The Siemens 3NJ6940-3BL11 is a pluggable bushing current transformer rated 600 A primary / 1 A secondary with Class 1 accuracy. It is the entry-level 1 A variant in the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. If you need to monitor current on a 600 A feeder and your meter or relay is installed some distance from the switchboard, the 3BL11 delivers the essential signal with the right secondary for long cable runs — at the most economical price point in the 600 A group.

When does Class 1 accuracy combined with 1 A secondary make sense?

This combination serves a specific installation profile: the CT is on a 600 A feeder in a remote switchboard, the monitoring equipment (PLC, BMS input, panel indicator) is in a central control room tens of metres away, and the measurement purpose is load confirmation and overload detection — not precision metering. The 1 A secondary handles the distance without burning the CT burden budget on wiring losses, and Class 1 provides more than enough accuracy for the task. If your application needs both long cable runs and precision measurement, step up to the 3BL12 (Class 0.5) or 3BL13 (Class 0.5 + legal-for-trade).

Is a 1 A CT harder to install than a 5 A CT?

No — the installation procedure is identical. The 1 A and 5 A variants share the same pluggable form factor (70 × 88.5 × 58 mm, 32 mm cable window). The only difference is the label on the secondary terminals and the current that flows in the secondary wiring. The same 1.5–2.5 mm² twisted-pair wiring used for 5 A CTs works for 1 A — with the advantage that voltage drop in the loop is 25 times lower, so you can afford longer runs or lighter-gauge conductors.

600/1 A Accuracy Variant Comparison

Frequently Asked Questions

Can I connect a 1 A secondary CT directly to a standard digital multimeter?

Check your meter input specification. Many modern digital energy meters and multifunction instruments accept both 1 A and 5 A inputs — selectable via software configuration or terminal choice. Older panel meters and some dedicated protection relays are fixed at 5 A. If your instrument is 5 A only, choose the 3BL21 instead, or insert a 1 A/5 A interposing transformer.

Does the 32 mm window fit a 600 A cable?

In practice, a 600 A low-voltage feeder typically uses 185–300 mm² conductors per phase. The outer diameter of a single-core 240 mm² cable is typically 26–30 mm — within the 32 mm window. However, always measure the actual outer diameter of your specific cable type, including any semi-conductive layers or outer sheath thickness, before ordering.