Many people familiar with HV protection schematics and wiring will be aware of the different wire numbering on CTs but may not know why it is used or what guidelines are followed for labelling. They are often simply copied from sheet to sheet without proper checks or understanding.

There is in fact a clear numbering and prefix guide that has been included into AS 2067(2016) in Informative Appendix E. This was not in the previous 2008 and many people are simply not aware of the guidelines to follow.

Below is an excerpt of the reference table used, which covers many of the commonly used prefixes and wire numbers of HV protection circuits. Busbar protection circuits and current transformer for instruments are often overlooked and end up with prefixes that differ from the guidelines.

Another commonly overlooked part of the wiring guideline is E2.2 section C which refers to secondary windings in the same panel. Often these secondary circuits are named with a different prefix. For example, if the secondary side of a transformer CTs are wired back to the same panel for differential protection, the secondary side CTs should all have an additional 500 added to the wire numbers.

Phase wiring would be A10 / A30 / A50 / A70 on the primary set of CTs and A510 / A530 / A550 / A570 on the secondary set.

Below is an citation of the applicable standard, with relation to bringing both CTs back into the same panel. Hopefully after reading this guide you’ll now know what numbers to use or where to look in the standards to follow best practice with any future schematics. If in doubt have a look at AS2067, but make sure it’s the 2016 Edition!

AS2067 Appendix E 2.2 Section c) - Where a number of similar leads from separate units are taken to a common panel, e.g. bus-zone protection, summation metering, suffixes A, B, C, etc. should be used to distinguish them. Where two associated equipments are mounted on one panel, e.g. generator and unit transformer, HV and LV sides of one transformer, all leads of the subsidiary or lower-voltage equipment of the two should be distinguished by adding 500 to the numbers of wires in the common panel and associated interconnecting cores only.

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