The Siemens 3NJ6940-3BJ13 is a pluggable bushing current transformer rated 400 A primary / 1 A secondary, with Class 0.5 accuracy and legal-for-trade calibration. Part of the SENTRON 3NJ6940 accessory family for 3NJ6 in-line fuse-switch-disconnectors, it is the calibrated 1 A choice for 400 A feeders — combining billing certification with the long-distance signal integrity of a 1 A secondary. The tool-free plug-in format and 32 mm cable window are shared with all 3NJ6940 variants.

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

The 1 A secondary preserves measurement integrity when the CT-to-meter distance exceeds 10–15 m. In a calibrated billing chain, every component must deliver its rated accuracy — and a 5 A CT with long secondary wiring can lose precision to I²R losses before the signal ever reaches the meter. The 3BJ13 eliminates that risk: the 1 A secondary reduces wiring losses by 96% compared to 5 A, keeping the measurement chain within its certified accuracy window regardless of cable length. For short cable runs where the meter sits in the same switchboard, the 5 A variant (3BJ23) is the conventional choice.

How does the 3BJ13 compare to other 400/1 A variants?

The 3BJ11 is Class 1 — for basic monitoring where accuracy is secondary. The 3BJ12 is Class 0.5 without calibration — precision sub-metering without billing. The 3BJ13 is Class 0.5 with legal-for-trade calibration — the only 400/1 A variant admissible for fiscal metering. Each step up adds capability (and cost) — choose the one whose capability matches your measurement obligation.

400/1 A Accuracy Variant Comparison

Frequently Asked Questions

Does a 1 A CT require a specific meter type?

Your meter must accept a 1 A current input. Most modern digital energy meters and multifunction instruments support both 1 A and 5 A — selectable via configuration. Legacy analogue meters and some single-purpose protection relays are 5 A only. Always verify the meter input specification before ordering a 1 A CT.

What is the maximum recommended secondary cable length for 1 A?

With 2.5 mm² twisted-pair copper wiring, a 1 A secondary can reliably drive distances of 50–100 m while staying well within the 5 VA burden budget — far beyond what a 5 A secondary can achieve with the same cable. Always calculate the total loop burden (meter input + wiring) and confirm compliance with the 5 VA rating.