Close and Latch Rating

The close and latch rating of a circuit breaker refers to the maximum fault current it can interrupt and still be able to close and latch into the connected equipment. This rating is important for ensuring the safety and reliability of the electrical system during fault conditions.

UL 1558 Specifications

UL 1558 is a safety standard for circuit breakers used in industrial control panels. It specifies that circuit breakers rated for 85kA interruptions must have a close and latch rating of at least 65kA. This means that the breaker can safely close and latch into a fault up to 65kA.

Significance

The close and latch rating is significant because it ensures that the circuit breaker will not be damaged or welded closed during a fault condition. This is important to prevent catastrophic consequences, such as equipment damage, fires, and even explosions.

Automatic Throwover Scenarios

In automatic throwover scenarios, where a breaker is programmed to automatically close into a fault to maintain power supply, the close and latch rating becomes crucial. If the fault current exceeds the close and latch rating, the breaker may fail to close or latch properly, resulting in a loss of power and potential system instability.

Available Fault Current over 65kAIC

If the available fault current in the electrical system exceeds 65kAIC, it is recommended to use circuit breakers with a higher close and latch rating. This will ensure that the breakers can reliably operate during fault conditions and maintain the integrity of the electrical system.

Recommendation

For a UL 1558 switchgear spec with an 85kAIC interruption rating, it is advisable to specify a close and latch rating of 85kAIC as well. This will provide the highest level of safety and reliability for the electrical system, regardless of the available fault current. Flag for review