The Siemens 3NJ6940-3BK13 is a pluggable bushing current transformer rated 500 A primary / 1 A secondary, with Class 0.5 accuracy and legal-for-trade calibration. It belongs to the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. The 1 A secondary handles long CT-to-meter cable runs without burning the burden budget on wiring losses — making it the right choice when a 500 A feeder needs billing-grade measurement and the meter is installed in a remote control room.

Why choose a 1 A secondary for a calibrated billing CT?

In commercial and industrial buildings, the energy meter or data concentrator often sits in a central equipment room — tens of metres from the switchboard. With a 5 A secondary, I²R losses in long secondary wiring can erode the calibrated accuracy you are paying for. A 1 A secondary slashes those losses by a factor of 25, preserving the legal-for-trade measurement integrity over distance. If your CT-to-meter cable run stays under a few metres, the 5 A variant (3BK23) provides equivalent billing-grade performance with better noise immunity.

What does legal-for-trade calibration add to a 500/1 A CT?

The 3BK13 is factory-calibrated and conformity-assessed under metrology directives — the measurement chain it forms with a calibrated meter is admissible for invoicing. The 3BK12 shares the same 500/1 A ratio and Class 0.5 accuracy for precision sub-metering without billing requirements, at a lower cost and with shorter lead times. The 3BK11 offers Class 1 accuracy for basic monitoring over long cable runs. Choose the 3BK13 when an invoice depends on the kWh reading from a 500 A feeder with a distant meter.

500/1 A Accuracy Variant Comparison

Frequently Asked Questions

Does the 1 A secondary require different wiring than 5 A?

No special cable type is needed, but good practice for 1 A CT secondaries recommends twisted-pair conductors with at least 1.5 mm² cross-section for runs up to 20 m, and 2.5 mm² for longer distances. Always calculate total burden (meter input impedance + wiring resistance) and confirm it stays within 5 VA at rated secondary current.

Can I connect a 1 A CT directly to a standard energy meter?

Many modern digital energy meters accept both 1 A and 5 A input — selectable via software or terminal assignment. Older panel meters and some dedicated protection relays are fixed at 5 A. Verify your meter input specification before ordering. If your meter is 5 A only and cannot be changed, choose the 3BK23 instead.