A Load Break Switch (LBS) is a mechanical switching device capable of making, carrying, and breaking electrical currents under normal circuit conditions. Operating primarily in medium-voltage networks ($11\text{ kV}$ to $36\text{ kV}$), an LBS is designed to safely disconnect an active, energized electrical load without causing dangerous arcing or system disruption.

It occupies a critical middle ground in electrical engineering: it is more advanced than a standard disconnector/isolator (which can only be opened when there is zero current flowing), yet it is simpler and more cost-effective than a Circuit Breaker (which is designed to interrupt massive short-circuit fault currents).

1. How a Load Break Switch Works

When a mechanical switch opens while carrying an active load, the electrical current attempts to bridge the growing physical gap between the contacts. This ionizes the surrounding air, creating a highly destructive, high-temperature electrical arc.

An LBS handles this using a dual-contact system and an arc-quenching mechanism:

Main Contacts vs. Arcing Contacts

An LBS features two sets of contacts per phase:

1. Main Contacts: Made of high-conductivity silver-plated copper, these carry the continuous rated current when the switch is fully closed.

2. Arcing Contacts: Sacrificial contacts designed to open after and close before the main contacts. This ensures that any electrical arcing occurs strictly on these rugged, heat-resistant tips, protecting the main contacts from erosion.

Arc Quenching Mechanism

To extinguish the arc swiftly, an LBS typically utilizes one of two methods:

• Air Break with Arc Chutes (Interrupter Blades): As the switch opens, a spring-loaded auxiliary blade snaps open inside a narrow, slotted plastic or ceramic enclosure called an arc chute. The chute cools, stretches, and splits the arc into smaller segments until it is extinguished.

• SF6​ Gas Insulation: In modern, fully enclosed gas-insulated LBS units, the contacts open inside a sealed chamber filled with Sulfur Hexafluoride (SF6​) gas. Because SF6​ is highly electronegative, it absorbs free electrons rapidly, quenching the arc instantly.

2. LBS vs. Circuit Breaker (VCB): Key Differences

Understanding the distinction between an LBS and a Circuit Breaker is vital for proper power system design:

The LBS-Fuse Combination: Because a standard LBS cannot interrupt short-circuit currents, it is widely installed in tandem with high-rupturing capacity (HRC) or HT fuses. When a short-circuit occurs, the fuse blows within milliseconds to protect the network, while the LBS provides safe, visible manual isolation for maintenance.

3. Core Components of an LBS Panel

• Switch Frame & Insulators: A robust, epoxy-coated or galvanized steel structure supporting porcelain or polymeric insulators that isolate live parts from the grounded frame.

• Operating Handle / Mechanism: A spring-assisted mechanism operated manually via a mechanical lever or remotely via an electric motor. The spring ensures a “quick-make, quick-break” snap action, preventing the speed of the human operator from affecting arc clearing times.

• Earth Switch: A safety feature mechanically interlocked with the main switch. When the LBS is opened, the earth switch can be closed to safely dump any residual capacitive charge from downstream cables directly into the ground, ensuring a completely safe working environment for technicians.

• Fuse Tripping Mechanism: In LBS-Fuse combinations, a mechanical striker pin on the fuse interacts with the LBS trip mechanism. If any single-phase fuse blows, it mechanically trips the entire 3-phase LBS open, preventing the downstream system from running dangerously on only two phases (single-phasing).

4. Typical Applications

• Transformer Ring Main Units (RMU): LBS units are the backbone of RMUs in urban underground distribution grids, allowing operators to loop power through or isolate specific localized step-down transformers.

• Industrial Substation Incomers: Serving as the main manual isolating switchboard for factory substations before the power hits the main distribution boards.

• Sectionalizing Overhead Lines: Pole-mounted outdoor LBS units allow utility companies to isolate specific grid sectors for routine line maintenance without knocking out power to an entire town.

Our Professional LBS Services

We provide comprehensive engineering services for medium-voltage Load Break Switches to optimize your grid’s reliability, flexibility, and safety.

1. System Design & Custom Panel Fabrication

We build and supply tailored LBS panels (both air-insulated and compact SF6​ gas-insulated designs) compliant with IEC 62271-103 standards. We ensure precise interlocking logic between the main switch, fuses, and earth switches to mitigate operator errors.

2. Symmetrical Fault Sizing & Fuse Selection

Our power systems engineers analyze your network’s maximum prospective short-circuit levels to properly size the companion backup fuses, guaranteeing seamless discrimination between your local LBS and upstream substation circuit breakers.

3. Site Commissioning, Testing & Audits

Our field service engineers execute rigorous diagnostic routines, including:

• Contact Resistance Profiling: Verifying low-microhm continuity to prevent localized thermal damage under full factory load.

• Insulation Resistance (Megger) Testing: Confirming phase-to-phase and phase-to-earth integrity.

• Mechanical Interlock Verification: Ensuring the earth switch can never be closed while the main LBS contacts are active.

Optimize Your Network Infrastructure

A Load Break Switch is a highly efficient, space-saving, and cost-effective method to gain control over your power routing without the budget footprint of a full circuit breaker network. Contact our medium-voltage engineering team today to review your line diagrams and select the optimal switching architecture for your facility.