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Home / Technical Articles / Testing and commissioning of MV/HV switchgear (wiring, final inspection and checks)

Testing and commissioning precautions

This article continues the series of articles dedicated to the erection, testing and commissioning of MV/HV switchgear by describing the most important precautions and recommendations in various procedures and steps. Starting from the wiring of low voltage command and signal cables, filling CBs with SF6 gas, special attention is given to testing and commissioning checks (visual, mechanical, electrical, operational and insulation resistance).

Guide to testing and commissioning of MV/HV switchgears (Wiring, final inspection and checks)
Guide to testing and commissioning of MV/HV switchgears (Wiring, final inspection and checks) - photo credit: NSS Ltd.

If you didn’t already, I highly recommended to read first: Guide to erection and commissioning of MV/HV switchgear – Inspection, installation and assembly (click here). Actions and procedures explained there start from scratch in explaining the testing and commissioning.

Table of Contents:

  1. Small Wiring And Connection Of LV Cables
  2. Filling With Insulation Medium
  3. Earthing of Switchgear
  4. Final Inspection
    1. Final Inspection of Indoor Switchgear
    2. Common for Both Indoor and Outdoor Switchgear
  5. Testing and Commissioning
    1. Visual Inspection Check
    2. Mechanical Check
    3. Electrical Check
    4. Operational Check
    5. Insulation Resistance Check
    6. High Voltage Check
  6. Final Commissioning / Load Testing
  7. Dos & Don’ts

1. Small Wiring And Connection Of LV Cables

Small wirings are auxiliary circuits such as tripping, closing, indicating, metering and protection CT/VT circuits. The CT circuit should be made to form a dosed circuit as dangerous voltages may develop under open circuit conditions and may even puncture the CT insulation.

It must be ensured that the ratings of CTs with meters, relays, etc. are matching. The auxiliary wiring should be neatly supported, and loose wires should be avoided as they may interfere with other devices. Refer to the schematic and wiring diagram and connect the required cables of auxiliary supply controls, metering and protection. Always use proper glands and lugs.

It is necessary to complete the small wiring connections of the panels to the adjacent cubicle and to connect external multi-core cables to the terminal blocks.

The most common issues I experienced in wiring of a medium voltage switchgear
The most common issues I experienced in wiring of a medium voltage switchgear (photo credit: ABB)

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2. Filling With Insulation Medium

Depending upon the design of the switchgear, oil or SF6 gas is to be filled as an insulating medium. After drying out of the switchgear, proper oil level or proper pressure of SF6 gas needs to be maintained as per the manufacturer’s recommendations. In many designs, poles are filled with nitrogen gas above atmospheric pressure (1.5/2 times of the atmospheric pressure) to avoid breathing of moist air.

During the process of ensuring that the joints are oil-tight for the oil circuit breakers, the joint faces, gaskets and the operator’s hands should be dean before and during the assembly. Jointing material should be applied as directed and the bolts tightened up evenly not more than half-a-turn at a time and finally pulled up as tight as possible.

Similarly for air blast switchgear or switchgear having pneumatically operated mechanism, due care and attention should be given to joints in piping with air at pressure. Tests should be conducted to check for air leakage after installation in order to ensure correctness of such joints.

A thin soap solution can be used for checking air leakage.

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3. Earthing of Switchgear

Earthing points provided in switchgear are to be connected with the station earth bar/earth grid. Measurements of earth impedance are made to ensure that no dangerous voltages can arise due to fault currents flowing in the earth conductors.

All equipment should be solidly and effectively earthed to the earth connection of the switchboards.

11kV connectors terminating EPR cable into medium voltage switchgear
11kV connectors terminating EPR cable into medium voltage switchgear (photo credit: Thorne & Derrick)

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4. Final Inspection

After the switchgear erection has been completed, a final inspection is made covering various aspects of both indoor and outdoor switchgear. The details of this inspection are discussed below.

