher, recycler | Alternates continuously between on and off time, sometimes set separately | Signal lamp, periodic flushing |
| Star-delta | Star-delta | Two outputs: star contact opens after time t, delta contact closes after a pause of about 50 ms | Star-delta starting |
Note the difference between off-delay with and without auxiliary voltage. With the ordinary type the supply must stay on and you switch via the control input. A “true off-delay” relay keeps timing even when the supply is lost completely, thanks to an internal capacitor.
🏭 Where are time-delay relays used?
| Application | Function | Typical setting |
|---|---|---|
| Star-delta start of a three-phase induction motor | Star-delta timer | Star time 6–10 s, changeover pause about 50 ms |
| Fan run-on in HVAC or extraction | Off-delay | 1–10 minutes |
| Delayed fault alarm (level, pressure, flow) | On-delay | 2–30 s to ignore short spikes |
| Staggered motor restart after a power failure | On-delay per motor | A few seconds difference per group |
| Periodic flushing of a filter | Flasher | For example 30 s on, 4 hours off |
| Stairwell or corridor lighting in building automation | Pulse or off-delay | 1–5 minutes |
Staggered restart deserves attention: if all pumps start at the same time after a grid outage, the inrush currents of every direct-on-line motor add up and the main protection may trip again. A few time-delay relays with increasing delays solve this cheaply.
🔐 Can you use a time-delay relay for safety functions?
No, not as such. A standard time-delay relay has no monitored contacts and no dual-channel design, and it is not certified to ISO 13849-1 or IEC 62061. A delayed shutdown on an emergency stop must therefore be implemented with a certified component.
- Stop category 1 - IEC 60204-1 describes controlled braking with power available, after which power is removed. For a category 1 emergency stop you use a safety relay with off-delayed contacts or a safety PLC, not an ordinary time-delay relay.
- Standstill monitoring - a fixed time assumes the machine has stopped within that time. If that is uncertain, standstill monitoring belongs in the safety function.
- Risk assessment - the right solution follows from the risk assessment under the Machinery Directive and, from 20 January 2027, the Machinery Regulation.
🔧 How do you select and set a time-delay relay?
Work through these steps during selection and commissioning:
- Determine the function - on-delay, off-delay, pulse, flasher or star-delta. A multifunction relay reduces the spare-parts stock.
- Choose the time range - choose a range in which the required time sits well within the scale; at the very start of a scale the setting is least accurate.
- Check supply and control voltage - 24 V DC from the control power supply or 230 V AC; universal supplies simplify the choice.
- Size the contacts - check the switching capacity for inductive loads (AC-15 and DC-13) according to IEC 60947-5-1, especially when the relay directly switches a contactor coil.
- Define reset behaviour - decide what happens after a supply interruption: does timing restart or not?
- Document the set value - record time and range in the schematic and fit a sealable cover if needed, so nobody turns the knob unnoticed.
- Test at commissioning - measure the actual time with a stopwatch or test instrument and compare it with the design value.
The product standard for time-delay relays is IEC 61812-1 (EN 61812-1 in Europe). The third edition was published in June 2023 and replaced the 2011 edition; its title is now Time relays and coupling relays for industrial and residential use, because coupling relays also fall within its scope. The standard defines, among other things, setting accuracy (the difference between the measured time and the value set on the scale) and repeatability, the two figures you use to compare time-delay relays in datasheets. Further new elements include requirements for risk assessment, routine tests and environmentally conscious design and, notably for OT convergence, cybersecurity requirements for use in industrial automation and control systems. This connects to IEC 62443, because modern multifunction time-delay relays are increasingly configured via NFC or an app.
🔄 Time-delay relay, PLC or drive?
| Solution | Advantages | Disadvantages | When to choose? |
|---|---|---|---|
| Time-delay relay | Simple, no software, immediately visible, independent of the PLC | Limited logic, set at the cabinet | One or a few fixed times, independent functions |
| PLC timer | Flexible, centrally adjustable, logging possible | Depends on PLC program and availability | Complex sequences or many timers |
| Logic module | Intermediate form with timer and counter functions | Still software and configuration management | Small controls with a few related functions |
| Soft starter | Stepless run-up, built-in ramp time | More expensive than star-delta | Pumps at risk of water hammer, lower grid load |
| Variable frequency drive | Speed control with its own ramp times and timers | Highest cost, harmonics | Variable speed required |
Siemens introduced LOGO! in 1996 precisely to replace separate relays, time-delay relays and counters. Even so, many designers deliberately choose a hardware time-delay relay for critical parts: it keeps working when a PLC stops or is reprogrammed. In ladder logic, the TON timer is literally the software successor of the time-delay relay in the circuit diagram. For the hardwired version, the relay’s datasheet remains the reference for compliance checks.
❓ Frequently asked questions
What is the difference between on-delay and off-delay?
With on-delay, the contact of a time-delay relay switches on only after the set time once the control signal appears. With off-delay, the contact switches on immediately and stays on for the set time after the signal disappears. You use on-delay for alarm delays and off-delay for run-on times.
Which standard applies to a time-delay relay?
The product standard for time-delay relays is IEC 61812-1, adopted in Europe as EN 61812-1. The current third edition dates from June 2023 and includes requirements for risk assessment, routine tests and cybersecurity. For the switching capacity of control contacts, IEC 60947-5-1 is also commonly referenced.
Why does a star-delta timer have a 50 ms pause?
A star-delta timer first releases the star contactor and only closes the delta contactor after about 50 milliseconds. During that pause the arc in the star contactor is extinguished, so no phase-to-phase short circuit occurs. On most versions, such as the ABB CT-SDS, the pause is fixed at 50 ms and you only set the star time; a few models offer an adjustable changeover time.
Can a PLC replace a time-delay relay?
A PLC can take over every time-delay relay function using the IEC 61131-3 function blocks TON, TOF and TP. That is flexible for complex sequences, but it makes the function dependent on the PLC program. For simple or independent functions, a hardware time-delay relay therefore often remains the more robust choice.
What is a true off-delay time-delay relay?
A true off-delay time-delay relay times the off-delay without auxiliary voltage, so it keeps working even when the supply is lost completely. The energy comes from an internal capacitor. An ordinary off-delay time-delay relay, by contrast, needs a permanent supply and starts timing via a separate control input.
Is a time-delay relay suitable for an emergency stop?
A standard time-delay relay is not suitable for an emergency stop or any other safety function, because it is not designed and certified to ISO 13849-1 or IEC 62061. For a delayed emergency stop in stop category 1, you use a safety relay with delayed contacts or a safety PLC. The choice follows from the machine’s risk assessment.
📌 In summary
A time-delay relay is a relay with an adjustable delay that implements time-dependent switching functions without software, from star-delta starts to run-on times and alarm delays. Choose the right function (on-delay, off-delay, pulse, flasher or star-delta), a suitable time range and contacts that can handle the load, and document the setting. IEC 61812-1:2023 is the product standard; safety functions require a certified safety relay or safety PLC, and complex logic belongs in a PLC.