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Home / Technical Articles / Eight important checks to do before powering up the dry-type transformer

8 Obligatory Checks

The transformer should be powered up only after checking the following items: name plate ratings, parallel operation, connections: cables, busbars, tap change connections, grounding connection, thermal protective device, impurities removal and insulation resistance test.

8 important checks to do before powering up the dry-type transformer
8 important checks to do before powering up the dry-type transformer (on photo: TRIHAL transformer by Schneider Electric)

star 1st Check (name plate ratings)

Make sure the name plate ratings are in accordance with the ratings foreseen for the place of installation.

Transformer nameplate with ratings and connections
Transformer nameplate with ratings and connections

star 2nd Check (parallel operation)

When transformers are to be operated in parallel, make sure they are connected with the right polarity. If there is only one transformer or more of them, but not operated in parallel, just skip to the next check.

Learn more about principles of transformers in parallel connection.

Parallel connection of transformers
Parallel connection of transformers

star 3rd Check (connections: cables, bus bars)

Make sure all connections to cables or bus bars are properly connected and well positioned.

Transformer low voltage connections to the busbars CANALIS
Transformer low voltage connections to the busbars CANALIS

star 4th Check (tap change connections)

Make sure that all connections at the tap change panel are firmly tightened and at the same position in the three phases.

Tap change connections
Tap change connections

star 5th Check (grounding connection)

Make sure the grounding mesh is correctly connected to the bolt provided for this purpose.

In addition, make sure the grounding mesh has been correctly executed at the right place foreseen in the project and shown in the drawing.

Earth connection MUST be checked
Earth connection MUST be checked

star 6th Check (thermal protective device)

In case of transformers fitted with a thermal protective device, check the connections of the circuit, making sure that the voltage is in accordance and that the alarm and shut-off contacts are connected to their corresponding loops.

Transformer thermal protection relay ZIEHL
Transformer thermal protection relay ZIEHL

star 7th Check (impurities removal)

Make sure there are no materials, equipment or any other impurities laid on the transformer, between the coils or obstructing the ventilation in the cooling channels.

star 8th Check (insulation resistance test)

It is always recommendable to check the insulation resistance by making measurements between the LV and HV windings and from the windings to the ground.

ONLY after these checks are made, the transformer can be connected to the system.

Voltage shall be applied while the transformer is set to no load, and such voltage shall be measured at the secondary winding to check for the corresponding output ratings. Operations under voltages other than the rated one can cause saturation significant loss increase, which could lead to over-heating and noise above the standard levels.

Warning! If the transformer is set to an improper tap, noises above the standard levels are to be expected.

The load should be applied progressively until the rated power .

  • Schneider Electric – Trihal Catalogue
  • Instruction Manual For Dry-type Transformers – WEG

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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. Sayed Osman
    Sep 27, 2019

    Kindly , I have a question about the difference between Cast resin and Cast impregnated dry type transformers .

  2. Manuel Bolotinha
    Mar 02, 2019

    Similar checks must also be done for oil type transformers.

  3. ashok
    Jul 17, 2016


  4. kEITH
    Aug 18, 2015

    Good Day.
    Need Help
    our 500kva 220V-440V step-up dry-type transformer is to supply a 145kW Emulsifier Machine.
    The problem is, when the machine starts, the voltage drops (220V drops to 210V ; 440V drops to 298V)
    Current ramps up to 2870Amps (primary side) and 1116Amps (secondary side – 440V).
    Due to this, the 1200Amps 220V MCCB trips off.
    (If we set the long-time trip setting of the breaker from 0.5 to 4 seconds (over exceeding the 3sec setting of Wye-Delta Starter) the breaker wont trip and the running current drops to normal FLA, as well as the voltage on each side)
    What seems to be the problem?Is the transformer undersized for the 145kW load considering the high starting current?
    Please share me some of your thoughts
    Thank you

  5. siti shahirah
    May 07, 2015


    I would like to ask one question about the bus bars of dry type transformer.
    I have found the difference of the size of bus bars on winding, palm and extension bus bars.
    Why is the size of bus bars on winding is difference with the palm and also with the extension bus bars? is it because the heat, losses or any other reasons?

    As for oil type transformers, the size of bus bars in the tank differ from outside the tank because of the density of oil (in the tank) and density of air (outside the tank).

    it would be much appreciated if you share the answer to me and to other viewers.

    Have a nice day!

  6. hamzath
    Jan 11, 2015

    Hi this is very important tool for A class contractors,

  7. Abhishek priyadarshi
    Oct 23, 2014

    Sir,Your all articles are very useful, interesting and informative.Can you post a article on Insulation Class?If you will post it,I feel very glad.
    Thank you.

  8. Pankaj Vitthal Chavan
    Jul 21, 2014

    Thanks gor this usefull information. Can you please post about transformer percentage impedance in simple words.

  9. MineroPequeno
    Mar 01, 2014

    I´d add as well to check the lenght of the bolts that connects the primary side. If they are too short, an arc can occur between the end of the bolt and the surface where the bolt should reach to. This arc produces burned área in the connection surface, that with the time results in high resistance that breaks the connection by heat increase.

  10. otto
    Jan 24, 2014

    Hi everyone
    can u please post something about insulation resistance..thanks

  11. Cherry Gupta
    Jan 24, 2014

    In sub zero temperature, further precaution in form of pre heating is required, as per the startup instructions provided by specific Transformer manufacturer. Most of the dry type transformers cannot start from the cold.

  12. Yan
    Dec 23, 2013

    Thank for this article, can you please post something about the maintenance works needed for dry-type transformers.

    Merry Christmas and a Happy New Year!


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