Learn how to draft the layout of an LV switchgear (sizing, designing equipment, installations)

LV switchgear layout: Design & Size

Low voltage (LV) switchgear is a broad terminology that includes a wide range of equipment like circuit breakers, switches, offload isolators, MCCBs, etc. within the 1 kV rating required to protect the LV system. The most common use of LV switchgear is in the LV distribution board which further comprises multiple segments like the main incomer, sub-incomers, feeders, etc. the size, configuration, and physical layout for switchgear vary as per specific requirements and standards.

In this article, we will discuss multiple factors to consider for sizing and designing the layout of LV switchgear for real-world application. The design engineer must go through various phases and iteration before finalizing the layout, which later is realized physically.

Original Equipment Manufacturers (OEMs) and independent manufactures produce thousands of LV switchgear assemblies each year, and properly designed layouts are mandatory to ensure that it meets the requirements.

Table of Contents:

1. Classification and critical features
2. LV switchgear introduction and functionality
   1. What differentiates switchgear from switchboards, MCCs, and PBDs?
3. Layout design and construction details
   1. Factors to consider while designing the switchgear layout
      1. Prevalent standards
      2. Component selection and clearance
      3. Temperature control and ingress protection
      4. Safety factors
   2. Power flow, interlink, and placement of components
      1. Placement of circuit breakers
      2. Provision for terminations, anchoring, and bends
   3. Housing, frameworks, and dimensions
   4. Single-line diagram to switchgear assembly, the final layout
      1. Taking a base of the single line diagram
      2. Determining the make and model of components
      3. Additional ancillary components
4. BONUS – AutoCAD drawings and pictures

1. Classification and critical features

The switchgear is of different type based on the voltage level at which it operates. HV switchgear operates at a voltage level of above 33 kV. MV switchgear operates at the voltage level of 33 kV and below. Likewise, LV switchgear operates at the voltage level of 1.1 kV maximum.

Irrespective of particular voltage levels, switchgear are known for their complete reliability, quick operation, and absolute isolation compared to other similar devices.

Figure 2 – MV switchgear cubicle (Left) and LV switchgear cubicle (right) inspection

Figure 1 displays the internals of cubicles of typical MV and LV switchgear. Physically, higher voltage creates extra complexity because of the higher risk of arc flashes and insulation failures. Apart from that, MV and LV switchgear shares a lot of similarities in terms of design and construction.

There has been substantial advancement in technology which started from the simplest form of switchgear, like a tumbler switch used widely for controlling small lights and household circuitry.

Switchgear protection plays a vital role in the modern power system- generation, transmission, and distribution. The modern advancement in technology has allowed multiple switchgear applications in the end-use level as well, which includes various industrial, commercial, and domestic purposes.

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2. LV switchgear introduction and functionality

LV switchgear is one of the most integral parts of low voltage power distribution and protection. They primarily comprise high-energy components capable of isolating or containing the downstream faults to secure the rest of the upstream network. Switchgear acts as a logical combination of electrical switching devices like circuit breakers, contactors, isolators, switches, etc working collectively to switch, control, measure, and protect the associated electrical circuits.

ACB is far more advanced than its counterparts and also equipped with additional features like Under Voltage Trip (UVT), automatic turn on with closing coils, etc.

Figure 2 – An example of low voltage switchgear layout

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2.1 What differentiates switchgear from switchboards, MCCs, and PBDs?

Switchgear, Switchboards, Motor Control Centers (MCCs), and Panelboard all are essential parts of LV power distribution, control, and protection. They primarily function to receive primary power and distribute it to downstream equipment as per requirements.

However, since we are focusing on the functionality of the latest form of LV switchgear in this article, it is important to point out what distinguishes it from the rest of its family members.

In most cases, incomers and main outgoing feeders in LV switchgear are equipped with ACBs while the downstream switchboards have MCCBs and MCBs for protection.

Figure 3 – LV distribution principle

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3. Layout design and construction details

Electrical switchgear often comes as an assembly or combination of multiple switching devices grouped in adjacent cubicles to facilitate power distribution and protection. Each component in the assembly has its function, starting from the enclosing body to the circuit breaker itself. Many additional components are added in between as per incoming and outgoing feeders in that case.

While doing so, the switchgear must meet the prevalent standards and must address many factors related to practical usages. First, let’s discuss those factors in a brief. Meanwhile, the figure below is the LV switchgear assembly installed in the power distribution room of an under-construction airport facility.

Figure 4 – LV Switchgear assembly for radial distribution in an under-construction distribution substation

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3.1 Factors to consider while designing the switchgear layout

3.1.1 Prevalent standards

While designing the layout of LV switchgear, the engineer must make sure they meet respective IEC standards. IEC 61439 applies to all the LV switchgear assemblies that do not exceed the voltage level of 1000V AC or 1500V DC. Apart from that, the electrical safety standards, standards for GI sheets of exoskeleton used in cubicles, standard cubicles size for easy ingress and egress, etc. are critical as well.

The latest revision and packaging of IEC 61439 include six different specific standards (IEC 61439-1 to IEC 61439-6). IEC 61439-1 is the most important in terms of LV switchgear, which lays the general rules for LV switchgear assemblies while other specific packages cover the rest of the portion.