Accuracy of metering systems and how it is really calculated

Over the last 3 years I have seen a huge increase in The Measuring Instrument Directive (MID) being specified for a number of projects, even though in some cases, there is no requirement for the Electricity sub meters being used in billing applications. We welcome this attitude and understand the benefits of such solutions that are available In today’s market. There is a huge increase in system integrators and management software that take measurements from metering points. But how accurate is the data is is reading ?

The conception of having an accurate system that has been MID certified by an external notified body gives a level of comfort and reassurance. There are three classes with regards to the accuracy A,B and C. A= 2%, B= 1% and C= 0.5%, but where this gets lost in translation is when you have to factor in Current Transformers (CT) and how you have to calculate the accuracy based on the potential error of the CT, cable run and the Meter.

If the consumption meter is Class B (1%) and the CT only has a Low Burden (Available VA) it could only be Class 3 which is 3% error, this gives you an overall error potential of 4%.

If the meter is Class B (1%) and the CT is Class 1 (1%) that is an overall error potential of 2%.

We have developed a true Class 1 Panel Mount “ Plug and Play” solution by where our meters are Class C (0.5%) and the CTs are Class 0.5 (0.5%) giving an overall accuracy of 1%. We are also improving our DIN rail CT operated meters and will be all retested to class C to insure all our products and systems as a standard will conform to Class C. We will also give the correct advice with regards to cabling from the CT to meter.

The obvious benefits is the accuracy Class being a true Class 1% system, but also interestingly on the lower end of the scale when the load is as low as 5% of the primary if you have a Class 1 CT rather than Class 0.5 it is twice as inaccurate. This is based on British standards with 1/5A secondary’s , It is not the case for 0.333mV as there is no standard for this and due to phase shift on mV it is our opinion that on the lower end it will be less accurate.

We do not want to give the misconception. That you have purchased a Class B meter, therefore our system is Class 0.5%, we strive to educate the industry that are not aware of this, what accuracy you are actually giving your clients and what effects it will have at the lower end of the scale when the load is at a small percentage of its overall potential.

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