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.

ACB Trouble. The generator does not connect to the main busbars. Troubleshooting

Greetings! In this article will discuss one of the problems with diesel generators, which is possible and rare, but happens on ships. In general, any problem with generators for an ETO is a sore and most discussed topic.

Also here we will briefly examine the conditions for synchronizing ship synchronous generators (alternators).