Overload Protection and Short-Circuit Protection for Motors: Correct Roles and Sizing Concepts

1) Overload vs Short-Circuit/Ground-Fault: Different Problems, Different Protection

Overload is a sustained overcurrent that is only moderately above normal running current (for example 110–150% of FLA). It is typically caused by mechanical load issues, low voltage, poor cooling, or phase loss. Overloads primarily overheat the motor windings over time.

Short-circuit and ground-fault currents are high-magnitude fault currents (often many times FLA) caused by insulation failure, damaged conductors, or wiring faults. These faults can destroy conductors and equipment quickly and can create arc-flash hazards.

Because the time and magnitude are different, protection is layered:

• Overload relay: protects the motor from overheating due to overload/phase loss; it is intentionally slower and does not clear high fault currents.
• Branch-circuit short-circuit and ground-fault protective device (SC/GF): fuses, circuit breakers, or motor circuit protectors clear high fault currents quickly to protect conductors and equipment.
• Contactor/starter: the switching device that the overload relay typically trips (opens the control circuit) to stop the motor.

2) Overload Relay Operation: Bimetal vs Electronic, and Trip Classes

Bimetal (thermal) overload relays

A bimetal overload relay uses a heater element and a bimetal strip. Motor current heats the element; the bimetal bends as temperature rises and eventually trips a mechanism that opens an auxiliary contact (commonly in series with the contactor coil). Key points:

• Thermal memory: if the motor was hot from a recent run, it trips faster on the next overload.
• Ambient sensitivity: surrounding temperature can affect trip time unless compensated.
• Simple and robust: common in traditional starters.

Electronic overload relays

Electronic overloads measure current (often via CTs) and use an algorithm to model motor heating. Many include features beyond basic overload protection:

Continue in our app.

You can listen to the audiobook with the screen off, receive a free certificate for this course, and also have access to 5,000 other free online courses.

Or continue reading below...

Download the app

• Adjustable trip class and more repeatable timing.
• Phase loss/imbalance detection, ground-fault sensitivity (model-dependent), and diagnostics.
• Remote reset and communications (model-dependent).

Trip classes: what they mean in practice

Trip class describes how long the overload relay will allow a high current (typically around 600% of setting) before tripping. The goal is to allow normal starting but still protect the motor if it fails to accelerate.

Important sizing concept: If a motor has a long acceleration time, a too-fast trip class can cause nuisance trips during starting. But choosing a slower class should be justified by the motor’s thermal capability and the application—slower tripping can increase risk if the motor stalls.

Step-by-step: selecting a trip class for long starting time

• Step 1: Identify whether the motor normally accelerates quickly or has a long start (high inertia load, high friction, loaded start).
• Step 2: Confirm the starting profile is normal for the application (not caused by a fault like low voltage or mechanical binding).
• Step 3: Choose the lowest trip class that allows normal starting without tripping (often Class 10 for typical loads; Class 20/30 only when needed).
• Step 4: If using an electronic overload, verify any “stall” or “jam” functions are enabled/adjusted appropriately so a non-accelerating motor is still protected.

3) Fuses vs Breakers vs Motor Circuit Protectors: Roles and Limitations

Short-circuit/ground-fault protection is not the same as overload protection. The upstream device is primarily there to clear faults and protect conductors, not to protect the motor from gradual overheating.

Fuses

• Strengths: very fast fault clearing, high interrupting ratings, consistent performance; current-limiting fuses can reduce let-through energy.
• Limitations: one-time operation; requires replacement; coordination and correct fuse type matter.
• Motor note: time-delay (dual-element) fuses are commonly used to tolerate motor starting current while still protecting against faults.

Circuit breakers (inverse-time and instantaneous trip)

• Strengths: resettable, widely used, can provide both thermal (long-time) and magnetic (instantaneous) trip elements.
• Limitations: may require higher settings to ride through inrush; not a replacement for a properly set overload relay for motor thermal protection.
• Motor note: the instantaneous (magnetic) trip can nuisance-trip on high inrush if not selected/adjusted correctly.

Motor Circuit Protectors (MCPs)

An MCP is typically a magnetic-only protective device intended for short-circuit protection of motor branch circuits. It is commonly used as part of a motor starter combination.

• Strengths: adjustable instantaneous trip to ride through starting current while still providing short-circuit protection.
• Limitations: generally does not provide overload protection by itself; must be used with an overload relay in the starter.

How they work together (coordination concept)

• The overload relay trips on sustained overcurrent and opens the contactor control circuit.
• The SC/GF device (fuse/breaker/MCP) clears high fault currents and protects conductors and equipment.
• Proper selection avoids nuisance trips during starting while still clearing faults quickly.

4) Setting Overloads Using Nameplate FLA and Application Factors

Overload settings are based on the motor’s nameplate full-load current (FLA) and the application. The objective is to protect the motor from overheating while avoiding trips during normal operation.

