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Home / Technical Articles / Major components you can spot while looking at opened LV and MV switchboards

MV/LV Switchboards in general

Depending on the size of the building or factory site and whether the supply is high voltage or low voltage, there may be requirements for both a main high voltage switchboard and one or more low voltage switchboards or just a single low voltage switchboard. The preferred name for the switchboard unit is a “Switchgear and Controlgear Assembly” (SCA).

Major components you can spot while looking at opened LV and MV switchboards
Major components you can spot while looking at opened LV and MV switchboards (on photo: Siemens gas insulated switchgear, 38KV, 1250 on left; LV switchboard and electrician performing testing on right)

The basic aim of the switchboard is to take the electrical power from the main supply source and then to feed or distribute power to the appropriate circuits within the building. The switchboard has to perform this function in such a way that there is proper control of power flow and proper electrical protection against the damaging effects of faults.

This protection is necessary to prevent personnel hazards and also equipment hazards and possible fires. It should be able to operate to isolate a faulty section in the minimum possible time consistent with the fault severity.

The switchboard should also be designed to present no danger of electric shock or injury to the operating personnel in the vicinity during normal or abnormal operation. Explosions in switchboards are a not infrequent occurrence which can cause significant injury to personnel.

In many cases, work is performed on the switchboard components while they are still live.


Components of MV and LV Switchboards

LV and MV are different despite they may look very similar. One thing they do the same is that they distribute electrical energy, but using different voltage levels. MV will always supply LV switchbord, never vice versa.

1. The major components of a MV switchboard:

    1. The incoming cables
    2. Outgoing circuit conductors
    3. Knife switch
    4. Load-break switch
    5. Earthing switch
    6. Circuit breaker
    7. Fuses
    8. Protection relays

2. The major components of a LV switchboard:

    1. Incoming and outgoing cables/busbars
    2. Isolate switch
    3. Internal busbars
    4. Load-break switch
    5. Circuit breaker
    6. Contactors
    7. Fuse switch
    8. Metering equipment
    9. Surge protection device

1. MV switchboard

1.1 The Incoming Cables

These may be either high voltage (HV) or medium or low voltage (MV or LV). For high voltage, they will normally be either impregnated paper insulation (unlikely these days), cross linked polyethylene (XLPE) or ethylene propylene rubber (EPR) insulated cable.

The last two types are the preferred types for new installations, with XLPE being the most common.

EPR cables are more flexible and are preferred for specialized applications such as trailing leads in mines. For low voltages the cables may be XLPE or elastomer (EPR) type cable.

In some cases busbars can be used instead of cables. This option is far more expensive than cables, but it also offers better reliability. If you ask me, I prefer busbar systems, where applicable of course.

33 KV incoming and outgoing cables
Figure 1 – 33 KV incoming and outgoing cables (photo credit: woottonandwootton.co.uk)

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1.2 Outgoing circuit conductors

These may be any of the following types:

  1. Insulated cables,
  2. Insulated busbars,
  3. Busbar trunking systems
  4. Mineral insulated metal-sheathed (MIMS) cables
  5. Fire-resistant cables
Switchgear layout. (a) Switchboard with one incoming feeder. (b) Switchboard with two incoming feeders.
Figure 2 – Switchgear layout. (a) Switchboard with one incoming feeder. (b) Switchboard with two incoming feeders.

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1.3 Knife Switch

Knife-switches are used in MV switchgear to isolate specific equipment or feeders for maintenance or other purposes such as earthing. They operate at no-load conditions by hand, or in remote-controlled installations, they are actuated by motor or compressed air.

Blades of knife-switch, mounted standing or suspended, must be prevented from moving spontaneously under their own weight. The physical size of this type of switches must be taken into consideration when deciding the dimensions of the switchgear.

Usually, the switchgear requires greater depth.

MV Knife Switch
Figure 3 – MV Knife Switch (photo credit: filnor.com)

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1.4 Load-break switch

Load-break switches are increasingly being used in MV distribution systems. For instance, ring main units use load-break switches in the two incoming feeders that connect the consumer’s substation to the network.

Load-break switches can be operated on load conditions. They have full making capacity and can handle all fault-free routine switching operations.

Two mechanisms can be used for load-break switch operation:

  1. Snap-Action Mechanism: A spring is tensioned that is released shortly before the switching angle is completed. Its force is used to move the contacts. The procedure is employed for both closing and opening.
  2. Stored-Energy Mechanism: It has one spring for closing and another for opening. During closing operation, the opening spring is tensioned and latched. The stored energy for opening operation is released by means of a magnetic trip or fuse.

Mainly two types of load-break switches are used: knife-contact type with/without fuses and slide-in type with/without fuses.

