Industrial Plug-in Relay: How to Select Coil, Contacts and Socket
Plug-in relays are the workhorses of control panels. They switch contactor coils, solenoid valves, lamps and signals, and they isolate PLC outputs from field devices. Because they plug into a base, a faulty relay can be swapped in seconds without touching the wiring. Picking the right one comes down to a few checks.
1. Coil voltage
Match the coil to your control supply: 24 V DC from a PLC or SMPS, or 110 / 230 V AC in traditional panels. Check whether the coil is AC or DC. A DC coil on AC (or the reverse) will hum, overheat or fail. The ComatReleco plug-in relay range covers coil voltages from 24 to 240 V AC/DC.
2. Contact rating and load type
The contacts must carry the load current and survive the way the load behaves when it switches:
- Resistive loads (heaters) are easiest.
- Inductive loads (contactor coils, solenoids, motors) cause arcing when switched off. Derate the contact rating and consider suppression (RC or diode).
- Lamp and capacitive loads draw a high inrush current.
- DC loads are harder to break than AC at the same current.
The ComatReleco range covers loads from 6 A to 50 A, so you can pick a relay with headroom instead of running a small relay at its limit.
3. Number of poles and contact form
Count how many separate circuits the relay must switch. Plug-in relays come with 1 to 4 change-over (C/O) contacts. Spare poles are useful for interlocks, indication or feedback to a PLC.
4. Socket and mounting
The socket is half the selection. Check the pin pattern (8-pin, 11-pin, 14-pin, square or round), DIN-rail or panel mounting, screw or spring terminals, and space for accessories such as hold-down clips, LED or suppression modules. Concord stocks matching ComatReleco multi-pin sockets, including 8-pin (S2B, S7-C), 11-pin (S3-B) and 14-pin (S9-M).
5. Indication and test features
An LED status indicator, a mechanical flag or a manual test button makes commissioning and fault finding much faster, especially in large panels.
Choosing within the ComatReleco range
| Need | Relay to look at | Key details |
|---|---|---|
| Compact general control, 2 C/O | C7-A2x | 8-pin, 2 × 10 A C/O, coils 24 / 110 / 220 V AC/DC |
| Inductive or high-inrush loads, 2 C/O | C2-A2x | 8-pin, 2 × 10 A C/O, AgSnO2 contacts, LED, dust-proof cover |
| 3 C/O, low coil power, wide temperature range | C3-G3x | 11-pin, 3 × 10 A C/O, 0.9 W coil, –40 to 60 °C |
| 4 C/O with mechanical flag | C4-A4x | 14-pin, 4 × 10 A C/O, AC or DC coils |
| 4 C/O, higher current | C9-A4x | 14-pin, 4 C/O up to 15 A at 250 V AC, LED and flag |
| PLC output / signal interface | C10-A1x, PLC slim relay module | Slim, 1 × 10 A C/O, DIN rail, LED |
Not sure which one fits? Tell us the coil voltage, the load and the number of contacts and we will match the relay and socket.
Interface relays for PLCs
PLC transistor outputs can only switch small currents. A slim interface relay between the PLC and the field device isolates the PLC, allows higher loads and different voltages, and makes a damaged output channel cheap to repair: you change a relay, not a PLC card.
Why a Swiss brand
In process plants, railways and utilities, a relay may operate millions of times. Contact material, coil insulation and mechanical life decide whether it lasts. ComatReleco is a Swiss relay brand, and Concord has supplied its relays to Indian industry since 1996.
Frequently Asked Questions
What is an industrial plug-in relay?
A control relay that plugs into a separate socket, so it can be replaced without disconnecting any wiring. It switches control loads such as contactor coils, valves and signals.
How do I choose the contact rating?
Use the load current with a safety margin, and derate further for inductive, lamp or DC loads, which are harder on contacts than resistive loads.
Can I use an industrial relay directly on a PLC output?
Yes, if the coil current is within the PLC output rating. For many outputs, slim interface relays or a PLC relay module keep wiring tidy and protect the PLC.
Talk to Concord
Concord Automation & Controls has supplied motor protection relays and industrial relays to Indian industry since 1996 and is the exclusive ComatReleco (Switzerland) distributor for India, Bangladesh and Mauritius. Share your motor, pump or panel details and we will recommend the right model: call or WhatsApp +91 96670 40511, email contact@concordrelay.com, or use our contact page.