Current Transformer Accuracy
Current Transformers (CT's) are used to transform the current to a level that is manageable so that it can be accurately measured. Depending on the application a certain level of accuracy is required. IEC 60044-1 (Instrument Transformers – Part 1: Current Transformers) defines the accuracy classes of the CT's.
CT Errors
The accuracy class of a CT is broadly presented as the percentage of error in the measurement. The standard accuracy classes are 0.1, 0.2, 0.5, 1, 3 and 5 for metering CT's. The are two factors that contribute to the accuracy class, the first is the accuracy of the measurement and the second is the displacement that the CT has on the phase.
Accuracy
The accuracy class translates to the maximum possible errors in the CT output. If a class 3 CT is being used on an 80 Amp load with a 100/5 CT, then the reading is guaranteed to be within 3%, i.e. between 77 and 83 Amps.
Phase Displacement
The CT can cause the measurement current to lead or lag the actual current. This can have an affect on the accuracy of the Power Factor, Real Power and Energy in the electricity meter readings. Typically, a lag will cause slightly higher than actual power factors and a lead will cause lower than expected PF.
Tables
Below are three tables that show the accuracy of a CT. The S Class CT's maintain their classification of accuracy down to 20% of the maximum load. Measuring CT's maintain their class of accuracy at the maximum load and the accuracy reduces marginally down to 10% of the maximum load. Moderate Accuracy CTs maintain their class of accuracy between 120% and 50% of the maximum load.
S Class CTs – High Accuracy
± Percentage current (ratio) error at percentage of rated current | Phase displacement (minutes*) at % of rated current | |||||||||
Class | 1 | 5 | 20 | 100 | 120 | 1 | 5 | 20 | 100 | 120 |
0.2 S | ± 0.75 | ± 0.35 | ± 0.2 | ± 0.2 | ± 0.2 | 30 | 15 | 10 | 10 | 10 |
0.5 S | ± 1.5 | ± 0.75 | ± 0.5 | ± 0.5 | ± 0.5 | 90 | 45 | 30 | 30 | 30 |
Measuring CTs – Good Accuracy
± Percentage current (ratio) error at percentage of rated current | Phase displacement (minutes*) at % of rated current | |||||||
Class | 10 | 20 | 100 | 120 | 5 | 20 | 100 | 120 |
0.1 | ± 0.4 | ± 0.2 | ± 0.1 | ± 0.1 | 15 | 8 | 5 | 5 |
0.2 | ± 0.75 | ± 0.35 | ± 0.2 | ± 0.2 | 30 | 15 | 10 | 10 |
0.5 | ± 1.5 | ± 0.75 | ± 0.5 | ± 0.5 | 90 | 45 | 30 | 30 |
1 | ± 3 | ± 1.5 | ± 1.5 | ± 1.0 | 180 | 90 | 60 | 60 |
Indication CTs – Moderate Accuracy
± Percentage current (ratio) error at percentage of rated current | ||
Class | 50 | 120 |
3 | ± 3 | ± 3 |
5 | ± 5 | ± 5 |
You can view our full selection of CT's by clicking on the link below. The product filters on the page allow you to choose the accuracy level required, along with other requirements such as ratio, CT type and aperture size.