4.1 Final Inspection of Indoor Switchgear

The following steps should be taken to inspect the indoor switchgear:

  • Check all chambers for complete cleanliness and the absence of foreign material, including tools used in the erection operation.
  • Check the tightness of hardware.
  • Check that labels are fitted and visible, wherever required.
  • Check that all lubricating points are lubricated properly.
  • Check that specified important settings are locked and intact.
  • Make a final check for the continuity of the earthing.
  • Check the operation of the breaker manually, covering the following:
  • Rack-in and rack-out operation, if provided;
  • Shutter operations;
  • Mechanical interlock operations and satisfactory operation of the interlock schemes;
  • Manual charging of the dosing mechanism;
  • Travel of the moving contact;
  • Proper contact wipe in isolating and fixed contact; and
  • Healthiness of the fuse and links.
Racking-in and racking-out operation of MV circuit breaker
Checking the mechanism, racking-in and racking-out operation of MV circuit breaker

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4.2 Common for Both Indoor and Outdoor Switchgear

The slow closing and slow opening operations should be checked as per directions given in Box 1 and Box 2 respectively.

Box 1 – Slow Closing Operation

Table 1 – System conditions at the beginning:

Circuit breaker/Interrupter:Open
Closing spring:Discharged
Motor supply:Disconnected
Tripping spring:Discharged

Open the cover of the operating mechanism and place the slow closing handle between the mechanism shaft and pin extension. Follow the instruction of manufacturer for slow closing after the mechanism is latched in closed position. The mechanical position indicator will show CLOSE.

Box 2 – Slow Opening Operation

Table 1 – System conditions at the beginning:

Circuit breaker/Interrupter:Closed
Closing spring:Discharged
Motor supply:Disconnected
Tripping spring and Contact Pressure springCompressed

For manual charging of the dosing spring, insert manual charging handle at the location provided inside the mechanism.

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5. Testing and Commissioning

Commissioning tests are conducted at site after installation of the equipment in order to ensure that:

  1. The equipment will perform its duties in service;
  2. Inter-connection with other apparatus is correct; and
  3. The test data are provided as records for future maintenance and service work.

The basic commissioning checks which are conducted at site are discussed below.

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5.1 Visual Inspection Check

The following steps should be taken for visual inspection:

  • Check that the serial number of the base frame and mechanism housing are the same.
  • Check that the nuts of foundation bolts are fully tightened and that the spring washers are used.
  • Check the tightness of the entire hardware of poles, structure, mechanism, etc.
  • Check that porcelains, etc. are free from any damage.
  • Check the levelling of the base frame and poles.
  • Check that all settings marked with a red paint line are undisturbed and properly locked with nuts and spring washer.
  • Check that all pins are locked with circlips.
  • Ensure that the gaskets for doors and covers are in good condition and pasted in proper places.
  • Check that cabling and earthing connections are properly made.
  • Ensure that porcelains are dean and the mechanism parts duly lubricated.
  • Check that the breaker does not operate manually or electrically if the key of the mechanical interlock (if any) is removed.
Visual inspection check of a medium voltage switchgear
Visual inspection check of a medium voltage switchgear (photo credit: Colombara Romeo)

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5.2 Mechanical Check

The following steps should be taken as part of the mechanical check:

  • Inspect physical and mechanical condition.
  • Inspect anchorage, alignment, grounding, and required area clearances.
  • Verify the unit is clean and all shipping bracing, loose parts, and documentation shipped inside cubicles have been removed.
  • Verify that fuse and circuit breaker sizes and types correspond to drawings and coordination study as well as to the circuit breaker’s address for microprocessor-communication packages.
  • Verify that current and voltage transformer ratios correspond to drawings.
  • Verify that wiring connections are tight and that wiring is secure to prevent damage during routine operation of moving parts.
  • Inspect bolted electrical connections for high resistance using one or more of the following methods:
    • Use of a low-resistance ohmmeter.
    • Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer’s published data.
  • Perform thermographic survey.
  • Verify operation and sequencing of interlocking systems.
  • Verify appropriate lubrication on moving current-carrying parts and on moving and sliding surfaces.
  • Inspect insulators for evidence of physical damage or contaminated surfaces.
  • Verify correct barrier and shutter installation and operation.
  • Exercise all active components.
  • Inspect mechanical indicating devices for correct operation.
  • Verify that filters are in place and vents are clear.
  • Perform visual and mechanical inspection of instrument transformers.
  • Inspect for physical damage, cracked insulation, broken leads, tightness of connections, defective wiring, and overall general condition.
  • Verify that circuit breaker ratings match drawings.
  • Verify correct functioning of drawout disconnecting contacts, grounding contacts, and interlocks.

