WB-CT309 v.2 Solid Current Transformer Strip

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This page describes the new device WB-CT309 v.2, for a description of the previous version see WB-CT309 v.1.

WB-CT309 v.2 transformer strip

Purpose

The strip is intended for use with WB-MAP meters as a replacement for split-core transformers. Solid-core transformers are often more convenient and more reliable when building a cabinet from scratch.

Differences from previous version

• the strip has a detachable screw terminal block;
• the board carries a bracket for DIN-rail mounting.

Specifications

Parameter	Value
Frequency	50/60 Hz
Mounting type	Solid, on the wire
Aperture diameter (max. cable diameter)	9.5 mm
Transformation ratio	1:2000
Phase angle, 1/1000 degree	120
Overall dimensions (L x W x H)	70 x 28 x 30 mm (without brackets)
Weight (with packaging)	75 g
Secondary winding open-circuit protection	No
Core material	Ferrite
Terminal blocks and wire cross-section
Recommended wire cross-section with a sleeve terminal	0.35 – 1 mm2 — single, 0.35 – 0.5 mm2 – double crimping
Length of a standard sleeve terminal	8 mm
Screw tightening torque	0.2 N∙m
For accuracy corresponding to class 0,5S
Rated primary current range	15 to 100 A
Rated primary current	15 A
Extended rated primary current at 25 °C	830 % (125 A)
Measured current	0.15 to 125 A

Datasheet for the transformers used: Файл:ZMCT123.pdf

The table lists characteristics that differ from those declared by the manufacturer. They were obtained by testing the transformers in the Wiren Board laboratory. To confirm them we regularly check devices from every batch.

Rated primary current (I_n) — the current in the primary circuit against which the accuracy class of the transformer is rated. The rated primary current defines the range of measured primary currents within which the measurement errors stay inside the limits permitted for the given accuracy class. For accuracy classes 0.2S and 0.5S that range is 0.01*I_n < I < 1.2*I_n, for accuracy classes 0.5 and 1 it is 0.05*I_n < I < 1.2*I_n.

Rated primary current range (I_{n_min}…I_{n_max}) — a characteristic that shows the measuring capability of the transformer more precisely. Laboratory testing showed that the transformers can measure primary current without losing accuracy over a wider range than the standards require for the accuracy classes. The rated primary current range is defined by the minimum (I_{n_min}) and maximum (I_{n_max}) rated primary current. In this case the range of measured primary currents for accuracy classes 0.2S and 0.5S is 0.01*I_{n_min} < I < 1.2*I_{n_max}, for accuracy classes 0.5 and 1 it is 0.05*I_{n_min} < I < 1.2*I_{n_max}.

Extended rated primary current — the maximum primary current beyond the rated range at which the measurement errors stay within the accuracy class. It is expressed as a percentage of the rated primary current (I_n). The extended rated primary current is in effect the highest primary current the transformer can measure with the accuracy of its class.

Measured current (I_min…I_max) — the range of primary currents the transformer measures with an error matching its accuracy class. The values of I_min and I_max are derived from the rated primary current range.

I_min

• for accuracy classes 0.2S and 0.5S is 1 % of the minimum rated primary current. I_min=0.01*I_{n_min};
• for accuracy classes 0.5 and 1 is 5 % of the minimum rated primary current. I_min=0.05*I_{n_min}.

I_max

• for accuracy classes 0.2S, 0.5S, 0.5 and 1 is 120 % of the maximum rated primary current. I_max=1.2*I_{n_max};
• if the transformer has an extended current range, I_max takes the value of the extended rated primary current.

Transformation ratio — the ratio of the rated primary current to the rated secondary current. The actual values of the transformation ratio and of the phase shift are individual to each transformer and are printed on its housing.

Below are the graphs of the WB-CT309 v.2 Current Transformer Strip current transformer errors against primary current, taken on a test bench. The measurements were made at a temperature of 25 °C.

• Current error and phase displacement of the ZMCT123 at 25 °C
• 

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°

Installation

Two strips mounted together

The pitch between transformers is 1 DIN module (17.5 mm). The strip is convenient to mount directly under the breakers. Two strips can also be butted together by flipping the right-hand one and connecting the wires in the other order (swap + and -, because the current direction is reversed) — this keeps the pitch between transformers. The strip can be mounted on a DIN rail or simply fixed to the cable with ties (slipped onto the cable itself to limit movement).

Operating without a connection to a WB-MAP meter is not recommended.

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.

Selecting a transformer by wire cross-section

The table below helps to select a current transformer by the cross-section and type of the cable. The table uses the following notation:

	can be used, optimal for a wire of this cross-section
	can be used, but not cost-effective
	cannot be used, does not fit the aperture diameter

Conductor cross-section, mm2	Cable type	Conductor diameter with insulation (max.), mm	Transformer models
			ZMCT102w	KCT-6	ZMCT205D	ZMCT134	WB-CT309 (ZMCT123)	ZEMCTK05-14	ZMDCT21	ZEMCTK04-13	CTSA024	CTSA035	ZEMCTK09-31G
0.5	ПВС	1.4
	ПуГВ	2.1
0.75	ПВС	1.58
	ПуГВ	2.4
1.0	ПВС	1.73
	ПуГВ	2.5
1.5	ВВГнг	2.3
	ПВС	2.08
	ПуГВ	3
2.5	ВВГнг	2.7
	ПВС	2.58
	ПуГВ	3.7
4	ВВГнг	3.4
	ПВС	3.26
	ПуГВ	4.2
6	ВВГнг	3.9
	ПВС	3.56
	ПуГВ	5.3
10	ВВГнг	4.7
	ПВС	4.75
	ПуГВ	6
16	ВВГнг	6
	ПуГВ	7.6
25	ВВГнг	7.7
	ПуГВ	9.6
35	ВВГнг	8.7
	ПуГВ	10.9
50	ВВГнг	10
	ПуГВ	12.6
70	ВВГнг	11.6
	ПуГВ	14.6
95	ВВГнг	13.6
120	ВВГнг	15.1

Known issues

No known issues.

Device revisions

The batch number can be found on a sticker on the side of the case or on the PCB.

Revision	Batch	Release date	Differences from previous revision
v2.4	v2.4A, v2.4B, v2.4C, v2.4C/1, v2.4D - ...	07.2025 - ...	First revision

Images and diagrams of the device

Overall dimensions

Corel Draw 2018 (font — Ubuntu): WB-Library.cdr.zip

Visio:

1. Wiren Board devices: WB-Visio-Lib.zip.
2. Panels, circuit breakers, contactors, and more by a third-party developer.

SVG: Файл:WB-CT-309 v.2.svg.rar