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Main distribution board (MDB)

MDB is a panel or enclosure that houses the fuses, circuit breakers and ground leakage protection units where the electrical energy, which is used to distribute electrical power to numerous individual circuits or consumer points, is taken in from the transformer or an upstream panel.

Medium voltage switchgear and low voltage distribution in a nutshell
Medium voltage switchgear and low voltage distribution in a nutshell

MDB typically has a single or multiple incoming power sources and includes main circuit breakers and residual current or earth leakage protection devices. A MDB is comprised of a free standing enclosure, a bus bar system, MCCB’s (molded case circuit breaker), metering and support equipment and required current transformers.

Panels are assembled in a systematic manner such as incomer section and outgoing section.

Main distribution boards are used to distribute and control the power supply in large buildings such as shopping malls, hospitals, universities, and hotels. The main distribution boards are generally installed after the main power source (eg. Transformers or generators) and the main idea of MDB is to divide the power supply into secondary outgoing feeders.

List of materials in Main Distribution Board

Moulded Case Circuit Breakers

The circuit breaker is a mechanical opening closing device, which is used for closing, breaking, separating circuit and carrying current under ordinary conditions and for automatically breaking the circuit under extraordinary conditions like short circuit and over


  • Over current releases (over current opening unit)
  • Under voltage releases (low voltage opening unit)
  • Shunt Trip releases (remote release unit)

Type of Circuit Breakers:

LV circuit breakers are manufactured in two different types depending on the release type. These are thermal-magnetic circuit and electronic circuit breakers.

Thermal – Magnetic Circuit Breakers

Thermal Protection

Bimetal, which provides thermal protection consists of combination of two metals with different extension coefficients under heat. When bimetal is heated, it bends towards the metal with less extension. In this way, a notch that assists opening of the breaker mechanism is released to disable the breaker.

Bending speed of bimetal is in direct proportion of the current passing through the breaker.

Thermal – Magnetic Circuit Breaker
Thermal – Magnetic Circuit Breaker
Magnetic Protection

Another function of the breaker is to protect the connected circuit against short circuits. Short circuit occurs as a result of contact of phase-ground. Breaker should perform instant opening to protect load it is connected to.

The part fulfilling this function is a mechanical opening mechanism that operates with magnetization caused by the magnetic area formed by the short circuit current.

Circuit Breakers with Electronic Over Current Release

The feature discriminating electronic circuit breakers from thermal-magnetic breakers is to control the overcurrent releases with electronic circuit. Electronic control is performed via microprocessor.

During design of the electronic circuit, worst possibilities to encounter in operation have been taken into consideration.Β In high circuit currents, direct opening has been ensured without operating electronic circuit. In this way, possibility of failure in the electronic circuit has been eliminated.

Current Transformers

Low voltage current transformers

Consists of three parts as primary winding, secondary winding and magnetic core which those windings are wound on. There is no primary winding in current transformers without busbar in primary. Instead, primary winding is formed by passing busbar or cable through toroidal core of the transformer.

LV current transformer
LV current transformer

Measurement current transformers

Measurement current transformers have been formed to feed measurement tolls, counters, relays and other devices operating with similar techniques. These are the transformers which insulate such devices from high voltage networks and which reduce currents out of limits of measurement devices to measurable values.

Air Type Circuit Breakers (ACB)

Air type circuit breakers are used for protection of generators with large powers, motor, capacitor groups and transformers, as well as general protection of factories, shopping malls, business centers.

Air circuit breaker
Air circuit breaker

Drawout Type Circuit Breaker

Circuit breakers are automatically turned on during pull and push of the drawer via lever. When drawer-type switcher fail, they can be quickly replaced with the spare one.

Power Logic

Power Logic (energy analyzer) is the name given to monitoring devices that provide continuous monitoring of electrical energy with current technologies, keeping measurement data continuously recorded and transmitting the records by communicating with remote devices and software when necessary.

While the importance of power quality and energy has been increasing day by day, it has become a priority to provide uninterrupted electricity to consumers with high quality and efficiency. The efficiency of electrical energy depends on the amount of visible energy that can be used as energy. The quality is ensured by eliminating the harmonic components it contains to a large extent.

Single phase and three phase voltage and current rms values, active and reactive (capacitive or inductive) power values, active and reactive energy values, power factor, network frequency, average and maximum powers, demand and harmonic distortions can be measured by energy analyzer.

The measured values can be stored in the memory of the energy analyzer, can be displayed on the screen, can be transferred and stored to computer environment by means of communication protocols or can be monitored remotely by means of communication protocols.

Title:Medium voltage switchgear and low voltage distribution in a nutshell – Umut ERSOY at HACETTEPE UNIVERSITY
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Medium voltage switchgear and low voltage distribution in a nutshell

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  1. Simson kanyemba
    Sep 02, 2020

    All good

  2. Ali
    Sep 01, 2020

    I intended any kind of electrical bussines in deelhi as i am electrical engineer more than 20 years experience .please help ang guide me

  3. Faiq Hussain
    Sep 01, 2020

    Dear we have a switch yard 220kv, 132kv in which different range of current transformer are installed, almost 35 years old, now in rainy season three no C.Ts has blast. CnDF test carried out of C.Ts which is below satisfactory.
    Kindly guide us about this situation plz.

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