The Siemens 3NJ6940-3BK23 is a pluggable bushing current transformer rated 500 A primary / 5 A secondary, with Class 0.5 accuracy and legal-for-trade calibration. It belongs to the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. The 500 A primary rating matches typical feeder sizes in commercial and light industrial distribution — and the calibration makes it admissible for billing on those circuits. Like every variant in the series, it plugs into the disconnector with no tools and accepts cables up to 32 mm.

What makes the legal-for-trade 3BK23 different from the 3BK22?

Both are 500/5 A Class 0.5 CTs. The difference is certification: the 3BK23 is factory-calibrated and conformity-assessed under metrology directives, making measurements from a 3BK23 + calibrated meter chain legally admissible for billing. The 3BK22 delivers the same Class 0.5 accuracy for internal energy tracking, sub-metering, and operational monitoring — without the certification cost and longer lead time. If no invoice depends on the kWh reading, the 3BK22 is the practical choice. If one does, the 3BK23 is the only choice.

When does a 500 A CT fit better than 600 A or 400 A?

The 500 A rating targets a common feeder size in European low-voltage distribution: a 500 A fuse-switch-disconnector protecting a cable in the 185–240 mm² range. If your feeder is fused at 500 A, a 500 A CT is properly matched — it operates in its optimal accuracy band near rated current. A 600 A CT on a 500 A feeder will spend most of its life below 85% of rating, where accuracy may degrade slightly. A 400 A CT on a 500 A feeder risks saturation during normal operation. Match the CT primary rating to the feeder protection rating for best measurement performance.

500/5 A Accuracy Variant Comparison

Frequently Asked Questions

Does the 3BK23 fit any 3NJ6 disconnector or only specific frame sizes?

All 3NJ6940 bushing CTs share a common mechanical interface compatible with the full range of 3NJ6 in-line fuse-switch-disconnectors. The CT is not frame-size-specific — the same 3BK23 fits a 3NJ6 250 A, 400 A, or 630 A disconnector, as long as the cable passing through the CT window does not exceed 32 mm diameter.

What is the actual measurable current range of a 500/5 A CT?

The 3BK23 maintains its rated Class 0.5 accuracy from approximately 20% to 120% of rated primary current — roughly 100 A to 600 A. Below 100 A, accuracy gradually degrades. For continuous loads significantly below 100 A on a 500 A feeder, consider whether a lower-ratio CT (e.g. 400/5 A 3BJ22) provides a better match to the actual load profile.