Electrical Relay Selection Guide for Industrial Control Systems
A guide to electrical relay selection for industrial control systems. Covers electromechanical relays, solid-state relays, protection relays, and selection criteria.
Relay Types
Electromechanical relay (EMR): Moving contacts, reliable, but limited life (1 million operations).
Solid-state relay (SSR): No moving parts, long life (>10 million operations), fast switching, but heat generation.
Reed relay: Hermetically sealed, fast switching, low contact resistance, but low current rating.
Protection relay: For electrical protection (overcurrent, earth fault, differential). Usually digital/numerical.
Electromechanical Relay Selection
Coil voltage: 5V, 12V, 24V DC, 110V AC, 220V AC, 380V AC (match control voltage).
Contact rating: Voltage (AC/DC), current (resistive/inductive load).
Contact configuration: NO (normally open), NC (normally closed), changeover (CO).
Mounting: Plugin (easy replacement), soldered, PCB mount.
Solid-State Relay (SSR) Selection
Input: Control voltage (3-32V DC), control current (10-20mA).
Output: Voltage (24-480V AC, 5-100V DC), current (1-100A).
Zero-crossing: Reduces EMI (electromagnetic interference). Preferred for resistive loads.
Heat sink: SSR generates heat. Must use proper heat sink (based on current and ambient temperature).
Application Examples
Motor control: Use contactor (not relay) for motor switching.
Lighting control: Use relay (EMR or SSR) for switching lights.
Heating control: Use SSR (long life, no arcing).
Signal isolation: Use reed relay or optocoupler.
Protection: Use digital protection relay.
Selection Mistakes to Avoid
1. Undersized contact rating (relay welding or burning).
2. Wrong coil voltage (relay not operating or burning).
3. Not considering inrush current (contact welding).
4. SSR without heat sink (overheating, failure).
5. Not providing snubber circuit (EMI, contact arcing).