The Siemens 3NJ6940-3BJ12 is a pluggable bushing current transformer rated 400 A primary / 1 A secondary with Class 0.5 accuracy. Part of the SENTRON 3NJ6940 accessory family for 3NJ6 in-line fuse-switch-disconnectors, it delivers precision measurement on 400 A feeders where the energy meter or data logger is installed in a central equipment room — not next to the switchboard. The 1 A secondary ensures that wiring distance does not erode the measurement accuracy you are paying for.

When is 400/1 A the optimal CT specification?

Three conditions point to the 3BJ12: (1) your feeder is protected at 400 A — matching the CT primary to the protection rating keeps operation in the accurate band; (2) your meter or relay accepts a 1 A input and sits more than 10 m from the switchboard — distance where a 5 A secondary starts to lose precision to wiring losses; (3) you need Class 0.5 accuracy for internal energy allocation or efficiency tracking — but billing is not involved. If all three apply, the 3BJ12 is exactly the right specification.

What is the difference between 3BJ11, 3BJ12, and 3BJ13?

All three share 400/1 A. The 3BJ11 is Class 1 — adequate for basic monitoring, alarms, and panel indication. The 3BJ12 is Class 0.5 — for sub-metering, cost allocation, and any application where measurement trustworthiness matters to operational decisions. The 3BJ13 adds legal-for-trade calibration — for billing. Each step answers a different measurement obligation at a different price point.

400/1 A Accuracy Variant Comparison

Frequently Asked Questions

Can I use this CT for a 400 A feeder that occasionally peaks at 420 A?

Yes. The 3BJ12 maintains Class 0.5 accuracy up to 120% of rated current (480 A). Occasional peaks to 420 A are within the specified operating range and will not damage the CT or degrade measurement accuracy.

Does the 1 A secondary mean lower signal quality than 5 A?

At very short distances (under 2 m), 5 A provides marginally better signal-to-noise ratio against electromagnetic interference. Beyond 5–10 m, the I²R losses in 5 A wiring outweigh that advantage — and 1 A becomes the electrically superior choice. Both provide clean, proportionate current signals when used within their intended cable length.