From Google AI:
LFB3070R "series rated" current limiting fuse
The LFB3070R is a current limiter module designed to be used in a "series rated" combination with specific molded case circuit breakers. It is not a standalone fuse, but an accessory that increases the overall short-circuit interrupting rating (AIC) of the breaker it is installed with.
LFB3070R overview
Manufacturer: Westinghouse, which later became part of Eaton/Cutler-Hammer.
Function: It is a 15-70 Amp, 3-pole, 600 VAC current limiter for use with FB-series circuit breakers.
Mounting: The LFB3070R bolts onto the load side of a standard FB circuit breaker to increase its interrupting capacity.
Interrupting capacity: When used in a series-rated setup, this module can raise the combination's interrupting rating to 200,000 AIC (Amperes Interrupting Capacity).
How "series rated" works
A series-rated system involves two or more overcurrent protective devices (OCPDs) connected in series that have been tested and certified by a Nationally Recognized Testing Laboratory (NRTL) to work together safely.
Higher interrupting rating: The combination of devices can be applied where the available fault current is higher than the interrupting rating of the downstream OCPD alone. The upstream device, typically with a higher individual interrupting rating, assists in clearing the fault.
Upstream protection: In a series-rated system, the LFB3070R acts as the "upstream" protector to the standard FB circuit breaker. For high-level fault conditions, the LFB3070R's current-limiting fuses melt, effectively dissipating the energy and limiting the peak current that the downstream breaker must handle.
Cost efficiency: Series-rated systems are often used for cost savings because they allow for the use of less expensive, lower-interrupting-rated circuit breakers downstream.
Labeling and limitations: The panelboard or electrical enclosure must be clearly marked by the manufacturer to indicate the specific tested combination of devices. There are also limitations on using series ratings where motor loads are connected between the protecting and protected devices.
Note: For a critical system where high-level fault conditions could cause a main device to open and kill power to an entire panel, a fully-rated system is generally preferred over a series-rated one.
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