When an engineer or technician refers to relay positions, they are talking about two distinct but interconnected concepts: the internal contact configuration (Form A, Form B, Form C) and the physical terminal pinout layout on the relay base or socket. Misunderstanding either will result in a circuit that fails to switch, or worse, a welded contact that fails to drop a motor or heater load. This guide maps out the exact contact forms, physical pin positions, and load-specific derating rules you need to specify, wire, and troubleshoot electromechanical relays safely.
Decoding Relay Positions and Contact Forms
Electromechanical relays are categorized by their 'Form' designations, which define the number of poles and the default resting state of the contacts. According to the IEC 61810-1 standard for electromechanical elementary relays, these forms dictate how the common (COM), normally open (NO), and normally closed (NC) terminals behave when the coil is de-energized.
Below is a spec-sheet comparison of three industry-standard DIN-rail and PCB relays, showing how their physical relay positions translate to real-world electrical ratings. Note that the breaking capacity drops significantly when moving from purely resistive loads to inductive loads.
| Manufacturer / Series | Form / Positions | Coil Voltage | Resistive Rating (AC1) | Inductive Breaking Capacity |
|---|---|---|---|---|
| Omron G2R-1-E | Form A (SPST-NO) | 24V DC | 16A @ 250VAC | 4A @ 250VAC (cos φ=0.4) |
| Omron G2R-2 | Form C (DPDT) | 120V AC | 5A @ 250VAC | 2A @ 250VAC (cos φ=0.4) |
| Finder 40.52 | Form C (DPDT) | 24V DC | 8A @ 250VAC | 3A @ 250VAC (cos φ=0.4) |
| Schneider RXM4AB1BD | Form C (4PDT) | 24V DC | 6A @ 250VAC | 2A @ 250VAC (cos φ=0.4) |
When mapping out relay positions on a standard base, Form A (SPST-NO) uses two positions: Line in, Load out. Form C (SPDT) uses three positions: Common, NO, and NC. Physically, on an 8-pin octal base, the coil positions are almost always pins 2 and 7. On a 14-pin base, the coil sits at pins 13 and 14. Always verify the pinout diagram printed on the side of the relay casing, as forcing a 14-pin relay into a miswired socket will instantly short the coil supply.
Coil vs. Contact Wiring and Load Selection
A relay is essentially two isolated circuits sharing a magnetic core. The coil side (control circuit) and the contact side (load circuit) must be wired and protected independently. The physical relay positions for the coil are typically labeled A1 and A2 on DIN-rail sockets, or 13/14 (DC) and 2/7 (AC) on octal sockets. The contact positions are labeled COM (Common), NO (Normally Open), and NC (Normally Closed).
When wiring DC coils (e.g., 12V or 24V DC), the collapsing magnetic field generates a massive inductive voltage spike (kickback) when the control switch opens. This spike will destroy driving transistors, MOSFETs, or PLC outputs. You must wire a flyback diode (like a 1N4007) in reverse bias directly across the A1 and A2 coil terminals. Connect the diode cathode (striped end) to the positive A1 terminal and the anode to the negative A2 terminal.
Selecting the right relay requires looking past the bold '10A' printed on the front cover. That number only applies to purely resistive loads (like a heating element). The governing rating column changes entirely based on your load type. Use the decision tree below to determine which datasheet column governs your specific application.
| Load Type | Examples | Governing Rating Column | Derating / Selection Rule |
|---|---|---|---|
| Resistive | Heaters, incandescent bulbs | AC1 / DC1 (Resistive) | Use 100% of the nominal contact rating. |
| Inductive | Solenoids, contactor coils, valves | AC15 / DC13 (Inductive) | Derate to 30-40% of nominal rating. Check cos φ (power factor) in datasheet. |
| Motor (AC) | Fans, pumps, compressors | HP / kW Motor Rating or FLA | Must handle 6x inrush current (Locked Rotor Amps) for make capacity. Use a contactor for >1HP. |
| Tungsten / Lamp | LED drivers, halogen arrays | Tungsten / Ballast Rating | Derate to 20% of nominal rating due to massive cold-filament inrush current. |
If you are switching a 24V DC solenoid valve that draws 2A, you cannot use a relay rated for '10A at 24VDC' if that rating is purely resistive. You must check the DC13 inductive column; if it is only rated for 1A at 24VDC inductive, the relay will fail. For high-current inductive loads, use the relay to switch the coil of a heavy-duty contactor, keeping the high-energy arcing away from the relay's small physical contacts.
Testing, Troubleshooting, and Replacement
When a circuit fails to actuate, you need a systematic approach to isolate whether the failure is on the control side (coil) or the load side (contacts). For comprehensive relay testing methodologies, refer to resources like All About Circuits or manufacturer application notes from Macromatic.
How to Test Dead (De-energized)
Always lock out and tag out (LOTO) the main breaker and verify zero voltage with a non-contact voltage tester and a multimeter before performing dead tests.
- Coil Resistance Test: Set your multimeter to Ohms (Ω). Measure across A1 and A2 (or pins 13/14). A healthy 24V DC relay coil (like the Omron G2R) should read between 600Ω and 700Ω. A 120V AC coil will read much higher (typically 4,000Ω to 8,000Ω). If you read 'OL' (Open Line), the internal coil wire is broken. If you read near 0Ω, the coil is shorted.
- Contact Continuity Test: Measure across COM and NC. It should read less than 1Ω. Measure across COM and NO; it should read 'OL'.
- Manual Actuation: Use a small flathead screwdriver to press the manual test button on the relay armature. While depressed, COM-to-NC should read 'OL', and COM-to-NO should read less than 1Ω. If the NO contacts read high resistance (e.g., 5Ω or more) when manually closed, the contacts are pitted or carbon-fouled.
How to Test Live (Energized)
If the dead tests pass but the circuit still fails, you must test under power. Wear appropriate PPE and use CAT III or CAT IV rated meter probes.
- Coil Voltage Check: Set the meter to AC or DC Voltage. Measure directly across A1 and A2 while the PLC or switch is commanding the relay ON. You must read within 10% of the nominal coil voltage (e.g., 21.6V to 26.4V for a 24V DC coil). If voltage is present but the relay doesn't pull in, the mechanical armature is jammed.
- Contact Voltage Drop: With the relay energized and the load connected, measure the voltage between the COM terminal and the NO terminal. A healthy closed contact will show a voltage drop of less than 50mV. If you read line voltage (e.g., 120V or 240V) across the closed COM and NO terminals, the contacts have failed to make a physical connection, or the mechanical linkage is broken.
When to Repair vs. Replace
In modern industrial and electronics contexts, the physical relay positions on PCB-mounted or standard DIN-rail plug-in relays (like the 14-pin Harmony series) are never repaired. They are sealed units. If the coil is open, or the contacts are welded shut from an inductive arcing event, you replace the entire relay cartridge. Attempting to file down pitted contacts on a 10A signal relay alters the contact pressure and spring tension, leading to unpredictable future failures and potential fire hazards.
The only exception is large, open-frame industrial contactors (e.g., 100A+ motor starters). On these massive units, the main power poles are separate, replaceable components. If the arc chutes are melted or the main silver-alloy contact pads are severely pitted, a technician can unbolt and replace just the contact poles and the arc chutes, provided the coil and mechanical linkage are still within spec. However, for anything under 40A, the labor cost of troubleshooting and the risk of a secondary failure always dictate a straight swap of the relay and, if the terminals show heat discoloration, the socket base as well.
