Supply system of Industrial plant

The transmission power lines will enter a distribution substation, where the voltage will be stepped down to distribution levels, where it will be distributed for use by industrial, commercial, and residential customers. At these load points (industrial, commercial, and residential customers), a transformer further reduces the voltage from 11 kV to 400 V to provide the last mile connection through 400 V feeders (also called low-tension feeders) to individual customers, either at 200 V A (as a single-phase supply) or at 400 V (as a three-phase supply).

Supply system of Industrial plant
Supply system of Industrial plant pic sources: Electrical Engineering Portal


11 kV Feeder Line to End User
Since transmission voltage levels cannot be used by consumers, it is required to step down the transmission voltage to more usable voltages. The transmission power lines will enter a distribution substation, where the voltage will be stepped down to distribution levels, where it will be distributed for use by industrial, commercial, and residential customers. Distribution networks are typically of two types: radial or Secondary selective (networked). A radial feeder leaves the station and passes through the service area with no normal connection to any other supply. This is typical of long rural lines with isolated load areas.

A secondary selective system, having multiple connections to other points of supply, is generally found in more urban areas. These points of connection are normally open but allow various configurations by the operating utility by closing and opening switches. Operation of these switches may be by remote control from a control center or by a lineman. The benefit of the networked model is that in the event of a fault or required maintenance, a small area of the network can be isolated and the remainder kept in supply. This system is usually used in Industries.

A lightning arrester (also called a lightning diverter) is a device used on electric systems to protect the insulation and conductors of the system from the damaging effects of lightning. A lightning arrester is placed where wires enter the transformer. A DO fuse (drop-out fuse) is a combination of a fuse and a switch used in primary overhead feeder lines and taps to protect distribution transformers from current surges and overloads.

An overcurrent caused by a fault in the transformer or customer circuit will cause the fuse to melt, disconnecting the transformer from the line. Time of Day metering (TOD), also known as Time of Usage (TOU) or Seasonal Time of Day (SToD), involves dividing the day, month, and year into tariff slots, with higher rates at peak load periods and low tariff rates at off-peak load periods. While this can be used to automatically control usage on the part of the customer (resulting in automatic load control), it is often simply the customer’s responsibility to control his own usage, or
Pay accordingly (voluntary load control). This also allows the utilities to plan their transmission infrastructure appropriately. It consists of a current transformer (CT) and a potential transformer (PT), as shown in figure (a).

A Stepdown The transformer steps down the input voltage, i.e., the secondary voltage is less than the primary voltage. A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Different types of CBs that are generally used are oil circuit breakers, air-blast circuit breakers,  vacuum circuit breakers, and SF6 circuit breakers. Since a transformer is the most important and expensive device, both the primary and secondary sides of the transformer are provided with circuit breakers. The power for loads is supplied through a transformer.

Figure (a): Single line diagram for supply system of industrial plant

Protection Devices

The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault while leaving as much of the network as possible in operation. Thus, protection schemes must be applied with a very pragmatic and pessimistic approach to clearing system faults. The devices that are used to protect the electrical systems from faults are called protection devices.


A fuse is a current-interrupting device that breaks or opens the circuit (in which it is inserted) by fusing the element when the current in the circuit exceeds a certain value. A fuse consists of a metal strip or wire fuse element, of a small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow.

The size and construction of the element are (empirically) determined so that the heat produced for a normal current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.

HRC fuse

HRC fuse or High Rupturing Capacity Fuse is the type of fuse, where the fuse wire or element can carry short circuit heavy current for a known time period. During this time if the fault is removed, then it does not blow off otherwise it blows off or melts. The enclosure of the HRC fuse is either glass or some other chemical compound. This enclosure is fully airtight to avoid the effect of the atmosphere on the fuse materials. The ceramic enclosure has a metal end cap at both heads, to which fusible silver wire is welded. The space within the enclosure, surrounding the fuse wire or fuse element is completely packed with a filling powder.

This type of fuse is reliable and has inverse time characteristics, which means if the fault current is high then the rupture time is less, and if the fault current is not so high then the rupture time is long. When the overrated current flows through the fuse element of a high rupturing capacity fuse the element is melted and vaporized. The filling powder is of such a quantity that the chemical reaction between the silver vapor and the filling powder forms a high electrical resistance substance which very much help in quenching the arc.

HRC Fuse(pic source:Electrical concept)

Miniature Circuit Breaker (MCB)

An MCB, or miniature circuit breaker, is an electromagnetic device that embodies a complete enclosure in a molded insulating material. The main function of an MCB is to switch the circuit, i.e., to open the circuit (which has been connected to it) automatically when the current passing through it exceeds the value for which it is set. It can be manually switched ON and off, similar to a normal switch, if necessary. MCBs are time-delay tripping devices, in which the magnitude of the overcurrent controls the operating time. This means these get operated whenever overload exists long enough to create a danger to the circuit being protected. Therefore, MCBs do not respond to transient loads such as switch surges and motor starting currents. Generally, these are designed to operate at less than 2.5 milliseconds during short circuit faults and 2 seconds to 2 minutes in case of overloads (depending on the level of current).

MCBs are used to perform many functions, such as local control switches, isolating switches against faults, and overload protection devices for installations or specific equipment or appliances. Under overload conditions, the current through the bimetal causes it to raise its temperature. The heat generated within the bimetal itself is enough to cause deflection due to the thermal expansion of metals. This deflection further releases the trip latch, and hence the contacts get separated. In some MCBs, the magnetic field generated by the coil causes a pull on the bimetal such that Its deflection activates the tripping mechanism.

Miniature Circuit Breaker(MCB) pic source: Finolex

Moulded Case Circuit Breaker (MCCB)

MCCBs are molded-case Circuit breakers with protection facilities for overcurrent and earth faults. It has a variable range of 50% to 100% operating current. They can be wired for remote as well as local operation both.They are manufactured for fault levels of 16 KA to 50 KA and an operating current range of 25 A to 630 A. They are used for applications with larger power flow requirements.

Moulded Case Circuit Breaker(MCCB) pic source:c&s Electric Blog
Moulded Case Circuit Breaker(MCCB) pic source:c&s Electric Blog

Switches/ Main Breaker

A major circuit fuse is also known as main breaker. Its function is to supply electricity to all circuits and outlet. Main switch is a master switch that shuts off all power in circuit.

Main Distribution Boards (MDB)

An 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. An 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, MCCBs, metering and support equipment, and the required current transformers. Panels are assembled in a systematic manner, such as in the incomer section and the outgoing section.

Sub-Main Distribution Boards (SDB)

The MDB then feeds SMDBs, which is installed generally at the point where a large distribution cable terminates and several smaller sub-circuits start.These are the switchboards that although similar construction, are larger than a final distribution board circuit. The boards are installed midway through the power distribution system, at the point in a large distribution cable ends, and several smaller starting sub-circuits.


A luminaire is defined as a complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps and ballast (where applicable), and to connect the lamps to the power supply. Luminaires can be of the traditional type, such as a recessed or surface- mounted incandescent, fluorescent or other electric-discharge luminaires. Luminaires can also be of the non-traditional type, such as fiber optics with the light source at one location and the fiber core or light pipe at another. In this case, the fiber cable is providing the light distribution for the lighting system. There are also a number of products coming into the market place that employ light-emitting diodes (LEDs) as the light source.

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