Core concept: set to the motor, not to the measured running current

Measured running current can be lower than FLA under light load, or higher due to voltage issues or mechanical problems. The overload relay is intended to protect the motor at its rated capability, so the nameplate FLA is the starting point.

Step-by-step: basic overload setting workflow

• Step 1: Read the motor nameplate FLA. Use the FLA for the actual connection voltage.
• Step 2: Identify service factor (SF) and temperature rise (if provided). These influence allowable overload setting in many practices and codes.
• Step 3: Choose the initial overload setting. Common practice is to start at 100% of nameplate FLA unless the application and motor rating allow a higher setting.
• Step 4: Run the motor under normal load and verify. Check that steady-state current is reasonable and that the overload does not trip during normal duty cycle.
• Step 5: If tripping occurs, do not immediately increase the setting. First determine whether the trip is legitimate (mechanical overload, single-phasing, low voltage, etc.).

Application factors that affect the correct setting

• High ambient temperature or poor ventilation: may require a more conservative setting or improved cooling.
• High cycling / frequent starts: increases heating; may require careful trip class selection and conservative settings.
• High inertia loads: may require a higher trip class (longer start allowance) rather than simply increasing the current setting.
• Altitude: reduced cooling can increase motor temperature rise; consider derating guidance from the motor manufacturer.

Practical example: what “too high” looks like

If an overload is set significantly above nameplate FLA to stop nuisance trips, the motor may run for long periods above its thermal limit without tripping, shortening insulation life. A better fix is usually to address the cause (load, voltage, phase imbalance) or adjust trip class appropriately.

5) Reset Modes and Safety Considerations

Manual reset vs automatic reset

• Manual reset: requires a person to reset after a trip. This is often preferred for safety because it forces investigation before restart.
• Automatic reset: the overload can reclose after cooling. This can be useful in some unattended processes but can create hazards if the motor restarts unexpectedly.

Reset timing and “thermal memory”

Many overloads will not reset until the element/model indicates the motor has cooled sufficiently. This helps prevent repeated overheats. Electronic overloads may provide a calculated “% thermal capacity used” indication.

Safety checklist before resetting

• Verify the cause is understood (not just “it tripped”).
• Check for jammed or driven-load issues before re-energizing.
• Consider unexpected restart hazards: conveyors, fans, pumps, and machinery with pinch points.
• Use proper lockout/tagout when investigating wiring, contactors, or motor leads.

6) Troubleshooting: Nuisance Trips vs Legitimate Overload Conditions

When an overload relay trips, treat it as a symptom. The goal is to determine whether the motor is actually overheating (legitimate trip) or whether the protection is misapplied/misset (nuisance trip).

Step-by-step troubleshooting workflow

• Step 1: Identify what tripped. Confirm it is the overload relay (not the breaker/MCP/fuse). Check indicators, trip flags, or relay diagnostics.
• Step 2: Record settings and nameplate data. Note overload dial setting, trip class, reset mode, and motor FLA/SF.
• Step 3: Measure operating current on all phases (or line and neutral for single-phase). Compare to nameplate FLA and check balance between phases.
• Step 4: Measure voltage at the starter under load. Low voltage can increase current and heating and can lengthen acceleration time.
• Step 5: Inspect the mechanical load. Look for binding, clogged pumps, tight bearings, misalignment, belt tension issues, or process changes increasing load.
• Step 6: Check for single-phasing/phase loss (three-phase). A lost phase can cause high current in remaining phases and rapid overheating; many electronic overloads detect this directly.
• Step 7: Check starter and wiring conditions. Loose lugs, burned contacts, or high resistance connections can cause voltage drop and heating.

Common causes and what they look like

Overload setting incorrect

• Too low: trips during normal load or during normal start even though voltage and mechanics are fine.
• Too high: motor runs hot, may smell, insulation life reduced; overload rarely trips even when load is excessive.

Jammed load / mechanical overload

• Symptoms: current rises above normal and stays high; motor may slow, belt may slip, pump may be blocked.
• Action: isolate and correct the mechanical issue; do not “solve” by increasing overload setting.

Single-phasing (three-phase)

• Symptoms: one phase current drops or goes to zero; remaining phases increase; motor may sound different and overheat quickly.
• Action: check fuses, breaker poles, contactor poles, and supply; verify phase-loss protection features and wiring.

Low voltage

• Symptoms: higher current for a given load, longer acceleration time, possible overload trip during start or under load.
• Action: measure voltage at the motor/starter during start and run; check feeder sizing, transformer taps, and connections.

Distinguishing overload relay trips from breaker/fuse trips

When adjusting is appropriate (and when it is not)

• Appropriate: trip class too fast for a verified normal long start; overload setting slightly misaligned with nameplate FLA; ambient compensation needed.
• Not appropriate: increasing settings to mask mechanical overload, phase loss, low voltage, or failing motor bearings—these should be corrected at the source.