Solid State Overload Relay
A solid state overload relay is an electronic motor-protection device that monitors current electronically and opens the control circuit when it detects an overload condition, often also detecting phase loss or phase imbalance. It is typically installed in a motor starter to protect a three-phase motor by de-energizing the contactor coil before motor overheating causes damage or a fire risk.
On the shop floor, the relay is wired in series with the motor starter control circuit, usually through CT-based sensing on each motor phase, and its normally closed auxiliary contact is placed so that a trip opens the contactor coil circuit and stops the motor. Technicians set the relay to the motor nameplate FLA, select an appropriate trip class, and verify reset and trip operation using test/reset functions and continuity checks on the NC trip contacts. It is used where motors are exposed to frequent starts, varying loads, or conditions where electronic diagnostics, phase-loss protection, and more precise trip settings are valuable.
- Incorrect FLA setting: If the relay is not set to the motor nameplate full-load amperes, it may nuisance-trip or fail to protect the motor properly.
- Bypassing or miswiring the NC trip contact: If the relay's trip contact is not correctly placed in the contactor coil circuit, a fault may not stop the motor, defeating the protection function.
- Assuming it replaces short-circuit protection: A solid-state overload relay does not clear high-fault current the way a fuse or breaker does; treating it as complete motor protection is a serious design and compliance error.
Does a solid state overload relay protect against single-phasing?
Yes, many models do; ABB explicitly states phase-loss trip logic, and other industrial models cite phase-loss and phase-imbalance detection as standard electronic-protection features.
Is it the same as a solid-state relay used for switching heaters or lamps?
No. A general SSR is a semiconductor switching device, while a solid-state overload relay is a motor-protection device with sensing, timing, and trip logic integrated into the starter circuit.
What does trip class mean?
Trip class is the time-delay profile used to let a motor start without nuisance tripping while still tripping on prolonged overload; industrial overload products commonly use class settings such as Class 10, 20, or 30 depending on motor start characteristics.