Under-Voltage (UV) Coils in Generator Circuit Breakers: Purpose and Common Failures

What is a UV Coil? An Under-Voltage (UV) coil is an essential safety component used in circuit breakers, particularly in generator and power distribution systems. Its primary purpose is to automatically trip (open) the circuit breaker when voltage drops below a certain threshold, protecting the system from operating under unsafe or unstable conditions.

Purpose of a UV Coil

1. Automatic Disconnection at Low Voltage. If the system voltage drops significantly (typically below 70–80% of nominal), the UV coil de-energizes, which causes the breaker to trip. This prevents motors and sensitive equipment from operating with insufficient voltage, avoiding damage.
2. Fail-Safe Mechanism. UV coils are often used as a fail-safe, ensuring that a breaker cannot remain closed (or be closed) unless there is adequate control voltage present — typically coming from the generator itself.
3. Interlocking and Synchronization Systems. In marine and industrial generator systems, UV coils are used in automatic transfer switches (ATS) and synchronization schemes to ensure proper breaker operation based on the system’s logic.

Why Do UV Coils Commonly Fail?

Despite being simple in construction, UV coils often fail prematurely due to a few recurring issues:

1. Continuous Energization

UV coils are usually designed for intermittent duty—energized briefly during breaker closure. If the coil remains powered continuously (common in incorrect installations), it overheats and eventually burns out.

2. Voltage Instability or Overvoltage

Fluctuating control voltage, especially transients, surges, or harmonics, can exceed the coil's rating and damage its internal winding insulation.

3. High Ambient Temperatures and Vibration

In marine or industrial environments, vibration, heat, and humidity accelerate insulation degradation, especially if the coil is not vibration-resistant or lacks sufficient cooling.

4. Improper Installation

Connecting a UV coil directly to a constant power source without going through appropriate control logic can lead to premature failure. It should only be energized during the breaker closing sequence.

5. Use of Non-Genuine or Incompatible Parts

Replacing a failed UV coil with an aftermarket or incorrect voltage-rated coil increases the risk of repeated failures.

UV coils

How to Avoid UV Coil Failures

• Use UV coils only as specified by the breaker manufacturer.
• Ensure correct wiring: the coil should not be continuously energized.
• Monitor and stabilize control voltage quality.
• Inspect coils regularly for signs of overheating or damage.
• Avoid operating in high-temperature or high-vibration environments without proper protection or ventilation.

UV coils play a crucial role in protecting generator systems and ensuring safe electrical operation. Their failures are almost always preventable with correct design, installation, and periodic inspection. Understanding how they work—and how they fail—helps engineers and electricians keep power systems running reliably and safely.