The Siemens 3NJ6940-3BJ21 is a pluggable bushing current transformer rated 400 A primary / 5 A secondary with Class 1 accuracy. It is the entry-level variant in the 400 A group of the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. For 400 A sub-main and feeder circuits where knowing the approximate load is sufficient — overload alarms, BMS analogue inputs, local panel indication — it provides the essential measurement function at the lowest cost in its group.

When is Class 1 accuracy the right fit for a 400 A feeder?

Class 1 (±1% at rated current) covers the vast majority of non-billing measurement tasks: confirming a 400 A feeder is not overloaded, detecting phase imbalance, providing a 4–20 mA signal to a building management system, or driving a local ammeter. These functions decide based on threshold crossing — not on the exact ampere value to sub-percent precision. If your organisation allocates energy costs internally based on measured consumption, step up to the Class 0.5 3BJ22. If billing is involved, the calibrated 3BJ23 is required.

Does a 400 A CT fit a feeder that peaks around 350 A?

Yes — and it is better matched than a 500 A CT would be. The 3BJ21 maintains its rated Class 1 accuracy from roughly 20% to 120% of rated current (80–480 A). A feeder peaking at 350 A operates at 87.5% of rating, well within the specified accuracy band. A 500 A CT on the same feeder would operate at only 70% of rating, where accuracy may begin to degrade.

400/5 A Accuracy Variant Comparison

Frequently Asked Questions

Can I upgrade to a higher-accuracy variant later?

Yes. All 3NJ6940 CTs share identical mechanical dimensions and plug-in interface. Upgrading from a 3BJ21 to a 3BJ22 or 3BJ23 requires no changes to the disconnector, cabling, or meter — simply unplug the old CT and plug in the replacement.

Is the 5 A secondary compatible with standard energy meters?

Yes. The 5 A secondary and 5 VA burden rating are industry-standard values. Virtually all digital and analogue energy meters, multifunction instruments, and protection relays accept a 5 A current input. Verify that your total secondary burden (meter input + wiring resistance) stays within 5 VA.