7070A-I AC Current Ratio Bridge

  • Calibration of Current Transformers
  • Calibration of Instrument Transformers with Different Rated Ratios Using One Single Reference Standard
  • Calibration of Current Transformers (CT) Up to 10000A When Used With MI 7020 and/or 7200 Series of Precision Current Transformers and/or Current Comparators
  • Short Measurement Times with High Accuracy and Low Inherent Burdens
  • Instrument Applications are Valid for Single Phase Measurements. Three Phase Transformers Measurements Performed by Testing Each Phase Separately
  • Test Frequency from 15 to 100 Hz
Features & Benefits

The accurate measurement of electric energy in high voltage, high current distribution systems relies on 3 key components, a watthour meter, a high voltage transformer and a high current transformer to step down the high voltage and high current to accurate low levels for input to the KWh meters or other electrical measuring devices. However, if the instrument transformers are not calibrated with similar accuracy, then the precise measurements made by the measuring devices will be with high uncertainty and misleading. For high voltages and currents, this inaccuracy can have a significant impact on the result. Therefore, calibration of the instrument transformers has become increasingly important.

Measurements International (MI) has many years of experience in high-voltage and high current engineering. Beginning with single instruments such as automated high voltage capacitance bridge, high voltage dividers and wattmeter to high complex systems like power calibration system, automated load loss measurement (ALMS) system and isolating current transformer (ICT) system. MI is also 17025 accredited for both in-house and onsite calibration of these measurements. This capability authenticates not only our suitability as a state-of-the-art instrument developer but also in calibration of instrument transformers.

The Model 7070A series of AC ratio bridges developed at MI are fully automated for fast and accurate measurements of instrument transformer errors. The 7070A is easily integrated in complex system to achieve calibration of voltage transformers up to 800kV and current transformers to 10,000 amps. The 7070A will set new standards of measurement in the calibration of instrument transformers.

The 7070A multi-range design allows to cover a wide range of measurements ratio with minimum number of reference standard when calibrating current transformer.

MI 7070A Models

Three different models are available that operate with MI or other supplier of standards related to the calibration of both voltage transformers and current transformer. These models consist of the following:

Model 7070A-I

The model 7070A-I is an AC Current Ratio Bridge and its main application is the calibration of current transformers using a reference CT, such as the MI 7020 series or other manufacturer reference CT. The 7070A-I has 10 input current ranges as follows: 5, 2, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.01 and 0.005A. Higher testing currents can be achieved using external precision CTs. The high number of input current ranges gives the flexibility of using one single ratio reference CT to test current transformers of different rated ratios. Figure 1 shows the setup diagram for testing current transformers.

Figure 1. Model 7070A-I Current Transformer Configuration

Measurement Systems

MI designs and supplies system solution for current and voltage transformer measurements customized for specific requirement. Some of the system components are listed below:
1. 7020 Series standard current transformers
2. 7200 series of current comparators
3. 2500 series of voltage dividers
4. Model CG series of high voltage capacitors
5. Electronic Burdens
6. Computer and printer for external control and data recovery.

Model No. 7070A-I
Input Channels 2
Current Measurements
Input Ranges per Channel (A) 0.005, 0.01 ,0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5
Current Accuracy (ppm of FS)
@ 50/60 Hz (23° C ± 5°C)
Ratio Accuracy (ppm)
(23°C ± 5°C)
For 0.5 ≤ Ix/Is ≤ 2.0
For 0.5 ≥ Ix/Is ≥ 2.0
Phase shift Accuracy (min)
(23°C ± 5°C)
For 0.5 ≤ Ix/Is ≤ 2.0
For 0.5 ≥ Ix/Is ≥ 2.0
Linearity (ppm) ≤20
Input Impedance (Ω) ≤3.5
Isolation (Vp-p) 600
Frequency Range (Hz) 15 to 100
Voltage Measurements
Input Ranges per Channel (V)  N/A
Voltage Accuracy (ppm of FS)
@ 50/60 Hz (23°C ± 5°C)
Ratio Accuracy (ppm)
(23°C ± 5°C)
Phase Shift Accuracy (min)
(23°C ± 5°C)
Linearity (ppm) N/A
Input Impedance (kΩ) N/A
Frequency Range (Hz) N/A
Power Measurements
Line to Neutral (ppm of FS)
@ 50 or 60 Hz (23°C ± 5°C)
Line to Line (ppm of FS)
@ 50 or 60 Hz (23°C ± 5°C)
Operating Environmental Conditions
Temperature (°C) 15 to 40
Relative Humidity 10 to 80
Non Condensing

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