An off-delay time delay relay (also known as delay-on-break, release delay, or interval-on-release) is an electromechanical or solid-state control device designed to keep a circuit active for a specific duration after the control signal is removed. When coil voltage is applied, the output contacts change state immediately. When coil voltage is removed, the internal timer starts counting down. Once the preset time elapses, the contacts revert to their normal, unenergized state.
This function is critical for applications like HVAC blower run-on cycles, cooling fan delays after heavy machinery stops, or lubrication pumps that need to run briefly after a motor shuts down. Below is a comprehensive guide to selecting, wiring, and testing these components on the bench and in the panel.
Off-Delay Relay Rating Table: Coil vs. Contact Specifications
The most common mistake when specifying a time delay relay off delay unit is confusing the coil control voltage with the contact switching voltage. The coil side (terminals A1 and A2) dictates what signal triggers the timer, while the contact side (typically labeled 15/18/16 or COM/NO/NC) dictates what load the relay can safely switch.
| Model Example | Coil Voltage (Control) | Contact Form | Resistive Rating | Inductive / Motor Rating | Breaking Capacity |
|---|---|---|---|---|---|
| Macromatic TR-60521 | 120V AC | SPDT (1 Form C) | 10A @ 240V AC | 1/3 HP @ 120V AC | 3A @ 24V DC |
| Schneider 82222 | 24V DC | DPDT (2 Form C) | 5A @ 250V AC | 2.5A @ 250V AC (cos φ=0.4) | 0.2A @ 110V DC |
| Omron H3Y-2 | 24V AC/DC | DPDT (2 Form C) | 5A @ 250V AC | 2A @ 250V AC (Inductive) | 1A @ 30V DC |
| Finder 80.01 | 12-24V AC/DC | SPDT (1 Form C) | 16A @ 250V AC | 5A @ 250V AC (cos φ=0.4) | 4A @ 24V DC |
Which rating column governs this load? You must match the load's power factor to the correct column. A purely resistive load (like a heating element) uses the Resistive Rating. A load with a magnetic field (like a solenoid valve or contactor coil) uses the Inductive Rating, which is heavily derated due to the high voltage spike generated when breaking the circuit. Motor loads require checking the specific HP (Horsepower) or FLA (Full Load Amps) rating, as motors draw 5x to 7x their running current during startup.
Coil vs. Contact Wiring and DC Flyback Protection
Wiring an off-delay relay requires strict separation of the control circuit (coil) and the load circuit (contacts). Terminals A1 and A2 are universally standard for the coil. Terminals 15 (Common), 18 (Normally Open), and 16 (Normally Closed) are standard for IEC-style contact numbering.
The DC Coil Flyback Rule: If your coil control voltage is DC (e.g., 24VDC from a PLC output), you must install a flyback diode (like a 1N4007) across the A1 and A2 terminals. Wire the cathode (the striped end) to the positive terminal (A1) and the anode to the negative (A2). When the DC control signal is removed, the collapsing magnetic field in the coil generates a high-voltage reverse spike. Without the diode, this spike will destroy the PLC's solid-state output transistor or cause severe arcing across the control switch contacts. AC coils do not require this diode, as the alternating zero-crossings naturally extinguish the arc.
Load Selection Decision Path: Resistive, Inductive, and Motor
Selecting the right time delay relay off delay unit depends entirely on the electrical characteristics of the load being switched. Below is a decision framework to ensure your relay contacts do not weld shut or degrade prematurely.
