Modern current transformers (CT) produce a secondary output current that is in-phase with and proportional to the primary input current (ie, divided by the CT ratio) when supplying current to a circuit with burden that does not exceed the burden rating. In a normal meter, the burden (resistance) is very low, but long secondary leads or degraded wiring contact integrity can increase the installed resistance. Furthermore, as in any equipment exposed to the environment, CTs can internally degrade or even fail.
The PowerMaster offers three methods for testing the integrity of an installed CT:
1) Burden Testing – checks only the effect of additional burden (resistive load) on the CT secondary output current.
2) Ratio Testing – checks only the ratio and angle as measured from the actual CT primary and secondary currents.
3) Burden & Ratio Tesing – simultaneously checks the CT secondary current and ratio as additional burden is applied. If practical, Burden & Ratio Testing is recommended since it provides ratio data that can be directly compared to the nameplate value and the percentage ratio change plot gives unambiguous indications of CT burdened response.