How to Read CT/VT Nameplates: Key Fields and Typical Mistakes

2026-02-15 09:00:00

How to Read CT/VT Nameplates: Key Fields and Typical Mistakes

CT and VT nameplates contain the information you need to verify compatibility with your metering and protection system. Many commissioning issues happen because the installed CT/VT nameplate does not match assumptions in the design documents.

This guide highlights common nameplate fields and the typical mistakes to avoid.

CT Nameplate: Key Fields

Ratio: primary current / secondary current, including multi-ratio tap options if present
Secondary rating: 1A or 5A (or specified output type)
Accuracy class: metering class or protection class
Burden: rated VA for the secondary circuit
Terminals and polarity: P1/P2 and S1/S2 markings
Insulation level: withstand and impulse ratings where applicable

VT Nameplate: Key Fields

Primary voltage class: system rating and insulation level
Secondary voltage: required by meters/relays
Accuracy class: metering vs protection requirements
Burden: rated VA and the number of secondary windings if applicable
Terminal markings: for correct phasing and wiring

Common Mistakes

Wrong ratio installed: causes scaling errors in meters and relays.
Burden mismatch: wiring and device burden exceed rating, degrading accuracy.
Protection vs metering class confusion: wrong CT type used for relay scheme.
Polarity errors: cause directional and differential issues.

Commissioning Tip

Before energization, record CT/VT nameplate data and compare it against:

design single-line diagram
relay settings and scaling
meter scaling and reporting system configuration

Internal Links

CT products: Current Transformers.

VT products: Voltage Transformers.

Conclusion

Reading CT/VT nameplates correctly prevents the most common measurement and protection issues. Verify ratio, class, burden, and polarity before energization, and align the system scaling and relay settings with the actual installed equipment.