The Siemens 3NJ6940-3BK21 is a pluggable bushing current transformer rated 500 A primary / 5 A secondary with Class 1 accuracy. It is the entry-level variant in the 500 A group of the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. For applications where knowing the approximate load on a 500 A feeder is sufficient — overload alarms, BMS integration, panel ammeters — it provides the essential measurement function at the most economical price point.

When is Class 1 the right accuracy choice for a 500 A circuit?

Class 1 (±1% at rated current) is entirely adequate for protection-oriented measurements: confirming a feeder is not overloaded, detecting phase loss, providing a 4–20 mA signal to a building management system via a transducer, or driving a local panel ammeter. These functions make decisions based on whether current is above or below a threshold — not on the exact ampere value. If your 500 A feeder needs precision sub-metering for internal cost allocation, step up to the Class 0.5 3BK22. For billing, the calibrated 3BK23 is required.

Does a lower-accuracy CT still integrate with modern digital monitoring systems?

Yes. The 5 A secondary is the universal analogue interface for energy management systems. A Class 1 CT connected to a modern digital meter still provides usable data for trending, alarm generation, and load profiling. The accuracy class sets the ceiling on measurement certainty — it does not limit what you can connect to.

500/5 A Accuracy Variant Comparison

Frequently Asked Questions

Can I upgrade from the 3BK21 to a Class 0.5 variant later?

Yes. All 3NJ6940 variants share the same mechanical dimensions and plug-in interface. Swapping a 3BK21 for a 3BK22 or 3BK23 requires no changes to the disconnector, the primary cabling, or the CT mounting — just unplug the existing CT and plug in the replacement.

Does a Class 1 CT meet IEC standards for switchgear metering?

Yes. Class 1 is a recognised accuracy class under IEC 61869-2 (the successor to IEC 60044-1) for current transformers used in measuring circuits. It meets the requirements for industrial switchgear metering where billing accuracy is not mandated.