CP1553: Meters and Current Transformers minimum accuracy classes

The metering Codes of Practice (CoPs) detail the standards and minimum accuracy classes for certain Metering Equipment, i.e. for Meters, current transformers (CTs) and voltage transformers (VTs).

Currently, the minimum accuracy class required for a Meter in CoP5 ‘the metering of energy transfers with a Maximum Demand of up to (and including) 1MW for Settlement Purposes’ is class 2 (or class A), which means that the Meter must have an accuracy within ±2% over most of its current range. However, the overall accuracy limit for a CoP5 Metering System is ±1.5%. This makes it difficult for Meter Operator Agents (MOAs) to assure overall accuracy of a Metering System is met, if they do not, or cannot, provide Calibration Certificates, for certain items of Metering Equipment. Calibration Certificates detail the actual errors obtained through testing. This can result in the Technical Assurance Agent (TAA) assigning a category 2 non-compliance for overall accuracy not being maintained.

Additionally, the minimum accuracy class for a CT in CoPs 3 ‘the metering of circuits not exceeding 10 MVA for Settlement purposes’, 5 and 10 ‘the metering of energy via low voltage circuits for Settlements Purposes’ is class 0.5. This standard requires class 0.5 CTs to be tested to a minimum of 5% of rated measuring current. This means that when a CT measures current below 5% of rated measuring current, its errors, and therefore its contribution to the overall accuracy of the Metering System, will not be known. Knowing the CT errors at currents below 5% provides assurance that overall accuracy is more likely to be maintained when CTs operate at low such loads.