# HT Motor Relay Settings: Complete Protection Guide for High Voltage Motors

**URL:** <https://forumautomation.com/t/ht-motor-relay-settings-complete-protection-guide-for-high-voltage-motors/14119>\
**Category:** Electrical\
**Tags:** motor-control\
**Created:** [May 25, 2026, 3:10pm UTC](https://forumautomation.com/t/ht-motor-relay-settings-complete-protection-guide-for-high-voltage-motors/14119 "2026-05-25T15:10:08Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [May 25, 2026, 3:10pm UTC](https://forumautomation.com/t/ht-motor-relay-settings-complete-protection-guide-for-high-voltage-motors/14119/1 "2026-05-25T15:10:08Z")

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**[HT Motor](https://forumelectrical.com/knee-point-voltage-calculation-for-ht-motor/)** Relay Settings are important for protecting **High Tension (HT) motors** from faults such as

- Overload,
- Short circuit,
- Earth fault,
- Overheating,
- Voltage imbalance and
- Rotor jamming.

Proper relay settings improve motor reliability, reduce downtime and prevent costly equipment failures.

**Common Protection Relays for HT Motors**

Common Protection Relays for HT Motors are

1). **[Over Current Protection (50/51)](https://forumelectrical.com/overcurrent-earth-fault-e-f-protection-testing-method-statement/)**

2). Earth Fault Protection (50N/51N)

3). Thermal Overload Protection (49)

4). Under Voltage Protection (27)

5). Over Voltage Protection (59)

6). Negative Sequence / Single Phasing Protection (46)

7). Locked Rotor Protection (48)

8). Differential Protection (87M)

**1). Over Current Protection (50/51)**

Protects the motor from overload and short circuits.

- **Instantaneous Over Current (50):** 6 to 8 × Full Load Current (FLC)
- **Time Over Current (51):** 1.1 to 1.5 × FLC

Example: For a motor with **100 A FLC** , the OC setting may range from **110 A to 150 A**.

**2). Earth Fault Protection (50N/51N)**

Protects against leakage current to ground.

**Typical Setting:** 10% to 20% of motor FLC.

**3). Thermal Overload Protection (49)**

Protects motor windings from the overheating.

**Trip Classes:**

- Class 10 → Normal load
- Class 20 → Heavy starting
- Class 30 → Very heavy starting

**4). Under Voltage Protection (27)**

Trips the motor during the low voltage conditions.

**Typical Setting:** 80% to 85% of rated voltage with a **2–10 second delay**.

**5). Over Voltage Protection (59)**

Protects insulation from excessive voltage stress.

**Typical Setting:** 110% of rated voltage.

**6). Negative Sequence / Single Phasing Protection (46)**

Protects against phase imbalance & single phasing.

**Typical Setting:** 10% to 15% negative sequence current.

 ![1](https://cdn.forumautomation.com/original/2X/b/bfa1cad8d7b1258dbc067beda64c4132ee8349a0.jpeg)

**7). Locked Rotor Protection (48)**

Protects the motor when the rotor gets jammed.

**Typical Setting:** 2.5 to 3 × FLC with **2–5 second delay**.

**8). Differential Protection (87M)**

Used in large HT motors to detect internal winding faults using **[CT comparison](https://forumelectrical.com/exploring-the-function-of-current-transformers-in-electrical-systems/)**.

Before configuring HT motor relays collect:

- Motor Rated Power
- Rated Voltage
- Full Load Current (FLC)
- Starting Current
- Starting Time
- CT Ratio
- Motor Duty Type

**Typical HT Motor Relay Settings**

| **Protection** | **Typical Setting** |
| --- | --- |
| Over Current | 120% FLC |
| Earth Fault | 10% FLC |
| Under Voltage | 80% Rated Voltage |
| Over Voltage | 110% Rated Voltage |
| Locked Rotor | 250% FLC |
| Thermal Overload | Class 10 / 20 |

**Precautions**

- Always verify motor nameplate data.
- Follow OEM and plant standards.
- Coordinate settings with upstream protection.
- Test relay operation before commissioning.

**Applications**

HT motor relay protection is widely used in:

- Thermal Power Plants,
- Cement Plants,
- Steel Plants,
- Water Pump Houses and
- Mining Industries.

**Proper HT motor relay settings help prevent failures, improve reliability and extend motor service life in industrial applications.**