Relays, contactors, circuit breakers and fuses
Switching and protection devices in control cabinets: relays vs contactors, utilization categories, coil suppression, circuit breakers and fuses, and how they interface with PLC I/O.
Relay or contactor
| Relay (control relay, interface relay) | Contactor | |
|---|---|---|
| Switches | Control circuits, small loads | Motors, heaters, power loads |
| Contacts | Low current (≤ 10 A typ.), often changeover | Main poles for high current + auxiliary contacts |
| Typical use | Interface between PLC outputs and other voltages, potential separation | Motor starters, load switching |
Utilization categories (IEC 60947) describe what a contact may switch: AC-1 resistive, AC-3 squirrel-cage motors (start, switch off running), AC-15 / DC-13 electromagnetic coils. A contact rated 10 A AC-1 may switch much less as DC-13.
Coil suppression
Switching off a coil produces a voltage spike that damages PLC transistor outputs and causes interference. Suppress at the coil:
| DC coils | AC coils |
|---|---|
| Freewheeling diode (slow release!) or diode + Zener / TVS (faster) | RC snubber or varistor |
Many contactors offer plug-in suppressors. Check PLC output modules: some have internal suppression, which is not enough for large coils on long cables.
Circuit breakers and fuses
- Miniature circuit breakers (MCB) protect cables against overload and short circuit; trip characteristic (B, C, D) defines the magnetic trip current (D for high inrush).
- Fuses react fast, can protect semiconductors (gR/aR types), need replacement.
- Motor protection circuit breakers combine short-circuit and adjustable thermal overload protection.
- Selectivity: the protective device closest to the fault should trip first.
Interface to the PLC
- Read the auxiliary contact of a contactor/breaker as feedback, not the PLC output.
- Use interface relays for loads that exceed output ratings or need potential separation.
- Feed "breaker tripped" contacts to PLC inputs for diagnostics.
Connections
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Sources
Updated 2026-10-02