The Siemens 3NJ6940-3BJ23 is a pluggable bushing current transformer rated 400 A primary / 5 A secondary, with Class 0.5 accuracy and legal-for-trade calibration. Belonging to the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors, it enables billing-grade energy measurement on 400 A feeders — a common size in commercial sub-main distribution and light industrial supply panels. The 32 mm cable window accepts standard 120–185 mm² feeder conductors.

What makes the calibrated 3BJ23 different from the 3BJ22?

Both share 400/5 A and Class 0.5 accuracy. The 3BJ23 adds factory calibration and conformity assessment under metrology directives — the measurement chain of 3BJ23 + calibrated meter is admissible for billing. If you invoice tenants, departments, or external customers based on the energy measured through this CT, the 3BJ23 is required. For internal energy tracking and sub-metering, the 3BJ22 provides the same measurement precision without the certification overhead, shorter lead time, and lower cost.

Why 400 A instead of 500 A or 300 A?

Match the CT primary rating to the feeder protection rating. A 400 A fuse-switch-disconnector is typically paired with 120–185 mm² cables supplying sub-main distribution boards, floor panels, or individual production equipment. A 400 A CT operates in its optimal accuracy band at the currents that feeder actually carries. A 500 A CT on a 400 A feeder will operate below 80% rating most of the time (lower accuracy); a 300 A CT risks saturation during normal peaks. The 3BJ23 is purpose-matched to 400 A fused feeders.

400/5 A Accuracy Variant Comparison

Frequently Asked Questions

Is the 3BJ23 compatible with all 3NJ6 disconnector frame sizes?

Yes. The 3NJ6940 pluggable CT interface is common across the entire 3NJ6 in-line disconnector range. You can install a 400 A CT on a 630 A frame disconnector if the actual feeder current does not exceed 400 A — the CT measures the cable current, not the disconnector rating.

Does legal-for-trade calibration require periodic re-verification?

The 3BJ23 is factory-calibrated and conformity-assessed. Under normal operating conditions it does not require periodic re-calibration. National metrology regulations vary — confirm with your local authority whether re-verification intervals apply in your jurisdiction.