Electric Motor Protection: Which Fault Do the Overload Relay, Fuse and MPCB Handle?

Here is a scene maintenance teams know well: a burnt-out pump motor, and in its panel an overload relay that never tripped. The relay works fine. Its dial is set to 32 A, while the motor nameplate says 21 A. For weeks the motor drew around 28 A, comfortably below the setting, and the winding slowly cooked until the insulation failed. The panel looked protected, but the setting had been copied from the contactor size rather than from the motor.

Protecting a motor is less about having devices in the panel and more about knowing which fault each one responds to, and setting each one to the right value. The overload relay, the fuse, the motor protection circuit breaker (MPCB), the phase monitoring relay and the sensors embedded in the winding each watch a different threat. None of them replaces the others.

What actually destroys a motor

Before choosing devices, separate the hazards. Most motor losses in the field come down to one of four situations:

• Slow overload: a worn impeller, an overloaded conveyor, an over-tensioned belt. Current sits 10–30 % above rated for hours. The shaft keeps turning while the insulation ages.
• Short circuit or earth fault: a loose terminal, a crushed cable, a punctured winding. Current jumps to many times rated within milliseconds.
• Lost phase or unbalance: a blown fuse or a burnt contact. A three-phase motor keeps running on two phases and overheats.
• Lost cooling: a clogged fan cowl, high ambient temperature, or long operation at low speed on a drive. The current looks normal; the winding temperature does not.

That last case matters: no device that measures current can see it directly. One device alone never gives complete protection.

Setting the overload relay

A thermal overload relay weighs current against time. It tolerates short excursions such as starting current and trips faster the longer an overload lasts. That behaviour is only useful if the set current is right, and the value comes from the nameplate line that matches the voltage the motor is actually connected to. On a 400 V supply, a 230/400 V motor runs in star and you use the 400 V current; a 400/690 V motor runs in delta and you use the 400 V (delta) figure. For a line-by-line walkthrough, see our guide to reading an IE3 motor nameplate.

Trip class deserves equal attention. Class 10 generally suits pumps and fans that reach speed within a few seconds. A high-inertia fan, a mixer starting full, or a crusher needs longer to accelerate. Without Class 20 or 30 the relay trips before the motor reaches speed, and the usual "fix" on site is to turn the dial up, which leaves the motor unprotected.

Who handles the short circuit: fuse or MPCB?

A fuse is there to protect the cable and the switchgear, not the motor winding. In motor circuits the fuse has to ride through starting current. An aM fuse is designed for short-circuit duty only and must always be paired with an overload device; a general-purpose gG fuse may need to be one size larger in a motor feeder. Oversizing the fuse just to stop nuisance blowing at start lengthens the clearing time during a real short circuit and exposes the cable and contactor to damage.

An MPCB combines thermal and magnetic tripping in one unit. Its adjustable range should cover the motor's rated current somewhere near the middle. For small and medium ratings it saves panel space and wiring. Check separately that its breaking capacity matches the prospective short-circuit current at that point in the installation. Range selection and step-by-step adjustment are covered in choosing and setting an MPCB.

Worked example: an 11 kW four-pole pump motor

Take an 11 kW, 4-pole IE3 motor on a 400 V supply. The rated current can be estimated with I = P / (√3 × U × cos φ × η). The IE3 efficiency limit for this rating and pole count is 91.4 %. With a typical power factor of 0.84:

I ≈ 11,000 / (1.732 × 400 × 0.84 × 0.914) ≈ 20.7 A

This is only a sanity check; always set from the nameplate. Suppose the plate shows about 21 A:

• Direct-on-line start: choose an MPCB or overload relay whose range comfortably includes 21 A and set it to 21 A. Short-circuit duty is handled by an aM fuse or by the MPCB's magnetic trip.
• Star-delta start: if the relay sits after the main contactor, in the winding leg, it sees phase current, which is 1/√3 of line current: 21 × 0.58 ≈ 12.2 A. Set it to 21 A and the motor can run roughly 70 % overloaded without a trip. If the relay sits upstream in the supply line, set it to line current.

Never adjust a star-delta overload without checking its position on the drawing. Remember too that star-delta starting only works with motors that run in delta at 400 V, i.e. 400/690 V windings.

Phase loss: when the current reading fools you

Many overload relays and MPCBs detect a current difference between phases and trip sooner on phase loss. That works when the motor is loaded and current rises clearly. A lightly loaded motor that loses a phase may stay below the setting while the unbalanced current still heats the rotor. On sites with unstable supply, generator-fed construction sites or unattended pumping stations, a separate phase monitoring relay is sensible. It watches phase sequence, under- and overvoltage and unbalance. For larger machines, the numbered functions of multifunction relays are explained in protection relay functions 46, 37, 49 and 48.

Seeing inside the winding: PTC and PT100

A blocked fan cowl or long low-speed operation on a drive heats the winding without raising current. A PTC thermistor embedded in the winding sharply increases its resistance at a set temperature and drops the contactor through a dedicated thermistor relay. A PT100 measures temperature continuously, so you can define an alarm stage and a trip stage. On HEM Motor three-phase series, PTC and PT100 winding sensors are optional and must be specified with the order. Motors that have sensors fitted but never wired to a relay are surprisingly common in the field. See motor winding temperature monitoring for sensor choice and wiring.

Panel mistakes that keep coming back

• Setting the overload to the contactor or cable rating instead of motor current.
• Setting a star-delta relay to line current without checking where it is wired.
• Using Class 10 on a long-start load, then turning the dial up.
• Replacing a motor and keeping the old settings. The same kW with a different pole count means a different current.
• Leaving the PTC leads unconnected in the terminal box.
• Going up two fuse sizes because it blew during start.

Plan protection with the order

Protection is a system designed together with the motor, not an accessory added later. Once rated current, starting method, load acceleration time and ambient conditions are known, the relay, fuse and sensor choices become straightforward. A checklist of what to request alongside a new motor is in motor protection devices to order with the motor. If you need a three-phase electric motor with PTC or PT100 sensors, send us your application details.

Frequently Asked Questions

Should the overload relay be set slightly above nameplate current?

Normal practice is to set it equal to rated nameplate current, since the relay already tolerates short excursions. If the motor constantly draws more than nameplate current, the problem lies with the load or with motor selection, and raising the setting only hides it.

If there is a fuse, do I still need an overload relay?

Yes. A fuse is sized well above motor current so it survives starting, which means it will not react to a 20–30 % overload lasting hours. That slow overload is cleared by the overload relay or by the thermal element of an MPCB.

What current do I set on a star-delta overload relay?

If the relay is in the winding leg, after the main contactor, set it to about 0.58 times the nameplate line current. If it is upstream in the supply line, set it to line current. Check the panel drawing before deciding which applies.

Does a motor with PTC sensors still need an overload relay?

Yes, because the two watch different things. The PTC tracks winding temperature and catches loss of cooling, while the overload relay tracks current and can react faster to events such as a locked rotor. Used together they cover each other's blind spots.