The Siemens 3NJ6940-3BK12 is a pluggable bushing current transformer rated 500 A primary / 1 A secondary with Class 0.5 accuracy. Part of the SENTRON 3NJ6940 accessory family for 3NJ6 in-line fuse-switch-disconnectors, it combines precision measurement with a 1 A secondary — purpose-built for installations where the CT sits in a remote switchboard and the meter or data logger is in a central control room. The 32 mm cable window and plug-in interface are identical to every other variant in the series.

When does a 1 A secondary outperform 5 A?

The advantage is purely about distance. At 5 A, a 100 m run of 2.5 mm² copper wiring adds roughly 1.4 Ω of loop resistance — consuming over 30% of a 5 VA burden budget before the meter is even connected. At 1 A, that same wiring dissipates 25× less power, keeping the CT comfortably within its rated burden. If your CT-to-meter cable run is under 5 m — typical for a compact switchboard — the 5 A variant (3BK22) is simpler and provides stronger signal immunity. Beyond 10–15 m, 1 A becomes the pragmatically correct engineering choice.

What does the 3BK12 offer that the 3BK11 and 3BK13 do not?

The 3BK11 is Class 1 — adequate for basic monitoring and overload detection over long cable runs. The 3BK13 is Class 0.5 with legal-for-trade calibration — billing-certified but at a premium cost and with ~30-day lead times. The 3BK12 sits in between: Class 0.5 precision for trustworthy internal energy allocation and sub-metering, without paying for calibration you never use. It is the value-optimised choice for 500 A feeders with remote metering and no billing requirement.

500/1 A Accuracy Variant Comparison

Frequently Asked Questions

Is this CT compatible with any 3NJ6 disconnector frame size?

Yes. All 3NJ6940 bushing CTs share a common mechanical interface that fits the full range of 3NJ6 in-line fuse-switch-disconnectors — from 250 A to 630 A frames. The CT measures current in the cable passing through it, not the disconnector frame rating.

Does a 500 A CT work on a feeder that normally runs at 300–400 A?

A 500 A CT maintains its rated Class 0.5 accuracy from roughly 20% to 120% of rated primary current (100–600 A). If your feeder consistently runs at 300–400 A, you are within the sweet spot. If the load rarely exceeds 200 A, consider whether a 300 A or 400 A CT would provide a better match to your actual load profile.