Thermal Overload Relays

Thermal Overload Relay, The Component That Saves Your Motor

A motor starter without a thermal overload relay is incomplete. The contactor starts and stops the motor. The thermal overload relay is what actually protects it.

When a motor overloads, draws too much current for too long due to a jammed pump, a heavy mechanical load, single phasing, or a voltage drop, the winding heats up. Without protection, the insulation breaks down and the motor burns out. That is an expensive replacement and a site shutdown.

A thermal overload relay monitors the motor’s running current continuously using a bimetallic strip. When current stays above the set level for too long, the strip trips the relay, which opens the contactor control circuit and stops the motor before damage occurs. It also provides phase loss protection, which is one of the most common causes of motor failure in India.

Siemens 3UA is one of the most widely used and trusted thermal overload relay series in Indian industry. We stock 17 variants from 0.1A all the way to 10A plus, covering everything from small water pumps and conveyor drives to larger three phase industrial motors.

All HSN 8536, made in India, available up to 45% below MRP.

Bulk pricing for panel builders, electrical contractors and industrial buyers.

Fast delivery to Mumbai, Delhi, Pune, Bangalore, Hyderabad, Chennai and all of India.

💬 FAQ Section

Q1. What is a thermal overload relay and what does it do? A thermal overload relay is a protective device used in motor control circuits to protect electric motors from damage due to overcurrent, overload, and single-phase failure. It monitors the motor’s running current using a bimetallic strip. When the current exceeds the set threshold for a sustained period, the relay trips, opening the control circuit of the contactor and stopping the motor before the winding insulation is damaged.

Q2. What is the difference between a thermal overload relay and an MCB? An MCB (Miniature Circuit Breaker) protects wiring and equipment from short circuits and overloads using a fixed tripping curve. A thermal overload relay is specifically designed for motor protection, with an adjustable current setting matched to the motor’s full load current. It has a longer tripping time to allow for motor starting inrush current while still protecting against sustained overloads. MCBs and thermal overload relays serve different protection purposes and are used together in a complete motor starter panel.

Q3. What is Siemens 3UA thermal overload relay? Siemens 3UA is a widely used series of thermal overload relays designed for use with Siemens 3TF and 3TB contactors in motor control panels. Available in multiple current ranges from 0.1A to over 25A, the 3UA series provides bimetallic thermal protection with manual and automatic reset options, phase failure protection, and a test button for checking trip function during commissioning.

Q4. How do I select the correct thermal overload relay current range for my motor? Select a thermal overload relay whose adjustable range covers the full load current (FLC) of your motor. The FLC is typically found on the motor nameplate. As a general rule, choose a relay whose midrange setting matches the motor FLC. For example, for a motor with FLC of 3A, a relay with a range of 2.5A to 4A would be appropriate. Always set the relay to the motor’s nameplate FLC after installation.

Q5. What is phase loss protection in a thermal overload relay? Phase loss (also called single phasing) occurs when one of the three supply phases to a three-phase motor is lost due to a blown fuse, loose connection, or supply fault. The motor continues to run but draws excessive current on the remaining two phases, rapidly overheating the windings. A thermal overload relay with phase loss sensitivity detects this current imbalance and trips before motor damage occurs. All Siemens 3UA relays include phase loss protection.

Q6. What is the difference between manual reset and automatic reset on a thermal overload relay? A manual reset relay requires the operator to physically press the reset button after a trip, ensuring someone investigates the fault before the motor restarts. An automatic reset relay resets itself after cooling down and allows the motor to restart automatically. Manual reset is preferred for safety-critical applications and remote installations where uncontrolled automatic restart could be dangerous.

Q7. Where can I buy Siemens thermal overload relay at wholesale price in India? Elecload India stocks 17 variants of Siemens 3UA thermal overload relays at up to 45% below MRP with bulk pricing for panel builders, electrical contractors, and industrial buyers. Fast delivery across all major cities in India.