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5.3 Electrical Check

The following steps should be taken as part of the electrical check:

  • Check the wiring with the relevant scheme and wiring diagrams. Check the insulation resistance with a 500 V megger.
  • Check that the control, indication and auxiliary leads are connected to the respective terminal blocks.
  • Ensure the availability of AC and DC supplies.
  • Charge the dosing spring by motor. The charging time should not exceed 15 seconds.
    If the tinning is more than 15 seconds, check the voltage. If voltage is OK, then check the linkage and gear movement for free movement. At the end of the spring charging operation, the motor supply is automatically cut off by the limit switch.
  • Electrically close/open the circuit breaker through push buttons/control switch from the local panel.
  • Check the contact resistance between the upper and lower terminal pads with breaker/ interrupter closed under spring force. The value should be less than the manufacturer’s recommendation.
  • Check the insulation resistance across breaker open terminals and between lower terminal to earth with 2.5/5 kV megger. The value should not be less than 10,000 mega-ohms or as recommended by the manufacturer.
  • Check the opening and closing of breaker/interrupter with remote control.

Learn how to interpret interlocking schemes between MV cubicles (single line and wiring diagrams)

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5.4 Operational Check

The mechanism should be operated slowly to check that the contacts are in correct alignment for checking simultaneity. Manufacturers usually explain the slow closing and slow opening operation in the instruction manual, which should be followed. All switchgear are supposed to operate at the minimum and maximum specified voltage levels to ensure correct tripping and latching at these values.

Similarly for the pneumatic mechanism, the maximum and minimum pressure should be used, and the breaker should operate satisfactorily under any of these conditions.

Closing and tripping of the circuit breaker by local control and remote control should be checked including tripping by protective relays.

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5.5 Insulation Resistance Check

The following insulation tests should be conducted:

(a) Insulation of main circuit:

The insulation of the main circuit is associated with the main connection. The insulation resistance to earth of each phase and between phases should be measured. The measured insulation resistance of the switchgear should be more than 1,000 mega-ohm for rated voltage above 1,000 V and up to and including 33 kV and 10,000 mega-ohms for rated voltages above 33 kV.

For any reading that shows considerably lower values than these, investigation should be carried out to trace the source of the low readings. The drying out method of improving insulation resistance may be followed.

Irrespective of whichever method is employed for drying out, the general principle is to apply heat continuously for a considerable time so as to drive out any moisture, which may have become entrapped.

Primary Injection Testing Of Protection System For Wiring Errors Between VTs / CTs and Relays
Primary Injection Testing Of Protection System For Wiring Errors Between VTs / CTs and Relays (photo credit: Varitel Proyectos e Ingeniería SAS)

(b) Insulation of the auxiliary and control circuits:

Insulation resistance to earth of all secondary wiring circuits should be tested using a 500 V megger. Readings obtained for a single circuit or combinations of circuits should not be less than 1 mega-ohm.

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5.6 High Voltage Check

The following high voltage tests should be conducted:

The application of high voltage to switchgear is not universal but may be conducted. It is preferable that such tests be carried out with AC supply. Voltage should be applied in accordance with the values specified in the relevant standards for a particular system voltage. The frames and secondary windings of all current transformers and voltage transformers included in the test should be earthed.

These tests should be carried out on each phase in tandem with the remaining phases earthed and all units racked in service position and the breaker dosed.

