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Home / Technical Articles / The art of a low voltage switchgear design: The case study and practical experience

It’s not just about the sizing…

LV panels are metal-enclosed switchgear that provides a three-phase power distribution to supply electric power at voltages up to 1000 volts, current up to 10000 amps, and a frequency of 50HZ or 60HZ. LV panels are always connected at the power distribution transformer’s secondary (low voltage) side.

The art of a low voltage switchgear design: The case study and practical experience
The art of a low voltage switchgear design: The case study and practical experience

These panels are used widely in many projects to supply MCCs (Motor Control Centers), low voltage switchboards or sub-main distribution panels, and other branch circuits. They also provide isolation and protection from faults such as short circuits and overloads.

In order to select the suitable type of LV panel for any project, many considerations must be taken into account, like whether the panel is installed indoors or outdoors, bottom or top cable entry, form and type, etc., as will be explained in the following lines.

Also, this article will mention different forms of separation of a typical LV panel, as well as the internal and external layouts, LV panel power circuit diagram, and other major details.

Table of Contents:

  1. LV panel Compartments
    1. Circuit Breaker Compartment
    2. Busbar Compartment
    3. Cable Compartment
  2. LV panel Components
    1. Multifunction Meters (MFM)
    2. Ammeters and Voltmeters
    3. Restricted Earth Fault Relay (REF)
    4. Forms of Separation and Layouts
  3. LV Panel Design Analysis
    1. LV Panel Front, Bottom and Side View Analysis
    2. Power and Control Circuit Analysis
    3. Earthing of LV Panel
  4. BONUS! – Drawings in High Resolution (PDF)

1. LV Panel Components

1.1 Circuit Breaker Compartment

A breaker compartment in an LV panel is usually used to house Air Circuit Breakers (ACB) and Molded Case Circuit Breakers (MCCB) that can withstand higher ampere ratings rather than miniature (MCB) and earth leakage circuit breakers.

In each compartment, circuit breaker cells are used to house breakers. These compartments can normally hold up to four circuit breakers or more if arranged vertically, and each circuit breaker is individually compartmentalized from other breakers. The following picture shows an ACB cell part of the breaker compartment.

Withdrawable ACBs can easily be removed for maintenance purposes and fixed back again.

Figure 1 – Withdrawable air circuit breaker cell

Withdrawable air circuit breaker cell
Figure 1 – Withdrawable air circuit breaker cell

Go back to the Contents Table ↑

1.2 Busbar Compartment

It is usually located at the backside of the breaker compartment, which is also compartmentalized by solid barriers from the breaker compartment. It houses the main busbar system, which is connected to the fixed upper isolating contacts of the main switchgear apparatus through branch connections. The main busbars are made of high conductivity copper.

The busbar compartment of each panel is isolated from the busbar compartments of the neighbouring compartments. Depending on the current rating, it could be of a single or double busbar configuration.

Figure 2 – Busbar Compartment (Rear Side of an LV Panel)

Busbar Compartment (Rear Side of an LV Panel)
Figure 2 – Busbar Compartment (Rear Side of an LV Panel)

Go back to the Contents Table ↑

1.3 Cable Compartment

A typical cable compartment has hinged doors or removable cover that enable access to landing cable lugs for the aim of termination. A cable compartment arrangement may be of a rear or front access type depending on the type of switchgear panel installed or selected for the project.

In most cases, a front access panel is found where there is not much space in the substation room, but this is not the rule. In case of a lack of room, a panel is usually placed against a wall, similar to a switchboard. Some substations may have concrete trenches, which act as a pathway for cables to be laid on the floor inside the trench and to be terminated at the bottom of the panel.

In this case, a bottom entry cable LV panel is required. The following picture represents an LV panel installed on top of a trench. Cables are terminated at the terminal boxes through the bottom of the panel.

You will notice that the glands of the cables are fixed on the bottom plate of the LV panel.

Figure 3 – Termination of cables at the bottom entry of LV panel

Termination of cables at the bottom entry of LV panel
Figure 3 – Termination of cables at the bottom entry of LV panel

On the other hand, the top cable-entry type of LV panel gives us the advantage of dropping cables down from the panel’s top side.

In this type, cables are usually laid on cable trays and connected to the panel by fixing the cable glands at the top plate, as shown in the below figure.

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More Information

Mohammed Ayman

I earned my degree from Eastern Mediterranean University (Turkey, North Cyprus) in B.S Electrical & Electronic Engineering; shortly after, I began my career as an electrical site engineer in a mega-scale project in Qatar which allowed me to monitor and supervise electrical site installations. I indulged in the design field of the electrical low voltage distribution systems and have accomplished more than 10 projects with the compliance of the national codes & international standards.

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