# Voltage Unbalance Protection (ANSI 60)

**URL:** <https://forumautomation.com/t/voltage-unbalance-protection-ansi-60/14529>\
**Category:** Electrical\
**Tags:** electrical\
**Created:** [October 21, 2026, 4:00pm UTC](https://forumautomation.com/t/voltage-unbalance-protection-ansi-60/14529 "2026-10-21T16:00:56Z")\
**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:** [October 21, 2026, 4:00pm UTC](https://forumautomation.com/t/voltage-unbalance-protection-ansi-60/14529/1 "2026-10-21T16:00:56Z")

</div>

## Voltage Unbalance Protection (ANSI 60)

**Purpose:** To detect blown PT fuses, unbalanced loads, and phase failures.

## Method 1: NEMA Formula

**% Unbalance = (Maximum deviation from Vavg / Vavg) × 100%**

## Example

Given:

Vab = 415 V

Vbc = 405 V

Vca = 390 V

**Average voltage:** Vavg = (415 + 405 + 390) / 3 = 403.3 V

Voltage deviations:

|415 − 403.3| = 11.7 V

|405 − 403.3| = 1.7 V

|390 − 403.3| = 13.3 V (maximum)

**% Unbalance = (13.3 / 403.3) × 100 ≈ 3.3%**

## Method 2: Symmetrical Components (Modern Relays)

**% Unbalance = (|V₂| / |V₁|) × 100%**

Where:

V₂ = Negative-sequence voltage

V₁ = Positive-sequence voltage

 ![1](https://cdn.forumautomation.com/original/2X/8/82695f2594c97ea4819c9cebe0f64f1524326d64.jpeg)

## Typical Settings

Alarm: 1–3%

Trip: 4–5%

Delay: 2–10 s