# Difference between WTI and OTI in a Transformer

**URL:** <https://forumautomation.com/t/difference-between-wti-and-oti-in-a-transformer/14184>\
**Category:** Electrical\
**Tags:** transformer\
**Created:** [June 8, 2026, 7:46am UTC](https://forumautomation.com/t/difference-between-wti-and-oti-in-a-transformer/14184 "2026-06-08T07:46:09Z")\
**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:** [June 8, 2026, 7:46am UTC](https://forumautomation.com/t/difference-between-wti-and-oti-in-a-transformer/14184/1 "2026-06-08T07:46:09Z")

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## Difference between WTI and OTI in a Transformer

| **Parameter** | **WTI (Winding Temperature Indicator)** | **OTI (Oil Temperature Indicator)** |
| --- | --- | --- |
| **Full Form** | Winding Temperature Indicator | Oil Temperature Indicator |
| **Measures** | **[Transformer](https://forumelectrical.com/ht-transformer-dg-works-boq-with-checklist/)** winding temperature | Top oil temperature |
| **Sensing Method** | Combination of oil temperature sensing and CT-based heating simulation | Direct temperature sensing of transformer oil |
| **Location** | Mounted on transformer tank, connected with a heater element | Mounted on transformer tank with temperature bulb in oil pocket |
| **Purpose** | Monitors winding hot-spot temperature | Monitors top oil temperature |
| **Accuracy** | Represents actual **[winding](https://forumelectrical.com/tertiary-winding-in-transformers/)** temperature more closely | Represents oil temperature only |
| **Protection Function** | Provides alarm, cooling fan control, and trip signals based on winding temperature | Provides alarm, cooling fan control, and trip signals based on oil temperature |
| **Temperature Rise** | Usually higher than OTI due to load current heating | Lower than winding temperature |
| **Current Dependency** | Depends on transformer load current through CT | Independent of load current |
| **Importance** | Prevents winding **[insulation](https://forumelectrical.com/insulation-resistance-test-in-preventive-maintenance/)** damage | Prevents oil overheating |

## How WTI Works?

- Measures oil temperature.
- Uses a Current Transformer (CT) connected to the transformer load current.
- A heater coil inside the WTI simulates additional winding heat.
- Displays the estimated hot-spot winding temperature.

 ![1](https://cdn.forumautomation.com/original/2X/7/79c0b5b97c73a2f97acf676651979ddcd044c4d0.jpeg)

## How OTI Works?

- Uses a temperature bulb placed in an oil pocket.
- Directly measures the top oil temperature.
- No CT or heater arrangement is required.

## Typical Alarm and Trip Settings

| **Function** | **OTI** | **WTI** |
| --- | --- | --- |
| Alarm | 85°C – 90°C | 95°C – 105°C |
| Fan Start | 70°C – 80°C | 80°C – 90°C |
| Trip | 95°C – 110°C | 110°C – 120°C |

## Points to Remember

Actual settings vary according to transformer manufacturer and utility standards.

OTI monitors transformer oil temperature whereas WTI monitors the winding hot-spot temperature.

WTI is generally more critical because winding insulation life is directly affected by excessive winding temperature.