Review the secondary circuit behind the class label
An accuracy class is useful only with its stated application and conditions. Identify whether each CT core or VT winding supplies revenue metering, indication, power measurement or protection. Do not collect all functions under one generic instrument transformer description.
The applicable requirements for inductive CTs and VTs are addressed separately in IEC 61869-2 and IEC 61869-3. These scopes support a separate equipment schedule; citing them does not verify the accuracy of a complete project measurement chain. CT scope and VT scope.
Build a connected-burden schedule
List the input burden of every connected instrument or relay from its actual data sheet. Include the relevant secondary wiring and connection path in the designer's calculation. Record units and the conditions under which the device data apply.
| Circuit element | Information needed | Review decision |
|---|---|---|
| CT core or VT winding | Ratio, intended function and declared class conditions | Correct allocation to the required duty |
| Connected instruments | Actual input burden and quantity | Total device contribution |
| Secondary wiring | Length, conductor details and connection arrangement | Wiring contribution and voltage-drop implications |
| Test and terminal devices | Approved arrangement and relevant characteristics | Complete circuit rather than device-only assessment |
| Future additions | Planned instruments or remote measurement | Whether the original class assessment remains valid |
Have the responsible designer compare the actual circuit with the instrument transformer's applicable class and burden range. Neither an arbitrary large VA rating nor a very small connected burden is a universal accuracy solution.
Keep CT and VT checks distinct
A CT circuit and a VT circuit operate differently, so the same worksheet arithmetic should not be applied blindly to both. Ask for separate calculations and an explanation of which wiring losses or drops are included. The review should identify the burden at the relevant equipment terminals and any accuracy effect along the measurement path.
Protection duty also involves characteristics beyond a metering accuracy label. Request the information required by the protection designer for the selected relay and fault conditions. A precise meter reading under normal load is not proof of adequate protective performance during a fault.
Example: the meter moves to another building
Imagine a metering arrangement approved with the meter near the switchgear. A later drawing relocates it to a distant control room while keeping the original CT and VT schedules. The instruments have not changed, but the secondary wiring has.
The buyer should request revised burden and measurement-path checks before accepting the relocation. Alternatives may include a revised arrangement or another approved means of transferring measured data. Those choices require engineering and utility review where applicable; they should not be settled by adding terminals during installation.
Freeze the allocation with the drawings
The final schedule should identify each core or winding, the connected devices, wiring references and accepted accuracy requirement. Specify applicable test documentation and maintain the allocation through later modifications.
For a medium-voltage switchgear enquiry, send this schedule, instrument data and secondary layout through the project contact page. CT and VT selection within the assembly requires confirmation. This guide does not establish standalone instrument-transformer supply or universal compatibility with a listed panel.