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How to Test Current Transformers and Sub Meter Error

How to Test Current Transformers and Sub Meter Error

1) Testing Procedure

Testing on both occasions, including checking the stock before, and the new incoming stock was tested simply by connecting SCT-F24-100-5A CT to SDM120CT meter with a 50 Amp AC(rms) load with two turns through the CT. We tested both 50% and 100% full scale measurements as that is where we can see the accuracy and linearity is ok.

The load used was a Variac controlled purely resistive 50 Amp Load bank.

In order to verify the current reading I used a new NC14-200E1 AC/DC current Clamp from our stock.

2 Possible causes for the errors you are seeing

I mention both possibilities as you are drawing comparisons to utility grade meters

2a) Accuracy or CT and Meter

The accuracy for SDM120CT

Current/Voltage = 0.5%

Energy (kw or kwh) = 1%

The accuracy of SCT-F24-100-5A

Current/Voltage = 1%

Energy (kw or kwh) = Not applicable

If we account for the current measurement error then we would expect worse case error of 2%  -> can be in positive or negative direction meaning +/-2%

-However for CT Connected Submeters in general , the measurement range accuracy is specified for 10-120% of the input current range. Below 10% accuracy is not guaranteed. In your case 4 Amp is 4% of Ipn, so the accuracy could be higher than +/-2%. see below excerpt from the user manual. You can see the input range specified as 0.25-5(6) amp.  (10%-120%)


- Utility meters generally have internal CTs which are more accurate down low and I believe the NMI requirements are for accuracy 5-120% Current input range from memory. The higher error down low is party to do with the noise from CT at low currents. Direct connected meters can eliminate most of this noise through pairing of the CT and noise reduction/filter techniques of the known CT

2b) General error on Ipn

-> This is the most likely cause for higher error down low.

-The specified error is based on Ipn (the nominal CT/Measurement Current) which is 100 Amp in this case. -+/-1.5% error on 4 Amp would be 1.5 Amp error (remember it is based on Ipn).

Therefore an error of 37.5% for 4 Amp input  could be expected.

General Error on kW/kwh

-The specified error is based on P(nom) (the nominal power Measurement Current) which is 23kW  in this case. -+/-2% error on 23W would be 460Watt error (remember error is based on P(nom)).

Therefore an error of 37.5% for 4 Amp input  could be expected.

For further clarification feel free to contact the Fastron Team

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