Why the Himel HDR3S Thermal Overload Relay Is Essential for Motor Safety

Why the Himel HDR3S Thermal Overload Relay Is Essential for Motor Safety

4 July 2026 · 8 min read · Trans-Digi Global

A thermal overload relay is the device that switches a motor off when it draws more current than it should for too long, before the windings overheat. The Himel HDR3S does this with a bimetal element set to the motor’s full-load current, and it also trips on phase failure, the loss of one supply phase. Fitted to a contactor, it protects against the slow, heat-driven damage that a circuit breaker’s short-circuit protection is not designed to catch.

In this article

• Why motors need overload protection
• How the HDR3S works
• HDR3S specifications
• Understanding trip class
• How to select and set an HDR3S
• Overload relay or motor circuit breaker?
• Where the HDR3S is used in Singapore
• Common mistakes
• Frequently asked questions
• Protect the motor, not just the circuit
• Sources

Trans-Digi Global supplies the Himel HDR3s thermal overload relay from local Singapore stock, together with matching HDC3 contactors, as an authorised Himel partner.

Why motors need overload protection

Heat is what ends a motor’s life. The US Department of Energy’s motor and drive system sourcebook sets out the numbers:

• Winding failures account for about one-fifth of all motor failures, and the causes of insulation failure include “the high temperatures caused by a current overload.”
• Insulation life is roughly halved for every 10 °C increase in operating temperature.
• If a load exceeds the motor’s breakdown torque, the motor stalls, “causing it to rapidly overheat and risking insulation failure if it is not properly protected.”
• A 5% voltage unbalance can make phase currents differ by as much as 40%, which can quickly damage a motor.

An overload relay targets exactly these conditions: sustained over-current, a stalled or jammed load, and the current imbalance that follows a lost phase.

How the HDR3S works

Inside the relay, a bimetal strip carries (or is heated by) the motor current in each phase. As current rises above the set value, the strips heat and bend until they release a trip mechanism. That opens the relay’s normally closed auxiliary contact, which is wired into the contactor’s coil circuit, so the contactor drops out and the motor stops.

• Inverse-time response: a small overload takes longer to trip than a large one, so normal starting current does not cause a nuisance trip.
• Phase-failure sensitivity: if one phase is lost, the differential movement of the bimetals trips the relay faster than a balanced overload would.
• Manual or automatic reset, a test button, a stop button and a trip indicator, per the Himel datasheets.

HDR3S specifications

From the Himel product data sheet for the HDR3S2513 (9 to 13 A):

• Frame size: 25 A; 3 poles
• Protection: overload and phase failure
• Reset: manual and automatic; test button, stop button, trip indicator
• Auxiliary contacts: 5 A conventional thermal current; 1.57 A at 230 V AC-15, 0.9 A at 400 V AC-15, 0.14 A at 230 V DC-13
• Rated insulation voltage (Ui): 660 V; impulse withstand (Uimp) 6 kV
• Environment: operation −5 to 40 °C, storage −25 to 55 °C, up to 2,000 m, relative humidity 0 to 95%, IP20, pollution degree 3
• Standard and certification: IEC 60947-4-1; CB, CE, SEMKO

Himel’s Panel Builder brochure lists HDR3s frame currents of 25, 36 and 93 A and the IEC/EN 60947-4-1 standard, and Himel’s HDR3s product page gives trip class 10A. The 25 A-frame setting ranges we stock are below; each links to its datasheet on our product page.

Understanding trip class

Trip class tells you how long the relay allows a motor to run at a high overload, as at start-up, before it trips. IEC 60947-4-1 defines four classes for thermal overload relays: 10A, 10, 20 and 30. As Schneider Electric’s guidance on trip classes summarises, classes 10A and 10 suit normal-duty motors, class 20 is recommended for heavy-duty motors, and class 30 is needed for very long starts.

The HDR3S is a class 10A relay, which suits the majority of pumps, fans and general machinery that reach full speed quickly. For a high-inertia load with a long run-up, such as a large fan or a loaded conveyor, check the motor’s starting time against the relay’s trip curve before you commit.

