Reference · English

Current transformer selection and accuracy

revision 106006

Selected guidance for transformers used with WB-MAP3E. The maximum input current of WB-MAP3E is 60 mA.

Choose by current range and conductor size

The complete measurement chain is limited by both the transformer and the meter input: maximum primary current cannot exceed the permitted secondary input current multiplied by the transformer ratio. Accuracy limits vary across the range and worsen near its lower end. Published upper limits are confirmed at 25 °C and decrease as the transformer heats up.

Transformer	Class	Measured primary current	Aperture	Core
KCT-6	0.5S	0.2–27 A	5.5 mm	Split ferrite
KCT-6	0.5	0.25–27 A	5.5 mm	Split ferrite
KCT-6	1	0.25–30 A	5.5 mm	Split ferrite
WB-CT309 (ZMCT123)	0.5S	0.15–125 A	9.5 mm	Solid ferrite

These are transformer laboratory ranges, not permission to exceed the meter input limit. KCT-6 and WB-CT309 v.2 nominal ratios are 1:2000; use the individually marked ratio and phase correction on each transformer. At nominal 2000:1, 60 mA corresponds to 120 A primary. Do not directly connect a conventional 5 A-secondary transformer to the meter.

Accuracy classes

Ratio error changes the measured current. Phase error is the angular difference between primary and secondary vectors, with polarity chosen so an ideal transformer has zero error. Phase error does not change current magnitude but can significantly affect power and energy. S classes specify an additional low-current test point.

Class	At 1% rated current	At 5%	At 20%	At 100%	At 120%
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°

Entries show current-ratio / phase error limits reproduced from the source. Store each transformer's parameters against its actual input; see configuration registers.