A motor protection relay detects abnormal conditions and trips the contactor before damage occurs. Four functions cover the most common failure modes: thermal overload, short circuit, earth fault, and phase loss.
This guide explains how each protection function works, the settings formulas, and how to configure a complete motor protection scheme for any three-phase induction motor.
A motor without protection is running on borrowed time. The most common failure modes are all detectable: thermal overload, short circuit, earth fault from insulation failure, and single-phasing from a blown fuse.
A correctly configured motor protection relay catches all four.
The 4 Motor Protection Functions Every Engineer Should Know
Detects sustained overcurrent that exceeds the motor's thermal capacity. The relay integrates the I²t heating over time using the thermal image model. Trip time decreases as the overload multiple increases.
Protects against: mechanical overload (seized bearing, jammed pump), supply voltage dip causing high current draw, slow acceleration due to high load inertia.
Detects phase-to-phase or three-phase short circuit current. The pickup must be set above the motor starting current peak (typically 600 to 700% FLC) to avoid nuisance tripping during direct-on-line starting.
Protects against: winding insulation failure, cable fault between motor and switchgear, phase-to-phase fault at motor terminals.
Detects current flowing from a phase conductor to earth through degraded insulation or a direct fault. Measured using the residual current method (sum of three-phase currents) or a dedicated core balance CT.
Protects against: insulation breakdown, moisture ingress, cable damage, bearing current damage to motor frame.
Detects the loss of one supply phase. Single-phasing causes the motor to draw 150 to 200% of FLC on the two remaining phases, rapidly overheating the windings.
A standard thermal overload relay detects single-phasing only if the motor is loaded above about 75% of full load.
Protects against: blown fuse on one phase, open contact on one pole of the contactor, supply cable open circuit.
Motor Protection Relay: How Each Function Connects
This prevents a motor from being restarted immediately after an overload trip, which would force the already-hot windings to absorb even more heat during the next starting transient. The thermal image model is defined in IEC 60255-149.
How to Set Each Protection Function
Overload Protection Setting (ANSI 49)
The overload pickup current (Ip) is set based on the motor full-load current (FLC) from the nameplate. The standard setting range is 100 to 125 percent of FLC. For most applications, set Ip = 100 to 105 percent of FLC to provide the tightest protection without nuisance tripping.
The trip class is the maximum trip time at 6x pickup current. Class 10 trips in 10 s, Class 20 in 20 s, Class 30 in 30 s.
Use Class 10 for standard motors. Use Class 20 or Class 30 for high-inertia motors (large fans, compressors, ball mills).
Short Circuit Setting (ANSI 50)
The short circuit instantaneous pickup (Is) must be set above the motor starting current peak and below the minimum fault current at the motor terminals. The standard formula is:
For soft-starter or VFD-fed motors, the starting current is limited. The short circuit pickup can be set lower, at 4 to 6x FLC.
Always verify that the minimum calculated fault current at the motor terminals exceeds the pickup by at least 20 percent margin.
Earth Fault Setting (ANSI 51N)
The earth fault pickup (Ie) is typically 5 to 20 percent of FLC. A lower pickup detects insulation faults earlier but can cause nuisance tripping from zero-sequence imbalance in long cable runs.
For high-resistance earthing systems (IT or high-resistance neutral), the earth fault pickup must be calculated from the system earth fault current level. See the earthing resistance calculation guide for the formula.
Motor Protection Relay Setting Calculator
Worked Example: 11 kW Motor, 400 V, DOL Start
FLC = P / (sqrt(3) x V x eta x PF)
FLC = 11000 / (1.732 x 400 x 0.88 x 0.85) = 11000 / 515.8 = 21.3 A
Step 2: Overload protection (ANSI 49)
Pickup Ip = 1.05 x FLC = 1.05 x 21.3 = 22.4 A
Trip class: Class 10 (standard pump, low inertia)
Reset: automatic after thermal image cools to 100% (typically 5 to 15 min)
Step 3: Short circuit protection (ANSI 50)
DOL starting peak = approximately 6x FLC = 6 x 21.3 = 127.8 A
Short circuit pickup Is = 10 x FLC = 10 x 21.3 = 213 A
Verify: Is (213 A) is above DOL peak (128 A) and below min fault current at motor terminals
Step 4: Earth fault protection (ANSI 51N)
Earth fault pickup Ie = 10% x FLC = 0.10 x 21.3 = 2.13 A
Time delay: 0.2 s (to ride through transient zero-sequence current on starting)
Step 5: Phase loss protection (ANSI 46)
Set current unbalance threshold to 15%
Time delay: 2 to 5 s (to ride through momentary voltage unbalance on the network)
Correct vs Incorrect Motor Protection Relay Selection
- Overload pickup set at 100 to 105% of motor FLC from nameplate
- Trip class matched to starting inertia (Class 10 for pumps, Class 20 to 30 for compressors)
- Short circuit pickup above DOL peak (128% x FLC) and below minimum fault current
- Earth fault pickup at 5 to 10% FLC with short time delay (0.1 to 0.5 s)
- Phase loss detection enabled with 15 to 20% unbalance threshold
- CT ratio correctly matched so secondary current at FLC is within relay input range
- Protection relay tested with secondary injection before commissioning
- Overload pickup set too high (e.g. 130 to 150% FLC) to stop nuisance trips this removes thermal protection
- Wrong trip class (Class 10 on a compressor) causes nuisance trips during every start
- Short circuit pickup set below DOL starting peak relay trips on every motor start
- Earth fault disabled or set too high insulation damage goes undetected until complete failure
- Phase loss protection not enabled motor destroyed by single-phasing before thermal relay trips
- CT ratio wrong relay reads 50% of actual current and never trips on overload
- Relay not tested settings incorrect but fault not discovered until first motor failure
Secondary injection means applying a test current directly to the relay current input terminals, bypassing the CTs and the actual motor circuit. This verifies that the relay trips at the correct current and time without needing to force an actual fault condition. A relay with wrong CT polarity wiring will not detect earth faults even if the pickup and time settings are correct. Secondary injection catches wiring errors that parameter verification alone cannot find.
Watch: Motor Protection Relay Setup and Testing
Motor Protection Relay Questions Engineers Ask
Related Articles on This Site
- Protective Relays Explained
- Motor Full Load Current (FLC) Explained
- Calculate Short Circuit Fault Current
- MCB vs MCCB vs ELCB vs RCCB
- Earthing Resistance Calculation
External References
- Motor Protection Relay Application Guide | ABB
- Motor Protection Relay Settings Guide | Schweitzer Engineering Laboratories
What We Learn Today
- Four functions protect a three-phase motor: ANSI 49 thermal overload (pickup at 105% FLC, Class 10 to 30), ANSI 50 instantaneous short circuit (pickup at 8 to 12x FLC), ANSI 51N earth fault (pickup at 5 to 10% FLC, 0.1 to 0.5 s delay), and ANSI 46 phase loss (15 to 20% current unbalance threshold).
- The short circuit pickup must be above the DOL starting peak (approximately 6 to 7x FLC) and below the minimum fault current at the motor terminals. For VFD or soft-starter fed motors, the starting current is limited and the short circuit pickup can be set lower at 4 to 6x FLC.
- A standard thermal overload relay does not reliably detect single-phasing below 75% motor load. A dedicated phase loss or current unbalance protection function (ANSI 46) is required to detect single-phasing at all load levels and protect the motor windings before they overheat.