The Siemens 3NJ6940-3BL13 is a pluggable bushing current transformer rated 600 A primary / 1 A secondary, with Class 0.5 accuracy and legal-for-trade calibration. It combines three requirements that rarely appear together: billing-grade certification, precision measurement, and a 1 A secondary for installations where the meter sits at a distance from the switchboard. As part of the SENTRON 3NJ6940 range for 3NJ6 in-line fuse-switch-disconnectors, it plugs in with no tools and accepts cables up to 32 mm diameter.

Why would I need a 1 A secondary on a calibrated CT?

In many commercial and industrial buildings, the energy meter or data concentrator is installed in a central equipment room — metres away from the switchboard where the CT sits. With a 5 A secondary, the resistive losses (I²R) in long secondary wiring can erode measurement accuracy and, in extreme cases, exceed the CT burden rating. A 1 A secondary reduces those losses by a factor of 25 for the same cable gauge, preserving the calibrated accuracy that you are paying for. If your CT-to-meter cable run exceeds 10–15 m, the 3BL13 is the right tool for the job.

What does legal-for-trade calibration mean in practice?

It means this CT is factory-calibrated and conformity-assessed under metrology directives — the measurement chain it forms with a calibrated meter is admissible for billing. If your organisation invoices based on measured energy, or your local electrical code requires calibrated CTs for specific feeders, the 3BL13 meets that requirement. For the same 600/1 A performance without calibration, see the 3BL12.

600/1 A Accuracy Variant Comparison

Frequently Asked Questions

Can I use a 1 A secondary CT with a 5 A meter input?

Not directly. Most digital energy meters accept either 1 A or 5 A input via configuration, but if your meter is fixed at 5 A, you will need an intermediate 1 A/5 A interposing transformer — which adds cost, space, and an additional accuracy link in the measurement chain. Check your meter specifications before ordering.

Is the 3BL13 plug-compatible with other 3NJ6940 CTs?

Yes. All 3NJ6940 variants share identical external dimensions (70 × 88.5 × 58 mm) and the same plug-in interface for 3NJ6 disconnectors. The 1 A and 5 A secondary variants are mechanically interchangeable.