The 3NJ6940-3BH13 is the only 300 A variant in the SENTRON 3NJ6940 family that carries legal-for-trade calibration. With 1 A secondary, Class 0.5 accuracy, and factory conformity assessment, it is purpose-built for revenue-grade energy metering in low-voltage distribution — at a higher cost and with longer lead time than its non-calibrated siblings.

What does legal-for-trade calibration mean in practice?

A legal-for-trade (eichfähig) current transformer has undergone factory calibration traceable to national standards and passed a conformity assessment. This certifies that its ratio error and phase displacement remain within Class 0.5 limits under rated conditions, making it admissible for billing applications where the measured energy determines a financial transaction. Without this certification, a CT — even one meeting Class 0.5 accuracy — cannot be used for revenue metering in regulated markets. Expect approximately 30 days lead time for the calibration and documentation process, compared to 3 days for the standard Class 0.5 variant (3NJ6940-3BH12).

Why pair 1 A secondary with legal-for-trade calibration?

Billing meters are often installed in a central monitoring location, potentially tens of metres from the switchgear. A 1 A secondary reduces I²R losses in the connecting cable by a factor of 25 compared to a 5 A secondary — critical when every watt-hour counts for revenue accuracy. The combination of 1 A output with factory calibration ensures that the measurement chain from CT to billing meter maintains its certified accuracy over real-world cable distances.

Frequently Asked Questions

Does the calibration certificate come with the transformer?

Yes. Legal-for-trade CTs ship with a conformity assessment document. Verify with your supplier that the certificate is included — it must be retained for audit purposes in billing applications.

Can I use this CT for normal monitoring if I do not need billing?

Yes, but the 3NJ6940-3BH12 (300/1 A, Class 0.5, non-calibrated) delivers identical measurement performance at roughly half the cost and a fraction of the lead time. Only choose the 3BH13 if legal-for-trade certification is a hard requirement.