Voltage Imbalance: Ground Fault, Open Line, or Resonance?

Single-phase grounding, line break (open-phase), and resonance can all cause three-phase voltage unbalance. Correctly distinguishing among them is essential for rapid troubleshooting.

Single-Phase Grounding
Although single-phase grounding causes three-phase voltage unbalance, the line-to-line voltage magnitude remains unchanged. It can be classified into two types: metallic grounding and non-metallic grounding.

• In metallic grounding, the faulted phase voltage drops to zero, while the other two phase voltages increase by a factor of √3 (approximately 1.732).

• In non-metallic grounding, the faulted phase voltage does not drop to zero but decreases to a certain value, and the other two phase voltages rise—but by less than 1.732 times.

Line Break (Open-Phase)
A line break not only causes voltage unbalance but also alters the line-to-line voltage values.

• When a single-phase break occurs on the upstream (higher-voltage) line, the downstream (lower-voltage) system shows all three phase voltages reduced— one phase significantly lower, and the other two higher but close in magnitude.

• When the break occurs on the local (same-level) line, the broken-phase voltage drops to zero, while the voltages of the intact phases remain at normal phase voltage levels.

Resonance
Resonance can also lead to three-phase voltage unbalance, manifesting in two forms:

• Fundamental-frequency resonance: Its characteristics resemble those of single-phase grounding—one phase voltage decreases while the other two increase.

• Sub-harmonic or high-frequency resonance: All three phase voltages rise simultaneously.