| Load Type | Characteristics & Inrush | Relay Selection Criteria | Overcurrent Protection Strategy |
|---|---|---|---|
| Resistive (Heaters, Incandescent) | No inrush current. Current is stable and in phase with voltage (Power Factor = 1.0). | Use the standard AC Resistive rating. 10A heater needs a 10A+ resistive rated relay. | Standard Type B MCB or fast-acting fuse. Breaker rating must not exceed the relay's maximum continuous contact rating. |
| Inductive (Solenoids, Contactors) | Moderate inrush. High voltage spike on break (arc). Power Factor is low (0.4 to 0.6). | Derate resistive capacity by 50-70%. Use the AC-15 or Inductive column. Add an RC snubber across the load. | Standard Type B or C MCB. Ensure the breaker's interrupting capacity (kA) exceeds the available fault current. |
| Motor (Fans, Pumps, Compressors) | Massive inrush (Locked Rotor Amps can be 6x-8x FLA). High inductive kick on break. | Must have a specific HP or Motor/FLA rating. Never use a standard resistive rating for direct motor switching. | Do not use standard Type B breakers. Use Type C or Type D curve MCBs, or time-delay (slow-blow) fuses to tolerate the motor inrush without nuisance tripping. |
Note on Protection: Never treat fuses and miniature circuit breakers (MCBs) as interchangeable without considering the trip curve. A standard Type B MCB trips magnetically at 3x to 5x rated current. If you use a 10A Type B breaker to protect a 10A relay switching a 5A motor, the motor's 30A startup inrush will instantly trip the breaker. A Type C (5x-10x) or Type D (10x-20x) breaker, or a dual-element time-delay fuse, allows the inrush to pass while still protecting the relay contacts from a true short circuit. For deeper coordination data, reference the Macromatic Time Delay Relay catalog or the Schneider Electric timer relay documentation.
Bench and Live Testing: When to Repair vs. Replace
When an off-delay circuit fails, the issue is rarely the timing logic itself; it is usually welded contacts, a failed coil, or a blown upstream control fuse. Here is how to isolate the fault.
Dead Testing (Power Removed & Verified)
- Coil Integrity: Set your multimeter to Ohms (Ω). Measure across A1 and A2. A healthy 24VDC coil typically reads between 100Ω and 500Ω. A 120VAC coil will read much higher (1,000Ω to 10,000Ω). An 'OL' (Open Loop) reading means the internal coil wire is broken. A '0.0' reading means the coil is shorted.
- Contact Continuity: With no power applied, measure across COM (15) and NC (16). It should read less than 1.0Ω. Measure COM (15) and NO (18). It should read 'OL'. If COM-NO shows continuity while de-energized, the contacts are welded shut from a previous overload event.
Live Testing (Energized)
- Control Voltage: With the system running, measure AC or DC voltage across A1 and A2. It must be within ±10% of the relay's nominal coil voltage. If voltage is present but the relay doesn't click, the coil is dead or the internal solid-state timing IC has failed.
- Timing Verification: Measure voltage at the NO contact. Remove the coil voltage (A1/A2). Start a stopwatch. The voltage at the NO contact should remain present until the exact dialed-in delay time elapses, at which point it should drop to zero. If it drops immediately, the internal timing capacitor or release circuit has failed.
| Failure Symptom | Root Cause | Action: Repair or Replace? |
|---|---|---|
| Coil reads 'OL' (Open) | Coil wire burned out due to overvoltage or heat. | Replace. Rewinding a control relay coil is not field-serviceable or economically viable. |
| Contacts welded (NO reads continuity when off) | Switched a load exceeding the contact breaking capacity; arc melted the silver alloy. | Replace. Do not file or sand relay contacts. This removes the protective silver-oxide/cadmium plating and guarantees rapid failure. |
| Timer delay is erratic or zero | Internal timing capacitor dried out, or potentiometer wiper is dirty/oxidized. | Replace. While bench technicians can replace SMD capacitors, the cost of labor exceeds a new $40-$80 industrial unit. |
| High resistance across COM-NO (> 5 ohms) | Carbon buildup or oxidation on contact surface from switching low-current inductive loads. | Replace. In a pinch, switching a higher current dummy load a few times can 'burn off' light oxidation, but replacement is the only reliable fix. |
Ultimately, time delay relays are considered consumable control components. While heavy-duty contactors are designed to be rebuilt with new contact blocks and coils, DIN-rail time delay relays are potted or tightly packed with surface-mount timing ASICs and micro-potentiometers. When a time delay relay off delay unit fails its bench test, swapping it for a new, properly rated unit is the only code-compliant and reliable path forward.
