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Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

Low Voltage

After the system voltage is decreased to lower level (600V or less) which is more appropriate for use the power need to be distributed respectively to all distribution branch circuits. At the entrance every branch circuit is equipped with main branch overcurrent device. Those devices are positioned in low voltage service entrance units which can be divided on low voltage switchgear and low voltage switchboard units. lowvoltage1

These units are rated from 400 to 4000A or more at 600V or less. The difference between these two type low voltage service entrance units are the low voltage metal enclosed switchgear use exclusively power circuit breakers, both as main and branch overcurrent devices in contrast to low voltage switchboards which use molded case circuit breakers and fused switches for control of outgoing branch circuits and either molded case or power circuit breaker as main overcurrent device.


Both, low voltage switchgear and switchboards can be designed for either indoor or outdoor applications. They are most frequently designed for indoor use. Outdoor units usually consist of a standard indoor unit with a weatherproof enclosure assembled around it.

A regular maintenance for this type of equipment should includes cleaning, mechanical checks of all bus joints and cable connections for proper tightness, checking for broken or missing parts and insulation resistance testing. Because, dependable performance and long operating life of these units primarily depends on the performance of the circuit breakers used for the specific application a great care during maintenance program should given to the inspection of those apparatus.

An Air circuit breaker is usually used as main overcurrent device in those applications.
A regular maintenance includes:

  • frame cleaning and checking for physical damage
  • cleaning and inspecting of arc chutes,
  • cleaning, inspecting and adjusting of breaker contacts
  • inspecting, checking for mechanical condition and exercising all auxiliary devices and switches
  • breaker manual exercising
  • electrical trip operation testing
  • high potential testing
  • checking for settings and if necessary making calibration checking of all trip characteristics

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Molded case circuit breakers are designed to provide circuit protection (overload and short circuit) for low voltage distribution systems. This type of circuit breaker is assembled as an integral unit in a supporting and enclosing insulating housing. Different types and sizes of molded cases are assigned by symbols which indicate case characteristics. There is no uniformity in these identifications between manufacturers, so that should be verified during inspection.

Another division can be made by breaker trip element type which can be electro-mechanical or solid state. This trip element provide molded case circuit breaker with different types of tripping characteristics. Also, molded case circuit breaker can be equipped with ground fault protection for use as main or branch breakers.


Molded case circuit breaker are treat as very reliable and safe apparatus. The maintenance should include the next:

  • checking terminals for condition and tightness
  • visual inspection for apparent damage and signs of overheating
  • molded case dielectrically test
  • manually breaker operating and checking for action
  • trip operation test
  • verifying the fuse rating and continuity (for fused designs only)

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Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers