Metering Current Transformer

Metering Current Transformers (CTs) are used with measuring instruments, such as ammeters, voltmeters, kilowatt-hour meters, etc to Isolate the instruments from the power circuits

Accuracy Class

As per IEC standard: It is specify as Class 5, 3, 1, 0.5 0.2 and class 0.1

Class 5 & 3 is suitable for use in moving magnet analog metering.
Class 1 to class 0.5 is suitable for use with solid state digital metering.
Class 0.2, 0.2S, 0.5S & 0.5 are for revenue purpose. As indicated in the graphs below, class 0.2S & 0.5S CT is linear from 20% to 120% load. With extended low load accuracy up to 1.0%.
* Accuracy Limits Range: IEC std. requires that for class 0.1 to 1.0 CT, the minimum burden of measurement shall be 25% of its rated burden, subject to 1.0VA min.
e.g. A 15VA Class 1.0 CT is supposed to maintain its class 1 accuracy from 15VA to 3.75VA.

The below graphs indicate the accuracy and phase displacement for metering CT from class 0.1 to class

 

The below graphs indicate the accuracy and phase displacement for metering CT from class 0.2S to class 0.5S

 

Instrument Security Factor

Instrument Security Factor (FS): Usually 5 or 10.

It is similar to ALF in the case of protection CT. It is burden dependent.
In the event of system fault currents flowing through the primary winding of a CT, the safety of the apparatus supplied by the transformer is greatest when the value of FS is small.
Example: 400/5A class 0.5 metering CT is to be installed, FS required is <5.
A digital power-factor meter with burden of 0.5VA and an analog meter of 1.25VA are to be connected in series, 2.5mm2 pilot wires of 2M length is required:
The total burden is: 0.5 +1.25+ 0.75 = 2.5VA
For a 5A Sec. CT. 3.75 – 7.5VA CT maybe selected for used.
** 15VA CT is not suitable, as the accuracy range is from 15 to 3.75VA. - Out of range.
**As the FS of a 15VA CT is around 5.0 at its rated burden. Its FS at 2.5VA may be greater than 10. - Less protection.
** Oversize metering CT using low grade material will usually have high FS value.

PVC Insulated Ring Metering Current Transformer

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Encapsulated Metering Transformer

Current Transformer Ordering

The CT ratio, rated burden, accuracy class and security factor are required. Custom specification can be manufactured upon request. Accuracy and other properties of CT is burden dependent. DO NOT OVER SIZE.
e.g. 400/5A, 7.5-1.5VA C0.5, FS5 (for ratio = 400/5A, 7.5 to 1.5VA, class 0.5, FS =<5)
other information: ID=?mm min, OD=?mm max, Ht=?mm if non-standard.

Factory Inspection Report

Inter Win will provide factory inspection for both metering and protection CT produced. This request must be made known when ordering. Fees may be required.

Installation Note

Low voltage current transformer can be mounted on: 1. Current bus-bar(s), 2. Solid cable or multiple cables. 3. Plates.
It can be mounted in any position, horizontal or vertical or tilted. It should be securely fasten with holders or mounting bolt and nuts.
* Current transformer must be connected to its load at all times. Unused CT secondary terminals must be shorted at all times. Open circuited CT secondary will overheat. It also induced dangerously high voltage which can be fatal to life and damage to equipment.

Transport & Handling

Precision CT accuracy can be damaged due to impact. Care should be taken against impact and drops. Current transformer is a non-repairable product. No spare parts will be provided.