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
Typical Settings
Alarm: 1–3%
Trip: 4–5%
Delay: 2–10 s