Load-break switch position in MV cubicle
Figure 4 – Load-break switch position in MV cubicle

Where:

  1. Switchgear: Switch-disconnector and earthing switch in an enclosure filled with SF6 and satisfying “sealed pressure system” requirements.
  2. Busbars: All in the same horizontal plane, thus enabling later switchboard extensions and connection to existing equipment.
  3. Connection: Accessible through front, connection to the lower switch-disconnector and earthing switch terminals or the lower fuse-holders. This compartment is also equipped with an earthing switch downstream from the MV fuses for the protection units.
  4. Operating mechanism: Contains the elements used to operate the switch-disconnector and earthing switch and actuate the corresponding indications (positive break).
  5. Low voltage: Installation of a terminal block (if motor option installed), LV fuses and compact relay devices. If more space is required, an additional enclosure may be added on top of the cubicle.

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1.5 Earthing Switch

Earthing switches are commonly used and installed in switchgear. When isolating any of the feeders (incoming or outgoing) for maintenance, the feeder must be earthed by closing the earthing switch to discharge any static charge carried by the feeder.

Earthing switches are mounted separately ahead of the switchgear, or in the base of load-break switch or just underneath the circuit breaker.

The switchgear manufacturer must mechanically interlock the earthing switch with the circuit breaker or load-break switch to avoid severe symmetrical short circuit if they are closed simultaneously.

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1.6 Circuit breaker

The basic function of a circuit breaker is to break/make the continuity of the circuit. It is the consideration of the effect on the circuit of doing this, which principally dictates the choice of breaker.

Therefore, circuit breakers are mechanical switching devices able to make, continuously carry and interrupt currents under normal circuit conditions and also within a limited time under abnormal conditions, such as short circuits.

Compromise is needed both on economic grounds, taking into account probability of certain conditions, and on technical grounds involving consideration of lower or higher speeds, of heavy or low current operation and many other opposing influences (e.g. maximum operating voltage and current at location, system frequency, duration of short circuit, switching cycle and climatic conditions).

The basic elements of circuit breakers are operating mechanism, insulators, interrupting chamber(s), capacitor and resistor.

The main types of circuit breakers include the following:

  • Bulk oil
  • Minimum oil
  • Air
  • Air blast
  • Sulfur hexafluoride (SF6)
  • Vacuum
  • Explosive

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1.7 MV Fuses

They protect apparatus and equipment against the thermal and dynamic effects of short-circuits. The outstanding features of MV fuse-links are:

  • High breaking capacity
  • High current limitation
  • Low switching voltage
  • Quick breaking
  • Non-ageing
It is important for MV fuse-links that they must be operated at the voltage for which they have been rated. Accordingly, the operating voltage corresponds to the maximum rated voltage of the fuse-link. Owing to the switching voltage occurring during arcing, the fuse-link cannot be used at lower voltages without limitation.

Medium voltage fuses generally fit into two categories: expulsion fuses and current limiting fuses. The definitions per ANSI C37.40 are explained in the following technical article:

Rating Definitions Applied to Medium Voltage Fuses

Medium voltage fuses
Figure 5 – Medium voltage fuses (photo credit: nepsi.com)

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1.8 Protection relay

Using simple words to describe: The ultimate goal of protective relay is to disconnect a faulty system element as quickly as possible. Sensitivity and selectivity are essential to assure that the proper circuit breakers will be tripped, but speed is the “pay-off.”

These are used for the higher voltages, together with their associated instrument transformers (current transformers (CTs) and voltage transformers (VTs)).

Overcurrent protection units are used to activate timing relays so as to provide proper fault protection operation.

There are many different relays used to protect various substation equipment, starting from transformers, incomers, feeders, capacitors, generators, motors, etc.

Micom P643 protection relay (photo credit: Abdelrahman Fayyad via Linkedin)
Figure 6 – Micom P643 protection relay (photo credit: Abdelrahman Fayyad via Linkedin)

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2. Low voltage switchboard

2.1 The Incoming and Outgoing Cables/Busbars

Same as at MV switchboard, incoming cables can be cross linked polyethylene (XLPE) or ethylene propylene rubber (EPR) insulated type.

Generally, busbars are more often used at low voltage level than at medium voltages.

The incoming and outgoing cables in LV switchboard
Figure 7 – The incoming and outgoing cables in LV switchboard

Why I Prefer Busbar Trunking Systems More Than Cables
Figure 8 – Busbar Trunking Systems and Cables

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2.2 Isolating switch (section switch or isolator)

This device allow segregation of the switchboard or its component parts to allow maintenance work. It is manually operated (some types are equipped with automatic close/open mechanism), and is a lockable, two-position (open/closed) device.

It provides safe isolation of a circuit when locked in the open position. It is not designed to make or to interrupt current, and no rated values for these functions are given in the standards.

The isolator must be capable of withstanding the flow of short-circuit currents for a limited time (short-time withstand capability), usually 1 second. For operational overcurrent, the time is longer.

Therefore, LV isolator is essentially a dead system switching device to be operated with no voltage on either side of it, particularly when closing. This is because of the possibility of an unsuspected short circuit on the downstream side. Interlocking with an upstream switch or circuit breaker is frequently used.

Isolating switch
Figure 9 – Isolating switch

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2.3 Internal busbars

These may be rigid copper (or aluminium) bars (insulated or uninsulated) in large switchboards or simply insulated single phase cables in small switchboards.

