Its name describes its function precisely: a switch that breaks "load" current — the normal current drawn by loads — nothing more. Do not expect it to interrupt fault current; that is not its job. Understanding exactly where this switch's limits lie is the difference between safe use and disaster.
What is an LBS?
A Load Break Switch (LBS) is a switch that disconnects loads drawing normal current, not large fault currents. It does not respond to faults such as short circuits or overloads; instead, it provides safe make/break operation under normal load at a given distribution point, providing electrical isolation along with simple arc quenching resulting from interrupting load current.
Construction and Operating Principle
- Insulators at both ends support the contacts and isolate them from the frame.
- A mechanical closing mechanism (usually spring-operated) ensures a contact speed independent of the operator's hand.
- Arc-quenching chambers for the arc resulting from interrupting normal current — designed for load current arcs, not fault current arcs.
- Operated and switched manually in most cases, for maintenance purposes and isolating loads for safe work on the circuit.
- Marked with its rated current — 630 A is a common rating in ring main units.
Where is it Used?
- Distribution networks: on poles or ground-mounted, to disconnect and reconnect loads easily during switching operations and maintenance.
- Ring main units: in incoming, outgoing, and transformer feeder cells — see RMU components.
- Factories and industrial systems at medium voltage, for controlling and isolating current paths.
So How is Downstream Equipment Protected?
Because the LBS cannot interrupt fault currents, it is paired with a protective device: a fuse in the transformer cell (the well-known LBS-Fuse combination) handles short circuits and overloads, while the switch handles operational disconnection — details in medium-voltage fuses and a full comparison in LBS vs Circuit Breaker.
Maintenance
Simple and easy: cleaning the insulators, cleaning and lubricating internal mechanical components, and removing dust and dirt — this simplicity is one reason for its widespread economic adoption.
Sample answer: It can safely make and break the normal load current under load, with simple arc quenching during disconnection, providing operational isolation for maintenance and switching. It cannot interrupt large fault currents such as short circuits — it is not a protective device, it does not trip automatically on a fault, and therefore it is paired with a fuse that provides protection (the LBS-Fuse combination in transformer cells) or installed downstream of a circuit breaker. Using it beyond its limits — i.e., expecting it to interrupt a short-circuit current — is a serious error.
Confusing it with a circuit breaker due to similar external appearance at times. The difference is fundamental: the LBS's arc-quenching chamber is designed for load current only, and exposing it to a short-circuit interruption means an arc explosion it cannot contain — know your equipment before switching.
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