NOTE 1 When determining the factor of construction, laboratories have to cope with a high measuring uncertainty due to the necessity of integrating the e.m.f. and due to nonlinear parameters at accuracy limiting
conditions. Furthermore, only few laboratories are in the position to provide the required duty cycles, and these with limited precision only. As a consequence, the results of direct and indirect tests usually do not match nicely,
and unreliable values may result. Therefore, little experience exists in this field.
NOTE 2 The expression “transformer of the same type” implies that the ampere turns are similar irrespective of ratio, and that the materials and the geometrical arrangements of the iron core and the secondary windings are
identical.
7.3.5.206 Test for turns ratio error for class PX and PXR protective current transformers
For class PX and class PXR, the turns ratio error shall be determined in accordance with Annex 2F.
The test may be substituted by performing the measurement of the ratio error with a burden connected, subject to an agreement between manufacturer and purchaser.
The turns ratio error shall not exceed the limits given in 5.6.202.4.
7.3.201 Determination of the secondary winding resistance
The secondary winding resistance shall be measured for current transformers of the following classes, to prove compliance with the appropriate clauses:
class PR: clauses 5.6.202.3.7 and 6.13.202.4 (if parameter specified)
class PX, PXR: clauses 5.6.202.4 and 6.13.202.5
class TPX, TPY, TPZ: clause 6.13.202.6
An appropriate correction shall be made to meet 75°C or other such temperature as may have been specified.
For classes PR, PX and PXR, the value obtained when corrected to 75 °C shall not exceed the specified upper limit (if any).
7.3.202 Determination of the secondary loop time constant
The secondary loop time constant shall be determined at current transformers with the following classes, to prove compliance with the appropriate clauses:
class PR: clause 5.6.202.3.6 (if parameter specified)
class TPY clause 5.6.202.5.3
The measured value shall not differ from any specified value by more than 30 %.
7.3.203 Test for rated knee point e.m.f. (Ek) and exciting current at Ek
The rated knee point e.m.f. shall be verified and the exciting current at rated knee point e.m.f. Ek shall be measured for current transformers with the following classes, to prove
compliance with the appropriate clause:
class PX, PXR: clause 5.6.202.4
A suitable sinusoidal exciting voltage with rated frequency shall be applied to the secondary terminals of the full winding of the transformer, all other terminals being open-circuited, and the exciting current shall be measured.
The exciting voltage shall be measured with an instrument which has a response proportional to the average of the rectified signal, but calibrated in r.m.s.. The exciting current shall be measured using an r.m.s measuring instrument having a minimum crest factor of 3.
The excitation characteristic shall be plotted at least up to a voltage equal to 1.1 x Ek.
At a voltage equal to Ek, the knee point condition according to 3.4.215 shall be fulfilled.
The exciting current at a voltage equal to Ek (or at any stated percentage), shall not exceed the specified limit.
NOTE 1 For selectable-ratio current transformers with tapped secondary windings, the excitation characteristic for other than the maximum ratio may be calculated.