How to select and set an HDR3S

1. Read the motor nameplate and note the full-load current (FLC) at your supply voltage.
2. Pick the range that contains the FLC, ideally near the middle so you have adjustment either side. A 7.5 A motor, for example, sits within the 5.5 to 8 A (HDR3S258) and the 7 to 10 A (HDR3S2510) ranges.
3. Match the contactor frame. The HDR3s is designed for coordination with Himel HDC3 contactors; confirm the pairing for your rating.
4. Check the trip class against the motor’s starting time (class 10A for normal starts).
5. Set the dial to the motor FLC, not to the contactor or breaker rating.
6. Choose the reset mode. Manual reset is the safer default where an automatic restart could endanger people or equipment.
7. Coordinate with short-circuit protection. The relay does not interrupt short circuits; an upstream MCCB or motor circuit breaker does. How the devices share the fault duty is defined as Type 1 or Type 2 coordination in IEC 60947-4-1.

Overload relay or motor circuit breaker?

The combinations are not mutually exclusive. Many panels use a motor circuit breaker for isolation and short-circuit protection, with a contactor and overload relay for control.

Where the HDR3S is used in Singapore

Our HDR3s product page lists the typical applications: motor protection in motor control centres, industrial machinery, pumps, compressors and conveyors. In Singapore buildings that covers a long list of small motors that run every day without anyone watching them, such as transfer and booster pumps, ventilation and exhaust fans, cooling-tower fans and small process machinery. These are the motors most likely to suffer a slow overload from a clogged filter, a worn bearing or a failed phase, and the least likely to be noticed until they stop.

Larger building-services motors on our project references, such as chilled-water pumps, cooling towers and carpark ventilation fans, are usually controlled by VSDs or soft starters with their own electronic motor protection. Direct-on-line and star-delta starters for smaller motors still rely on a contactor and thermal overload relay.

Common mistakes

• Setting the dial to the breaker or contactor rating instead of the motor FLC. The relay then protects nothing.
• Raising the setting after nuisance trips without finding the cause. Repeated trips usually point to a real problem: low voltage, a lost phase, a jammed load or a long start that needs a different class.
• Automatic reset on equipment that can restart unexpectedly.
• Ignoring panel temperature. The HDR3S datasheet lists −5 to 40 °C for operation, so a hot, unventilated panel needs attention.

Frequently asked questions

What does a thermal overload relay protect against?

Sustained overload, stalled or jammed loads and, on the HDR3S, phase failure. It does not protect against short circuits; that is the job of an upstream MCCB, motor circuit breaker or fuses.

What trip class is the Himel HDR3S?

Class 10A, per Himel’s HDR3s product information. Classes 10A and 10 suit normal-duty motors.

What setting should I use?

The motor’s full-load current from the nameplate, at the supply voltage it runs on.

Which contactor does the HDR3S work with?

It is designed for coordination with Himel HDC3 contactors. Tell us the motor rating and we will confirm the matching contactor and relay.

Manual or automatic reset?

The HDR3S offers both. Manual reset is the safer default where an unexpected restart could cause harm; automatic reset can suit remote, unattended equipment where the control logic manages restarts.

Do you have stock in Singapore?

Yes. The HDR3s range is held in local stock with datasheets available. Contact us with the motor FLC and we will reply the same business day.

Protect the motor, not just the circuit

Send the motor nameplate or full-load current and we will confirm the HDR3S range, the matching contactor and the upstream protection. Browse the Himel range or contact our team.

Related reading: Why Contactor Selection Matters for Reliable Motor Control · The Himel HDM3 MCCB Explained · How the Himel HAVXS Expert VSD Minimises Motor Wear

Sources

• Himel product data sheet: HDR3S2513, thermal overload relay HDR3s-25, 9 to 13 A
• Himel product data sheet: HDR3S258, thermal overload relay HDR3s-25, 5.5 to 8 A
• Himel: Panel Builder brochure (relay panorama: HDR3s frames, trip class, standard)
• IEC 60947-4-1:2023, Contactors and motor-starters
• Schneider Electric FAQ: thermal overload relay trip classes under IEC 60947-4-1
• US Department of Energy: Improving Motor and Drive System Performance, A Sourcebook for Industry
• Himel: HDR3s thermal overload relay product page
• Trans-Digi Global: Himel HDR3s thermal overload relay product page
• Trans-Digi Global: Himel HDP6 motor circuit breaker product page