What is Load Break Switch (LBS)?

A load break switch is a disconnect switch that has been designed to provide making or breaking of specified currents.

This is accomplished by addition of equipment that increases the operating speed of the disconnect switch blade and the addition of some type of equipment to alter the arcing phenomena and allow the safe interruption of the arc resulting when switching load currents.

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Disconnect switches can be supplied with equipment to provide a limited load switching capability. Arcing horns, whips, and spring actuators are typical at lower voltages.

These switches are used to de-energize or energize a circuit that possesses some limited amount of magnetic or capacitive, such as transformer exciting current or line charging currents.

An air switch can be modified to include a series interrupter (typically vacuum os SF6) ) for higher voltage and current interrupting levels.

These interrupters increase the load break capability of the disconnect switch and can be applied for switching load or fault currents of the associated equipment.

A load break switch is a type of switching device used for voltages in the range of 12 to 36 kV and must have the following capabilities:

• Interruption of current equal to its continuous current rating at the system voltage and the power factor of the normal load
• Designed to possess enough insulation to isolate the circuit in closed position.
• Interrupting small capacitive and inductive currents which is essential for disconnecting the unloaded overhead lines, transformers, cables, etc.
• Carrying the maximum fault current for the duration required by the interrupting device to clear the fault.
• Making on the terminal fault at rated voltage.

The basic functional difference between Load break switch and circuit breaker is that the former cannot interrupt the short circuit currents. The following figure  shows the how the load break switches are employed in a substation.

Line diagram for Load break switch at substation

Load Break Switch

The AC high voltage load break switch is used in indoor or outdoor medium voltage systems at rated frequency 50/60 Hz. The load break switch is generally composed of disconnect blade, arc extinguishing chamber and operating mechanism. The arc extinguishing chamber will be made of insulating material with merits of high dielectric performance, and arc-endurances.

Generally, two types of load break switches (LBS) have been developed namely air-blast and SF6.In air-blast type, the interrupter heads same as those of used for isolation purpose in air-blast circuit breakers are employed for making and breaking currents. In SF6 type LBS, the gas serves as insulating and arc quenching medium.

The three-phase load break switch is installed on one sectional galvanized steel base, joint together with one drive axis to ensure closing and opening of three poles synchronously. The switch opens or closes under rated load current, without requiring any secondary protection device.

Load break switch switches the current by mechanically moving their contacts with an appropriate speed in order to make (close) or break (open) the current. It is exposed to mechanical, thermal and dielectric stresses during switching operation. Hence, for investigating and in detail exploring the interrupting capabilities of load break switch, they need to undergo various test duties according to IEC 62271-103. The essential parameters that are taken into consideration for analyzing the behavior of the load break switch during breaking tests are the current levels and the transient recovery voltage (TRV). 

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