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Home / Technical Articles / Learn how to analyse and check factory wiring diagrams of a MV switchgear

Introduction to wiring diagrams

As already written earlier regarding the LV switchboard, wiring diagrams are used to show the control and signalization principle of operation inside the switchboard. Usually, they are given in form of electrical circuits between two lines which are representing control voltage potentials. Of course, various types of signals can be transferred outside the switchboard, for the purpose of measuring, monitoring, or control, i.e. to be used for SCADA applications.

Learn to analyse and check factory wiring diagrams of a MV switchgear
Learn to analyse and check factory wiring diagrams of a MV switchgear

Within this article, we will be talking about wiring diagrams inside medium voltage (MV) switchgear. Basically, the same rules and principles apply here as well. However, there are some differences related to the fact that in the case of MV applications, primary equipment operates at medium voltage level, while control and signalization circuits are always of low voltage type.

An example of wiring diagram related to MV equipment will be given and discussed later within the article.

Table of content:

  1. The basics of MV switchgear
  2. Example of MV switchgear wiring diagram

1. The basics of MV switchgear

When we say “medium voltage” or just “MV” switchgear, we refer to a switchgear with “primary” equipment which operates at medium voltage level, typically at or close to 6 kV, 10 kV, 20 kV, and 35 kV in Europe. These are phase to phase, or “line” values of rated operation voltage.

Such switchgear will also contain so-called “secondary” equipment, used for control, measuring, signalization, and auxiliary purposes. So basically, MV switchgear consists of primary and secondary equipment.

Primary equipment is designed and used for rated voltage and current values specified for switchgear. These are medium voltage circuit breakers, switch disconnectors, fuses, surge arresters, measuring transformers, etc.

Secondary equipment is used for control and supervision of primary equipment, usually at 24 VDC, 48 VDC, 110 VDC, 115 VAC, or 230 VAC voltage levels. Protection and signalization relays, miniature circuit breakers, selector switches, auxiliary relays, signal lights, etc. are the example of secondary equipment.

Primary equipment is bulky, heavy, and expensive. It is placed inside the MV compartment, which can be further divided into a busbar compartment, circuit breaker compartment, cable compartment, etc. depending on content and topology.

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Miodrag Kokotovic

Graduated from Faculty of Electrical Engineering, within University of Belgrade, in the field of electrical power systems. Expert in electrical part of tender preparation, design, procurement, construction and commissioning of treatment plants and pumping stations, electrical power quality and energy management.

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