In large capacity switchboards each phase may have a number of conductor sections.

Bare LV busbars are very close together and are thus subject to high electrodynamic forces on short circuit and resonant force effects must be considered in determining supports.

The resonant frequency must be calculated to ensure it is not close to 100 Hz.

Internal busbar system inside LV switchboard
Figure 10 – Internal busbar system inside LV switchboard

Low voltage bare copper busbars
Figure 11 – Low voltage bare copper busbars

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2.4 Load break switch

Load break switch is a control switch, non-automatic, two-position (open/close), operated manually and sometimes provided with electrical tripping for operator convenience.

Load break switch is used to close and open loaded circuits under normal conditions. It does not provide any protection for the circuit it controls.

Its characteristics are determined by the frequency of switch operation (600 close/open cycles per hour maximum), mechanical and electrical endurance and current making and breaking capacity for normal and infrequent situations.

4 pole load break switch with visible breaking and a remote tripping function
Figure 12 – 4 pole load break switch with visible breaking and a remote tripping function

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2.5 Circuit breakers

The circuit breaker is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation – isolation, control and protection.

For low voltage (less than 1000 V) units, the circuit breakers are invariably of the air-break type using the “de-ion” principle, with isolated metal splitter grids. Modern switchboards have molded-case circuit breakers (MCCBs) for the higher current ratings (more than about 100 Amps) and miniature circuit breakers (MCBs) for the lower rating levels (less than 100 Amps).

Compact NSXm range of moulded-case circuit breakers
Figure 13 – Compact NSXm range of molded-case circuit breakers

MCBs would normally be used in the smaller sub-main and local switchboards in a building.

Low voltage circuit breaker consists of the following principal parts to carry out four essential functions:

  • Circuit-breaking components, comprising the fixed and moving contacts and the arc-dividing chamber.
  • Latching mechanism that becomes unlatched by the tripping device on detection of abnormal current conditions. This mechanism is also linked to the operation handle of the breaker.
  • Trip-mechanism actuating device (learn more here). It is either:
    • Thermal-magnetic device in which a thermal-operated bimetal strip detects overload conditions, while an electromagnetic striker pin operates at current levels reached in short circuit conditions, or:
    • Electronic relay operated from current transformers, one of which is installed on each phase.

Additional modules can be added to the circuit breaker to be adapted to provide further features such as sensitive detection (30 mA) of earth leakage current with CB tripping, remote control and indication (on-off fault) and heavy-duty industrial circuit breakers of large current ratings that have numerous built-in communication and electronic functions.

LV circuit breaker
Figure 14 – LV circuit breaker

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2.6 Contactors

The contactor is very simple device. It is actually a solenoid-operated switching device that is generally held closed by reduced current through the closing solenoid. Different mechanically latched types can be used for specific applications (e.g., motor starting, switching capacitors).

Contactors are designed to carry out numerous close/open cycles and commonly controlled by on/off pushbuttons. They may have auxiliary contacts, normally closed (N.C.) and normally open (N.O.) contacts, to be used for control functions.

The characteristics of contactors are specified by:

  1. The operating duration,
  2. The application in which to be used,
  3. The number of start/stop cycles per hour and
  4. Mechanical and electrical endurance.

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2.7 HRC fuses and fuse switch

These are also used in MV and LV switchboards for high level fault protection and, in many cases, there are combinations of HRC (high rupturing capacity) fuses and overload switches with limited interrupting capacity used (combined fuse-switch or CFS units) because of their economy.

It consists of three-switch blades, each constituting a double break per phase. These blades are not continuous throughout their length, but each has a gap in the center that is bridged by the fuse cartridge.

Fuse switch
Figure 15 – Fuse switch (photo credit: Edvard Csanyi)

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2.8 Metering equipment

The metering of a switchboards usually include: line and phase voltage, line current in each phase, total power, power factor metering.

The current is monitored by a current transformer (CT): in SWBs there may be two CTs, one for protection and one for metering.

Metering current transformers
Figure 16 – Metering current transformers (photo credit: marelexelectrical.com.au)

Auxiliary power supply metering
Figure 17 – Auxiliary power supply metering (photo credit: bpa.ru)

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2.9 Surge protection

Modern switchboards will also have some overvoltage surge protection designed into both the MV and LV sides to protect equipment against the effects of any over-voltage transients that may be generated within the system or conducted in from external sources.

Surge arrester type iPR 4P 10KA 3P-N by Schneider Electric
Figure 18 – Surge arrester type iPR 4P 10KA 3P-N by Schneider Electric

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Sources:

  1. UNSW Sydney (the University of New South Wales)
  2. Medium Voltage Technology Switchgear Application Guide by Siemens
  3. The art and science of protective relaying by C. Rusell Mason
  4. Electric distribution systems by Abdelhay A. Sallam and Om P. Malik

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

6 Comments


  1. Wasim Anwar
    Aug 06, 2019

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  3. vignesh
    Jun 13, 2019

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  5. zuproc66
    Jun 12, 2019

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  6. ADITYA SAHA
    Jun 10, 2019

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