Thermal Overload Relay | Motor Overload Protection Relay for Contactors (JR28s / LR2)

A motor that draws even 15–20% over its rated current will overheat and cook its winding insulation given enough time — but not fast enough to blow a fuse. A thermal overload relay is what catches that slow overload: it heats up in step with the motor and trips the control circuit before the motor is damaged.

What the JR28S does

The JR28S carries the motor current through internal bimetallic strips that bend as they heat. On a sustained overload they trip an auxiliary contact that drops out the contactor, disconnecting the motor. It includes phase-loss (differential) protection, ambient-temperature compensation, and a selectable manual or automatic reset. It mounts directly onto its matching contactor (or on a separate base plate) to form a complete starter.

Full current range (0.1–630 A)

Each frame size covers a set of overlapping adjustable current ranges — set the dial to your motor's full-load current:

Key specifications

Model code and mounting

The code reads JR28s-[frame] / [mounting], where the frame is the size (25, 36, 93, 150, 200, 630) and the mounting code is Z = stack-mount (piggyback) directly onto the contactor, or F = separate mount on a base plate. The JR28S is the CNC equivalent of the widely used LR2 / LRD relay footprint.

What the JR28S does not do

A thermal overload relay protects against overload only — the slow, sustained over-current. It does not break short-circuit faults; that needs an upstream circuit breaker or fuse in the starter. It also needs a contactor to actually switch the motor. For a single device that combines switching, overload and short-circuit protection, see the CNC YCKB7 integrated motor starter.

Downloads

• JR28S instruction manual — IEC 60947-4-1 (PDF)
• JR28S catalogue (PDF)
• CE certificate (PDF)

Frequently asked questions

What is a thermal overload relay?

It is a motor-protection device that trips when a motor draws too much current for too long. It heats up in step with the motor and, on a sustained overload, opens a contact that drops the contactor and disconnects the motor before it burns out.

How do I set the relay current?

Set the adjustment dial to the motor's full-load current from its nameplate, within the relay's range. Pick the frame and range that brackets that current — for example a 15 A motor sits in the 12–18 A range on a JR28s-25.

How does it mount?

It stack-mounts directly onto its matching contactor (code Z) for a compact starter, or on a separate base plate (code F). The matching-contactor column in the range table shows which contactor each frame fits.

Does it protect against a lost phase?

Yes — built-in differential phase-loss protection trips the relay if one phase is lost, which would otherwise quickly overheat a three-phase motor.

Manual or automatic reset — which should I use?

Manual reset is safer for most machinery, so a trip is investigated before restart. Automatic reset suits unattended equipment that should restart itself once it cools. The JR28S lets you select either.

Does it also break short circuits?

No. It handles overload only. Pair it with an upstream breaker or fuse for short-circuit protection, or use a CPS like the YCKB7 that includes both.