The Siemens 3NJ6940-3BL12 is a pluggable bushing current transformer rated 600 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 addresses a specific installation challenge: accurate current measurement when the meter is not sitting right next to the switchboard. The 1 A secondary minimises resistive losses in long CT secondary cables, preserving the precision that Class 0.5 promises.

Why choose 1 A secondary instead of the more common 5 A?

The 1 A secondary reduces I²R losses in the CT wiring by a factor of 25 compared to 5 A — for the same cable length and cross-section. This matters when the energy meter, data logger, or protection relay is installed in a central control room tens of metres from the switchboard. With a 5 A CT and a long cable run, secondary wiring resistance can consume a meaningful share of the CT burden budget, pulling the operating point away from the rated accuracy window. The 3BL12 solves that. For installations where the meter is within a few metres of the disconnector, the 5 A variant (3BL22) is the conventional choice with better noise immunity.

What separates the 3BL12 from the 3BL11 and 3BL13?

All three share the 600/1 A ratio. The difference is accuracy class and calibration. The 3BL11 is Class 1 — adequate for basic monitoring and protection. The 3BL12 is Class 0.5 — the precision tier for sub-metering and internal energy allocation where billing is not involved. The 3BL13 adds legal-for-trade calibration on top of Class 0.5 — for fiscal metering over long cable runs. Choose the 3BL12 when measurement accuracy matters to your operations but no invoice depends on it.

600/1 A Accuracy Variant Comparison

Frequently Asked Questions

Does the 1 A secondary require special cabling?

No special cable type is required, but good practice for 1 A CT secondary wiring recommends twisted-pair conductors with a minimum cross-section of 1.5 mm² for runs up to about 20 m — and 2.5 mm² for longer distances — to keep total secondary loop resistance within the CT burden budget. Always calculate the total burden (meter input impedance + wiring resistance) and confirm it does not exceed 5 VA at the rated secondary current.

Is the 3BL12 mechanically identical to the 5 A variants?

Yes. All 3NJ6940 bushing CTs share the same external dimensions (70 × 88.5 × 58 mm), the same 32 mm cable window, and the same plug-in interface for 3NJ6 in-line disconnectors. The secondary current rating has no effect on the mechanical form factor.