The Mechanics of an Off Time Delay Relay

An off time delay relay (frequently called a delay-on-break or true off-delay relay) is designed to keep its output contacts in their actuated state for a precise, adjustable duration after the input trigger signal is removed. Once the preset time expires, the contacts revert to their normal resting state.

This behavior solves specific mechanical and thermal problems on the jobsite. Common applications include HVAC fan run-on cycles to clear residual heat, conveyor belt coast-down sequences to prevent material spillage, and sump pump post-purge cycles to empty the discharge pipe and prevent check-valve water hammer.

Bench Note: Do not confuse this with an 'interval-on' (delay-on-make) relay. In an off-delay configuration, the contacts change state immediately when the trigger is applied, and the timing sequence only begins the moment the trigger is removed.

Sizing the Contacts: Which Rating Column Governs Your Load?

The most common mistake when specifying time delay relays is reading the maximum resistive ampacity (often 16A) and applying it to a motor or solenoid load. Inductive and motor loads generate massive inrush currents and arc-inducing back-EMF that will weld standard 16A contacts shut in a matter of weeks.

To size the relay correctly, you must look at the utilization category columns defined by IEC 60947-5-1. Here is how the rating columns govern real-world loads:

Load Type IEC Utilization Category Governing Rating Column Typical Derating Factor Example Application
Resistive AC-1 / DC-12 Maximum Thermal Current (Ith) 1.0 (No derating) Space heaters, incandescent lighting
Inductive Control AC-15 / DC-13 Inductive Breaking Capacity 0.2x to 0.3x of Ith Relay coils, solenoid valves, contactor coils
Squirrel Cage Motor AC-3 Motor FLA / HP Rating 0.15x to 0.2x of Ith Small conveyor motors, exhaust fans

If your off time delay relay is rated for 16A at AC-1, its AC-15 inductive rating will likely be around 3A to 5A, and its AC-3 motor rating will be roughly 0.5 HP (approx. 3A Full Load Amps). Always size the relay based on the lowest applicable column for your specific load type.

Coil vs. Contact Wiring and the DC Flyback Rule

Modern industrial off time delay relays (like the Finder 80 Series or Macromatic TR-60) separate the power/timing circuit from the output contacts. Understanding this split is critical for correct wiring.

The Coil / Power Side (A1, A2, and Y1)

Unlike a standard electromechanical relay where A1 and A2 simply energize a magnetic coil, a time delay relay's A1 and A2 terminals power an internal switch-mode power supply and microcontroller. For a true off-delay function, continuous power must be applied to A1 and A2. The trigger signal is applied to a separate terminal, usually labeled Y1. When Y1 receives voltage, the output contacts actuate. When Y1 drops to 0V, the internal timer starts counting down.

DC Flyback Protection: If you are switching a DC inductive load (like a solenoid or a DC motor) using the relay's output contacts, you must install a flyback diode (e.g., 1N4007) in reverse parallel across the load. While some modern relays have internal solid-state clamps on the coil side, the output contacts are raw electromechanical switches. Without a diode, the collapsing magnetic field of a DC load will arc across the opening contacts, pitting and destroying them prematurely.

The Contact Side (15, 16, 18)

The output side typically features a Form C (SPDT) contact arrangement:

  • 15: Common (C)
  • 18: Normally Open (NO) - Closes immediately when Y1 is triggered.
  • 16: Normally Closed (NC) - Opens immediately when Y1 is triggered.
Wire your load between the Common (15) and the appropriate throw (18 for off-delay run-on circuits).

The Selection Decision Tree: Pick Your Exact Part

Use this decision path to select the correct off time delay relay for your panel. Do not guess; follow the load type to the required specification.

