CPC 100 - Universal testing device for electrical diagnostics

CPC 100 - Universal testing device for electrical diagnostics on transformers, current transformers, voltage transformers, grounding systems, lines and cables, and circuit breakers (photo by www.omicron.at)

Polarity Detection

This is needed for identifying the primary and secondary phasor polarities. It is a must for poly phase connections. Both a.c. and d.c methods can be used for detecting the polarities of the induced emfs.

The dot method is used to indicate the polarities.

The transformer is connected to a low voltage a.c. source with the connections made as shown in the Figure 1 (a). A supply voltage Vs is applied to the primary and the readings of the voltmeters V1, V2 and V3 are noted. V1 : V2 gives the turns ratio.

If V3 reads V1−V2 then assumed dot locations are correct (for the connection shown).

Transformer polarity test scheme

Figure 1 - Transformer polarity test scheme

The beginning and end of the primary and secondary may then be marked by A1 − A2 and a1 − a2 respectively. If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 to a2 in the secondary.

If more secondary terminals are present due to taps taken from the windings they can be labeled as a3, a4, a5, a6. It is the voltage rising from smaller number towards larger ones in each winding. The same thing holds good if more secondaries are present.

Figure 1 (b) shows the d.c. method of testing the polarity. When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.

Reference: Electrical Machines I – Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao


Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.


  1. James
    Dec 18, 2013

    Umm.. if you apply a DC voltage to the primary it will put a charge on the secondary side.. this is basic electronics people…. just because it is constant 25kv doesnt mean it wont induce a voltage on the opposite side…. the difference here is that DC will use the Peak Voltage and A/C will use an RMS Value because with the cycles you loose that peak….

    Do you not remember putting 12vDC on a coil then letting your buddy hold the ends of the coil to get that inrush shock when you remove the current?!? So fun in school….

  2. Hamza Yunus
    Oct 28, 2013

    to author,
    i m a bit confused here, if one energizes the primary of a T/F with DC voltage, due to constant flux, emf should not b induced in the secondary. how come can we get deflection in mechanical voltmeter ?
    would you mind elaborating a bit, huh ?

  3. satyanarayana adabala
    Sep 02, 2013

    A Dc supply applying to primary of transformer the winding is damaged,how can apply the dc supply for polarity test

    • mhkawi
      Sep 03, 2013

      i think it is a pulse not continous dc supply

  4. mhkawi
    Aug 29, 2013

    i think it is better for knowing the polarity to use DC method but use ammeter not voltmeter
    , please i want more article to show how to design polarities of the CT

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