17.2 Motor Overload & Short-Circuit Protection

Motor Overload & Short-Circuit Protection

Quick Answer: A motor circuit carries two separate protective duties. Short-circuit protection (fuse or breaker) clears faults and is sized above starting current. Overload protection (thermal relay, magnetic relay or thermistor) is set to the motor’s full-load current and protects the windings from sustained overcurrent. One device rarely does both well.

Two Duties, Two Devices

The reason they are separated is the starting transient. A single device set low enough to protect the windings against a 15% overload would trip during every start; a device set high enough to allow starting would let the motor cook at 150% load indefinitely.

Thermal Overload Relays

The traditional bimetal relay places heater elements in each line. Current heats bimetal strips which bend and, after a time proportional to the heating, operate a contact that drops out the contactor coil. Because heating is an I²t process, the relay mimics the motor’s own thermal behaviour: a small overload takes minutes to trip, a large one takes seconds.

Setting discipline:

• Set the dial to the nameplate full-load current, adjusted for the actual supply voltage and connection.
• On a star-delta starter, the relay may sit in the delta leg and must be set to FLC / √3 — a classic capstone trap.
• Ambient temperature compensation matters when the starter enclosure is hotter than the motor.

Magnetic Overload Relays

Magnetic (electromagnetic) relays respond to current magnitude through a solenoid rather than accumulated heat, sometimes with an oil dashpot providing time delay. They are less common in modern general installations but appear in older switchgear and in applications needing an adjustable instantaneous element.

Thermistor (Direct Temperature) Protection

Positive temperature coefficient thermistors are embedded in the motor windings during manufacture. Their resistance rises sharply at a defined temperature, and a relay monitoring them trips the contactor.

Advantages over current-based methods:

• Protects against cooling failure — a blocked fan or blanketed motor overheats at normal current, which a thermal relay will never see.
• Protects motors on variable speed drives, where reduced speed reduces self-cooling but current looks normal.
• Protects against high ambient and frequent starting.

Thermistor protection is the correct answer whenever a stem describes overheating with normal current readings.

Trip Classes

Trip class states how long the overload relay will tolerate a start at 7.2 × FLC:

Selecting Class 10 for a high-inertia fan produces nuisance trips on start. Selecting Class 30 for a small pump leaves the windings exposed for three times longer than necessary during a genuine stall.

Single-Phasing — the Classic Motor Killer

If one line conductor opens (blown fuse, loose termination, broken conductor), a running three-phase motor keeps turning on two phases. The remaining windings carry roughly √3 ≈ 1.73 times their normal current and overheat rapidly, while the motor may still sound approximately normal to a passer-by.

Protection responses:

• A three-pole thermal relay senses the elevated current in the remaining phases and trips.
• A phase-failure relay monitors all three lines and trips on loss or severe imbalance.
• Thermistors catch the resulting winding temperature rise directly.

A stall or locked rotor produces the same overheating faster: locked-rotor current continues indefinitely because the rotor is not accelerating away from it.

Coordination With the Cable

The complete chain must satisfy the coordination logic from Chapter 7:

1. Ib — the motor full-load current, plus any allowance the design requires.
2. In — the short-circuit protective device rating, chosen above starting current.
3. Iz — the cable current-carrying capacity after derating, which must still be at least Ib and be protected against overload by the overload relay.

Note the important variation: on a motor circuit, overload protection of the cable is provided by the overload relay, not by the upstream short-circuit device. That is what allows In to sit above Iz in a properly designed motor starter arrangement — a point assessors probe because it appears to contradict the general rule.

Isolation for Mechanical Maintenance

Every motor needs a means of isolation for mechanical maintenance that is either adjacent to the machine or capable of being secured in the off position. Requirements to state on the practical:

• The isolator must break all live conductors and be clearly labelled with the machine it controls.
• It must be lockable, or the arrangement must otherwise prevent inadvertent reconnection.
• A VSD-fed motor may store energy in the DC bus — observe the manufacturer’s discharge time before working on terminals.
• Isolating the control circuit alone is never sufficient; the power circuit must be proved dead at the point of work.