WB-MAP: Accuracy classes and tolerances

Main parameters of the WB-MAP current inputs

Rated current for the WB-MAP measuring inputs Inom: 50 mA
Maximum current for the WB-MAP measuring inputs Imax: 1.25·Inom = 62.5 mA

When the meter works with an external current transformer, the primary current limit of the pair is set not only by the transformer but also by the meter input: it equals the Imax of the input multiplied by the transformation ratio of the transformer.

The actual ranges of primary current the transformers measure with their declared accuracy are given in the selection table below. It is the same table as in the «Selecting a transformer by current» section on each transformer's page; detailed specifications and error graphs are on the transformer pages.

Selecting a transformer by current

Transformer model	Measured current range by accuracy class, A				Aperture diameter (max. wire diameter), mm	Core material	Core type
	0.2S	0.5S	0.5	1
ZMCT102w	0.15…45	-	-	-	5	Ferrite	Solid
KCT-6	-	0.2…27	0.25…27	0.25…30	5.5	Ferrite	Split
ZMCT205D	-	0.06…15	-	-	6.5	Ferrite	Solid
ZMCT134	-	0.15…65	-	-	7.5	Ferrite	Solid
WB-CT309 (ZMCT123)	-	0.15…125	-	-	9.5	Ferrite	Solid
ZEMCTK05-14	-	0.15…90	0.25…90	-	10	Ferrite	Split
ZMDCT21	0.8…140	-	-	-	12	Ferrite	Solid
ZEMCTK04-13	-	0.25…152	-	-	16	Ferrite	Split
CTSA024	-	0.35…290	-	-	24	Ferrite	Split
CTSA035	-	5…755	-	-	35	Steel	Split
ZEMCTK09-31G	-	5…755	-	-	35	Steel	Split

The measured current range is the range of currents the transformer measures with an error no worse than the one its declared accuracy class allows. The range is derived from the rated primary current range given on each transformer's page: the lower bound is 1 % of the minimum rated primary current for classes 0.2S and 0.5S and 5 % for classes 0.5 and 1; the upper bound is the extended rated primary current.

• The permissible error is not uniform across the range — it grows towards the lower bound. The error limits are given on the WB-MAP meters: accuracy classes and tolerances page.
• The upper bounds are confirmed at a temperature of 25 °C. They drop as the transformer heats up — see the «Temperature degradation» section on the transformer's page.

Measurement errors and accuracy classes

Permissible error range for classes 0.5 and 0.5S

Two errors are rated for measuring current transformers: the current error and the phase displacement.

Current error, also called the ratio error, is the error that arises because the actual transformation ratio is not equal to the rated one. That is, because the transformer is not ideal, the measured current differs slightly from the current actually flowing in the conductor the transformer is installed on.

Phase displacement is the phase difference between the vectors of the primary and secondary current or voltage, with the vector directions chosen so that this angle is zero for an ideal transformer. Phase displacement does not affect current measurement. But even a small phase displacement introduces a noticeable error into energy and power measurement. That is why the accuracy class sets very strict requirements for it.

The error limits are set by ГОСТ Р МЭК 61869-2-2015 and ГОСТ IEC 60044-1-2013 and are rated at 4 or 5 points depending on the accuracy class.

Accuracy class — a characteristic of the transformer meaning that its current error and phase displacement stay inside the permissible error ranges set for that class under the specified operating conditions.

Accuracy classes rated at 5 measurement points are marked with the letter S. Such classes guarantee higher measurement accuracy at low loads.

The current error and the phase displacement must not exceed the values given in the table below.

Current error and phase displacement limits for measuring current transformers of accuracy classes 0.5S, 0.5 and 1

Accuracy class	Primary current, % of the rated value
	1	5	20	100	120
	Permissible error limits, ± current / phase
0.5S	1.5% / 1.5°	0.75% / 0.75°	0.5% / 0.5°	0.5% / 0.5°	0.5% / 0.5°
0.5	-	1.5% / 1.5°	0.75% / 0.75°	0.5% / 0.5°	0.5% / 0.5°
1	-	3% / 3°	1.5% / 1.5°	1% / 1°	1% / 1°