| Electromechanical control relay | Switch control signals or loads using an electrically operated contact mechanism. | Current through a coil creates a magnetic field that moves an armature and changes the contact state. Removing coil power lets a spring return the mechanism. | PLC output interfaces, machine sequencing, interlocking, and general control panels. | Available with normally open, normally closed, or changeover contacts. Contacts physically separate when open; mechanical wear and contact arcing are possible. | Check coil voltage and AC/DC type, contact configuration, load voltage and current, electrical life, switching frequency, and the load’s inrush current. |
| Solid-state relay (SSR) | Switch electrical loads electronically where frequent or quiet operation is useful. | An input signal drives semiconductor switching devices. AC-output designs commonly use triacs or thyristors; DC-output designs commonly use transistors. | Heaters, temperature-control systems, and applications with frequent switching and no moving contacts. | Silent operation and no mechanical contact wear. SSRs have an on-state voltage drop, can produce heat, and may have off-state leakage current. | Match the SSR to AC or DC load type, load current, inrush, switching mode, heat-sink needs, ambient temperature, and leakage-current sensitivity. |
| Safety relay | Monitor safety-related control circuits and help bring machinery to a defined safe state when a fault or stop condition is detected. | Internal circuits monitor inputs such as emergency-stop devices or guard switches; redundant switching paths and fault monitoring are used in suitable designs. | Emergency-stop circuits, machine guards, light curtains, and other machinery safety functions. | Some designs use mechanically linked, or force-guided, contacts to support detection of certain contact faults. Safety performance depends on the complete circuit and application design. | Select according to the required safety function and risk assessment. Verify input compatibility, reset and feedback requirements, contact ratings, and applicable machinery-safety requirements. |
| Thermal overload relay | Protect a motor against sustained overload conditions; it is generally used with a contactor to interrupt the motor control circuit. | Motor current heats a bimetal element or sensing system. An overload condition causes a trip mechanism to change the relay contacts. | Motor starters, pumps, fans, conveyors, and other motor-driven equipment. | Provides overload response with a time delay related to current and thermal behavior. It is not, by itself, a substitute for short-circuit protection. | Set the adjustable current range to the motor’s nameplate current and check trip class, phase-loss sensitivity, reset mode, ambient conditions, and coordination with short-circuit protection. |
| Monitoring relay | Detect abnormal electrical conditions and switch an output when monitored values cross configured limits. | Electronic sensing and comparison circuits monitor conditions such as voltage, phase sequence, phase loss, or current, then operate an output relay. | Supply monitoring, phase-failure protection, voltage-window alarms, and equipment interlocks. | Typically provides a status or trip output rather than directly switching a high-power load. Delay, hysteresis, and reset behavior vary by model. | Confirm the measured parameter, operating range, thresholds, response delay, supply requirements, output contact rating, and required recovery behavior. |