The insulation resistance test should be carried out before as well as after the high voltage test to ensure that there has been no reduction in value. If the second reading is lower, each separate component should be tested to find out where the lower figure has occurred.

The following precautions should be observed when conducting the high voltage test:

  • During the test, one terminal of the testing transformer is connected to earth and to the frame of the circuit breaker.
  • The test should be commenced at a voltage of more than one-third the test voltage.
  • The voltage should be increased from its initial value as rapidly as is consistent with its value being indicated by the measuring instrument.
    The full test voltage should then be maintained for the appropriate duration as the case may be and then reduced to its initial value and switched off.
  • While conducting the high voltage test, the manufacturer’s instructions should be followed. The HV test on auxiliary and control circuit should be carried out by looping together all the wires and applying 2000 V AC for one minute. All current transformer and voltage transformer earth connections should be removed for this test, and should be replaced immediately thereafter.

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6. Final Commissioning / Load Testing

Once all the checks on the equipment have been carried out, the operational tests are found to be satisfactory, the voltage test passed, and all the functional tests on the relays, etc. completed as per the specifications, standards and schemes, the equipment can be energized. When the circuits are on load, all the instruments and indicators can be checked for correct readings.

If the test blocks are available or the relays have test plug connections, it may be worthwhile to carry out a final check to see that the CT polarity is correct and that the instrumentation is giving correct reading. A typical pre-commissioning checklist is shown below for an outdoor circuit breaker.

Carry out a final check to see that the CT polarity is correct
Carry out a final check to see that the CT polarity is correct

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7. Dos & Don’ts

Dos and Don’ts for Site Work:

  • Do Plan Civil, Storage, Erection, Testing, Commissioning Activities
  • Do Not Do Unplanned Site Work ❗
  • Do Train Site Personnel
  • Do not Assign Work to Untrained Personnel ❗
  • Do Prepare Field Quality Documents
  • Do not Neglect Documentation and Records ❗
  • Do Take Safety Precautions
  • Do not Neglect Safety ❗
  • Do Receive and Check Packages on Arrival and Take to Foundation If Ready
  • Do not Neglect Inspection for Transit Damage ❗
  • Do Inspect and Store if Site is Not Ready
  • Do not Keep Packages and Breakers in Field ❗
  • Do Erect the Structure Vertically
  • Do not Erect Structure in Inclined Position ❗
  • Do Place Breaker Assembly on Structure
  • Do not Interchange Breaker and Structure ❗
  • Do Assemble the Operating Rod by Using Slow Closing Handle
  • Do not Disturb any Setting ❗
  • Do Check Mechanical Slow Open-Close Operations
  • Do not Operate Fast Open-Close to Begin with ❗
  • Do Carry out all Pre-commissioning Tests
  • Do not Energize Till Completion of all Tests ❗
  • Do Complete Commissioning Tests Before Energizing
  • Do not Forget Safety and Work Permits ❗

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  • Switchgear book by BHEL – Bharat Heavy Electricals Limited
  • Commissioning specifications for MV/LV electrical power equipment and systems – American National Standards Institute

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More Information

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.


  1. Technitab Solutions
    Nov 25, 2021

    It is very clear through this post that how testing and commissioning of the electric switchgear is important. You have made it more crystal clear that commissioning is the systematic process of verifying that a piece of equipment, system, or facility performs according to its design intent and the owner’s operational requirements. Increasing numbers of owners are recognizing the benefits of commissioning their electrical systems on-site.

  2. Mwenda
    Oct 25, 2021

    Interested to do a course

    Feb 08, 2021

    Great to know more about you

  4. Iraj Safianboldaji
    Nov 04, 2020

    Evry thing is god. Thanks

  5. Conrad
    Oct 29, 2020

    Where is your business premises

  6. Kyeyune vicent kisakye
    Oct 29, 2020

    Thank you so much for everything

  7. mohammed.allam
    Oct 28, 2020

    many thanks for these valuable information

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