The Siemens 3NJ6940-3BK11 is a pluggable bushing current transformer rated 500 A primary / 1 A secondary with Class 1 accuracy. It is the entry-level 1 A variant in the 500 A group of the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. If you need to confirm load status on a 500 A feeder from a remote monitoring point, the 3BK11 delivers the essential signal with the right secondary for distance — at the lowest cost in the 500/1 A group.

When is Class 1 plus 1 A secondary the right combination?

This pairing solves two practical constraints at once: the CT sits in a remote switchboard with the meter tens of metres away (1 A handles the distance), and the measurement purpose is load confirmation, overload alarm, or BMS integration — not precision metering (Class 1 is more than enough). For a 500 A feeder supplying general lighting, small power, or HVAC auxiliaries, this is the fit-for-purpose choice. Upgrade to the 3BK12 if you need Class 0.5 for internal cost allocation, or to the 3BK13 if billing compliance is required.

Does a 1 A CT work with a standard panel ammeter?

Check the ammeter scale. Traditional analogue panel ammeters with a 1 A full-scale input exist but are less common than their 5 A counterparts. Most digital panel meters and multifunction instruments accept both 1 A and 5 A inputs — check the device data sheet. If your instrument is fixed at 5 A, choose the 3BK21 (500/5 A Class 1) instead, or add a 1 A/5 A interposing transformer.

500/1 A Accuracy Variant Comparison

Frequently Asked Questions

Can I upgrade from Class 1 to Class 0.5 later?

Yes. All 3NJ6940 variants share identical external dimensions (70 × 88.5 × 58 mm), the same 32 mm cable window, and the same plug-in interface for 3NJ6 disconnectors. Swapping a 3BK11 for a 3BK12 or 3BK13 is a simple unplug-and-replace operation with no wiring or mechanical changes needed.

What happens if the secondary circuit is accidentally opened under load?

Never operate a current transformer with an open secondary while primary current flows. The CT will attempt to drive current into an infinite impedance, producing dangerous voltage peaks at the secondary terminals that can damage the CT insulation and create a safety hazard. Always ensure the secondary is connected to a burden or short-circuited before energising the primary circuit.