The Siemens 3NJ6940-3BL21 is a pluggable bushing current transformer rated 600 A primary / 5 A secondary with Class 1 accuracy. It is the entry-level variant in the 600/5 A group of the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. Like all models in the series, it clips onto cables up to 32 mm diameter and plugs directly into the disconnector — no tools, no extra wiring, no separate mounting enclosure.

When is Class 1 accuracy perfectly adequate?

Class 1 (±1% at rated current) covers the majority of non-billing measurement tasks in low-voltage distribution: confirming that a feeder is loaded within its rating, triggering overload alarms, providing an analogue signal to a building management system, or driving a simple panel ammeter. If your goal is to know whether a 600 A circuit is running at 200 A or 500 A — rather than exactly 437 A versus 442 A — Class 1 is exactly what you need. The 3BL21 gives you that capability at a significantly lower cost than the Class 0.5 variants (3BL22, 3BL23).

Is the 5 A secondary the right match for my installation?

The 5 A secondary is the most common standard in low-voltage switchgear and matches the current input on typical digital panel meters, PLC analogue modules, and protection relays. If your measuring instrument has a 5 A input — and the cable run between CT and meter is under a few metres — this is the straightforward choice. For installations with long secondary cable runs, the 1 A version (3BL11) cuts resistive losses in the secondary wiring by a factor of 25.

600/5 A Accuracy Variant Comparison

Frequently Asked Questions

Can I later swap a 3BL21 for a higher-accuracy variant?

Yes. All 3NJ6940 variants in the 600 A group share identical mechanical dimensions (70 × 88.5 × 58 mm), the same 32 mm cable window, and the same plug-in interface for 3NJ6 disconnectors. A future upgrade from Class 1 to Class 0.5 requires no mechanical rework — simply unplug the old CT and plug in the replacement.

Does a Class 1 CT still protect my equipment?

Yes. For protection purposes — overcurrent detection, short-circuit trip confirmation, thermal overload monitoring — Class 1 accuracy is more than sufficient. Protection decisions are based on detecting a threshold crossing (e.g. current above 630 A), not on knowing the exact ampere value to sub-percent precision.