Condition / Load Type Required Specification Concrete Part Recommendation
IF Load is Resistive (Heaters) AND Control Voltage is 24VDC AC-1 rating ≥ 10A, DC coil input Macromatic TR-63124 (True Off-Delay, 24VDC)
IF Load is Inductive (Solenoids/Contactors) AND Voltage is mixed (24-240V) AC-15 rating ≥ 3A, Multi-voltage input Finder 80.01.1.240 (Multi-function, 24-240VAC/DC)
IF Load is Motor (AC-3) AND FLA > 3A Relay cannot switch directly; must pilot a contactor Finder 80.01.1.240 (to pilot a Schneider LC1D09 contactor)

The Default Pick: If you are stocking a maintenance cart or building a general-purpose control panel and need one part to cover 90% of off-delay scenarios, buy the Finder 80.01.1.240. It costs roughly $75, accepts any voltage from 24VDC to 240VAC on both the power and trigger inputs, and features a multi-function dial that includes off-delay (delay-on-break) alongside interval and on-delay modes. It is the most versatile time delay relay on the market for panel builders.

Dead and Live Testing Procedures

Before energizing a newly wired panel, you must verify the relay's health and timing accuracy. Time delay relays fail silently; a blown internal fuse or dead timing capacitor won't show visible damage.

Dead Testing (Multimeter Only)

  1. Isolate Power: Lock out and tag out the panel. Verify 0V at A1 and A2.
  2. Coil/Input Impedance: Set your DMM to Ohms (Ω). Measure across A1 and A2. A healthy switch-mode input will read high impedance (often >100kΩ) or briefly spike as internal capacitors charge, then settle. If it reads 0Ω (dead short), the internal MOV or rectifier is blown. Replace the unit.
  3. Contact Continuity: Measure across 15 and 16 (NC). It should read < 1Ω. Measure across 15 and 18 (NO). It should read OL (Open Loop). If the NO contact shows continuity while dead, the contacts are welded shut from a past overcurrent event.

Live Testing (Energized Verification)

  1. Apply Continuous Power: Energize A1 and A2. The relay should remain in its resting state (15-16 closed, 15-18 open).
  2. Apply Trigger: Apply voltage to Y1. The output contacts should snap immediately (15-18 closes). Verify load operation.
  3. Verify Timing: Remove the trigger from Y1 while keeping A1/A2 powered. Start a stopwatch. The contacts must remain actuated until the dial setting expires, then snap back to resting. If the timing deviates by more than ±5% of the dial setting (e.g., a 60-second setting times out at 45 seconds), the internal timing capacitor has degraded. Replace the relay.

Repair vs. Replace: The Potted Epoxy Reality

When an off time delay relay fails, the question of repair versus replace is easily answered: Always replace.

Modern time delay relays are not serviceable. The internal PCBs are encased in thermally conductive epoxy or potted in polyurethane resin to protect the microcontroller and timing capacitors from industrial vibration, moisture, and corrosive atmospheres. Attempting to dig out the potting compound to replace a failed 555 timer IC or a bulging electrolytic capacitor will destroy the trace paths and compromise the dielectric isolation between the low-voltage timing circuit and the mains-voltage output contacts.

Branch Circuit Protection Note: If your replacement relay keeps failing due to short circuits on the load side, check your branch protection. Do not treat fuses and circuit breakers as interchangeable. If you are protecting a circuit that includes motor inrush, a standard Type B or Type C miniature circuit breaker (MCB) will nuisance-trip on the magnetic inrush. You must use a Type D curve breaker to tolerate the Locked Rotor Amps (LRA). Conversely, if you are protecting the relay's internal solid-state electronics from a dead short, standard thermal breakers are too slow; you must use fast-acting semiconductor fuses or Class CC/RK5 time-delay fuses sized to the relay's specific let-through current limits, as detailed in NEMA ICS 2.

By matching the correct utilization category to your load, wiring the trigger and flyback paths correctly, and protecting the branch circuit with the proper trip curve, an industrial off time delay relay will provide years of maintenance-free run-on and coast-down control.

For detailed wiring diagrams and multi-function dial settings, always refer to the manufacturer's technical datasheet for your specific part number before closing the panel door.