The Siemens 3NJ6940-3BL23 is a pluggable bushing current transformer rated 600 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. Slide it over a cable up to 32 mm diameter, plug it into the disconnector, and you have certified metering with no separate CT enclosure and no additional wiring.

What does legal-for-trade calibration give me that standard Class 0.5 does not?

Both the 3BL23 and the 3BL22 deliver Class 0.5 measurement accuracy — the difference is legal standing. The 3BL23 is factory-calibrated and conformity-assessed under metrology directives, making it admissible for billing applications (eichfähig). If you invoice tenants, departments, or external customers based on measured energy consumption — or if your jurisdiction mandates calibrated CTs for fiscal metering — this is the variant you need. For precision monitoring and internal sub-metering with no billing requirement, the 3BL22 provides the same accuracy at a lower cost and with shorter lead times.

When does a 5 A secondary make more sense than 1 A?

A 5 A secondary is the conventional choice for low-voltage metering: it matches the input of standard digital energy meters and protection relays, and delivers better noise immunity over short cable runs — typically up to a few metres between CT and meter. If your measuring device accepts 5 A input, or you are replacing an existing 5 A CT in a retrofit project, the 3BL23 is the direct drop-in. For installations where the CT-to-meter cable run exceeds 10–15 m — or where your meter specifically requires 1 A input — consider the 3BL13, which offers the same Class 0.5 calibrated performance with a 1 A secondary to minimise voltage drop in the secondary wiring.

600/5 A Accuracy Variant Comparison

Frequently Asked Questions

Does this CT work with any 3NJ6 switch disconnector?

Yes. All 3NJ6940 bushing current transformers share the same mechanical interface and plug directly onto any 3NJ6 in-line fuse-switch-disconnector. The CT slides over the load-side cable and seats into the disconnector body — no tools, no wiring changes, no adapter plates needed.

Does legal-for-trade calibration require periodic re-verification?

The 3BL23 is factory-calibrated and conformity-assessed. Under normal operating conditions — no severe overcurrent events, no physical damage — it does not require periodic re-calibration. National metrology regulations vary, so confirm with your local authority whether re-verification intervals apply in your jurisdiction.

What is the maximum cable size that fits through the CT window?

The 32 mm diameter window accommodates conductors up to approximately 240 mm² per phase in typical low-voltage distribution — sufficient for most 600 A feeder applications. Always check the actual outer diameter of your specific cable type against the 32 mm window before ordering.