The 3NJ6940-3BH21 is the 300 A / 5 A Class 1 variant in the SENTRON 3NJ6940 family of pluggable current transformers. Among the 300 A options, this is the most economical choice — a 5 A secondary for short-distance meter connections with Class 1 accuracy adequate for load monitoring and overcurrent protection.

When should I choose the 5 A secondary over 1 A?

The 5 A secondary is the conventional choice for industrial panel wiring. It produces a stronger signal with better noise immunity on short cable runs — typically up to 10 metres between the CT and the measuring instrument. Most standard panel ammeters, transducers, and energy meters are designed for 5 A nominal input, so this variant connects without intermediate converters or special-order meters. If your CT sits inside the same cabinet as your meter, 5 A is the straightforward choice.

Is Class 1 accuracy sufficient for 300 A monitoring?

Class 1 means the current error is within ±1% at rated primary current. For load monitoring, overload detection, and non-billing energy tracking, Class 1 is fully adequate. Applications that require billing-grade accuracy or precision sub-metering should consider the Class 0.5 variants (3NJ6940-3BH22 or 3NJ6940-3BH12). The 3BH21 delivers reliable measurement for everyday operational monitoring without the cost of higher accuracy grades.

Frequently Asked Questions

Is this CT compatible with all 3NJ6 switch disconnectors?

Yes. The 3NJ6940 series plugs directly onto any SENTRON 3NJ6 in-line fuse-switch-disconnector. The 32 mm cable bushing accommodates the standard cable cross-sections used with 300 A NH fuse holders.

Can I use this transformer with a digital multifunction meter?

Yes. Any meter with a 5 A current input is compatible. Verify the meter's burden does not exceed the CT's 5 VA rating when combined with the connecting cable impedance.