The Siemens 3NJ6940-3BJ11 is a pluggable bushing current transformer rated 400 A primary / 1 A secondary with Class 1 accuracy. It is the entry-level 1 A variant in the 400 A group of the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. For a 400 A sub-main or feeder where you need basic load confirmation from a remote monitoring point — and every euro of project budget counts — this is the fit-for-purpose choice.

Why choose 1 A with Class 1 accuracy?

This combination solves for cost and distance simultaneously. The 1 A secondary enables long CT-to-meter cable runs without signal degradation; Class 1 accuracy covers all non-precision measurement needs (overload alarms, BMS integration, panel indication). If your 400 A feeder supplies general loads where kWh precision is not tracked, the 3BJ11 is the specification that does the job with no over-specification. For internal energy cost allocation, the 3BJ12 adds Class 0.5 precision. For billing, the 3BJ13 adds calibration.

Does a Class 1 CT still work with modern digital monitoring?

Absolutely. Class 1 defines the measurement accuracy ceiling — it does not limit what you can connect the CT to. A 3BJ11 feeding a digital multifunction meter still provides current data for trending, alarm generation, and load profiling. The accuracy class governs how precisely that data reflects the actual primary current — not whether the data exists at all.

400/1 A Accuracy Variant Comparison

Frequently Asked Questions

Can I use a 400 A CT on a circuit fused at 500 A?

No. A 400 A CT on a 500 A feeder will be driven beyond its rated primary current during normal operation, risking saturation, measurement error, and potential thermal damage. The CT primary rating must equal or exceed the feeder protection rating. For a 500 A circuit, choose a 500 A variant (3BKxx series).

Will the 32 mm window accommodate 400 A feeder cables?

A 400 A low-voltage feeder typically uses 120–185 mm² conductors with outer diameters of roughly 18–26 mm — well within the 32 mm window. Always measure your specific cable outer diameter before ordering, particularly for armoured or multi-core cables.