A time delay relay isolates the control trigger from the load switching, adding a programmed pause before the contacts change state. Whether you are preventing a compressor from short-cycling or keeping a purge fan running after a machine stops, the physical wiring remains consistent across most industrial control panels. Below is the complete node-by-node trace, terminal mapping, and verification process for the most common format in the wild: the 8-pin octal plug-in relay.

The Decision Path: Picking the Right Relay Mode and Part

Time delay relays are not universal; the internal logic dictates whether the delay happens on energization, de-energization, or as a single pulse. Use this decision matrix to select the correct mode for your application, terminating in a concrete default pick for general control panel builds.

Application Need Timing Mode Recommended Part Number Approx. Price
Motor soft-start / prevent short-cycling ON-Delay Omron H3Y-2 (24VDC) $38
Cool-down fan / purge cycle OFF-Delay Macromatic TR-65821 $55
Pulse signaling / alarm flasher Interval (One-Shot) Schneider Zelio RE17RAMW $62
The Default Pick: For 90% of general HVAC and industrial control applications (like delaying a contactor coil after a thermostat or PLC call), you need an ON-Delay relay. The default, bulletproof pick is the Omron H3Y-2 8-pin 24VDC (Part #H3Y-2-24VDC). It handles 5A at 250VAC, fits standard DIN-rail octal sockets, and costs around $38. Buy this unless your sequence specifically requires the load to drop out on a timer.

Decoding the Time Delay Relay Wiring Diagram Symbols

Before tracing the wires, you must read the schematic correctly. The symbols on your drawing follow IEC 60617 and ANSI/IEEE 315 standards. Misreading the contact symbol is the number one reason a newly wired panel fails its sequence test.

  • The Coil: Represented by a standard rectangle. If it has a small 'x' or a clock face inside, it designates a time delay coil rather than an instantaneous control relay.
  • ON-Delay NO Contact: A standard Normally Open symbol (a line gap) with a curved arrow pointing away from the gap. This means "delays on make." The contact waits to close until the timer expires.
  • ON-Delay NC Contact: A standard Normally Closed symbol (overlapping lines) with a curved arrow pointing toward the overlap. This means "delays on break." The contact stays closed during the timing cycle, then opens.
Warning: Do not confuse the On-Delay arrow (pointing away) with the Off-Delay arrow (pointing toward the contact line). Wiring an Off-Delay relay when your schematic calls for On-Delay will cause your load to energize instantly and drop out later, potentially damaging motors or ruining process sequences.

8-Pin Octal Terminal and Pin Mapping Table

The physical Omron H3Y-2 and similar 8-pin octal relays use a standardized base. The pins are numbered on the bottom of the relay and mirrored on the wiring diagram printed on the side of the relay casing. Here is the exact mapping for a DC control circuit.

Pin Number Function Internal Connection Standard DC Wire Color
2 Coil (+) Power supply positive Red or White
7 Coil (-) Power supply negative / Ground Black or Blue
1 Common 1 (COM) Load power source Red
3 Normally Open 1 (NO) Switched load output Orange
5 Normally Closed 1 (NC) Switched load output Yellow
4 Common 2 (COM) Load power source Red
6 Normally Open 2 (NO) Switched load output Orange
8 Normally Closed 2 (NC) Switched load output Yellow

Note: Pins 2 and 7 are always the coil. Pins 1 and 4 are the commons. Pins 3 and 6 are the NO contacts. Pins 5 and 8 are the NC contacts. This layout is universal across almost all 8-pin octal relays, regardless of the manufacturer.

Node-by-Node Wiring Trace: Source to Load

Let's trace the current flow for an ON-Delay circuit controlling a 24VDC compressor contactor. We will explicitly track polarity and the ground return path.

1. The Control Circuit (Energizing the Coil)

  1. Source: Current leaves the 24VDC Power Supply positive terminal.
  2. Protection: Passes through a 2A DC supplementary breaker.
  3. Trigger: Flows through the initiating device (a thermostat switch, pressure transducer, or PLC discrete output).
  4. Relay Coil (+): Enters the relay socket at Pin 2.
  5. Relay Coil (-): Exits the relay socket at Pin 7.
  6. Ground Return: Returns to the 24VDC Power Supply negative terminal, which is bonded to the panel's DC ground bus.

Polarity Callout: On a DC coil, polarity matters. Reversing Pins 2 and 7 won't destroy the relay, but it bypasses the internal flyback suppression diode. When the coil de-energizes, the collapsing magnetic field will send a voltage spike back into your PLC output card, eventually frying the solid-state switch. Always wire positive to 2 and negative to 7 on DC coils.

2. The Load Circuit (Switching the Contacts)

  1. Source: 24VDC (or 120VAC, if your contacts are rated for it and the circuit is isolated) enters the relay socket at Pin 1 (COM 1).
  2. Switching: When the timer expires, the internal armature pulls in, connecting Pin 1 to Pin 3 (NO 1).
  3. Load: Current exits Pin 3 and travels to the load (e.g., the A1 terminal of a compressor contactor coil).
  4. Ground Return: Current exits the contactor coil at A2 and returns directly to the panel's DC ground bus.

Ground Path Callout: The relay coil and the relay contacts are galvanically isolated. The ground path for the load does not pass through the relay. Furthermore, the metal DIN-rail mounting clip on the relay socket must be bonded to the panel's main equipment grounding conductor via a 14 AWG green/yellow wire to ensure fault clearing, as per NFPA 79 industrial machinery standards.

How to Verify Every Connection with a Multimeter

Never assume a relay is wired correctly just because the wires are tight. Use a digital multimeter (DMM) to verify the circuit in three distinct stages. For comprehensive relay testing procedures and timing tolerance charts, refer to the Macromatic technical documentation library.

Check 1: Coil Integrity (De-energized)

Lock out and tag out the panel. Set your DMM to Ohms (Ω). Place the probes on Pin 2 and Pin 7. A healthy 24VDC coil will read between 1,000Ω and 2,500Ω. If it reads 0.0Ω, the coil is shorted. If it reads OL (Open Loop), the internal winding is broken. Replace the relay.

Check 2: Contact Resting State (De-energized)

Set your DMM to Continuity (the beep setting).
Probe Pin 1 (COM) and Pin 5 (NC). The meter should beep, confirming the NC path is closed at rest.
Probe Pin 1 (COM) and Pin 3 (NO). The meter should read OL, confirming the NO path is open at rest. If it beeps here, your contacts are welded shut from a previous overload.

Check 3: Timing and Switching (Energized)

Remove locks and energize the control circuit. Apply the trigger signal to start the timer. Set your DMM to Continuity and hold the probes on Pin 1 and Pin 3 (NO). Watch a stopwatch. When the stopwatch hits your dialed-in delay (e.g., 10.0 seconds), the meter should beep.
Diagnostic: If it beeps instantly, you either have the wrong relay mode (Off-Delay instead of On-Delay) or the timer dial is set to zero. If it never beeps, check for voltage drop across the coil; you may have a loose Pin 2 connection starving the coil of the minimum 19VDC required to pull in the armature.

Safety Tip: Always perform a Live-Dead-Live test with a Non-Contact Voltage Tester (NCVT) before touching terminals. Even in 24VDC control panels, 120VAC or 480VAC power circuits often share the same wire duct. Verify your meter on a known live source, test the target circuit dead, and verify the meter again on the live source to ensure the meter didn't blow a fuse during the test.

By sticking to the 8-pin octal standard and defaulting to the Omron H3Y-2 for ON-Delay applications, you eliminate the guesswork from control panel wiring. Trace the nodes, respect the DC polarity, and verify with your meter before closing the panel door.