The Siemens 3NJ6940-3BL22 is a pluggable bushing current transformer rated 600 A primary / 5 A secondary with Class 0.5 accuracy. Part of the SENTRON 3NJ6940 accessory family for 3NJ6 in-line fuse-switch-disconnectors, it delivers precision measurement in a tool-free, plug-in format. The 32 mm cable window accepts standard low-voltage distribution conductors, and the 5 VA burden is compatible with common digital energy meters and protection relays.

Why choose Class 0.5 over Class 1 for a sub-metering application?

Class 0.5 halves the maximum error compared to Class 1 — ±0.5% versus ±1% at rated current. For internal energy allocation, departmental cost tracking, or any scenario where measurement accuracy directly affects operational decisions, that tighter tolerance translates to more trustworthy data. The 3BL22 sits between the economical Class 1 3BL21 (sufficient for basic monitoring) and the legal-for-trade 3BL23 (required for billing). If you need precision without paying for certification you will never use, this is the right variant.

When is a 5 A secondary CT the right choice?

The 5 A secondary is the industry standard for low-voltage metering — it interfaces directly with the 5 A current input on most digital multimeters, energy analysers, and protection relays. Over short cable distances between CT and meter (up to about 5 m), 5 A provides strong signal integrity and noise immunity. If your installation places the meter farther from the disconnector, the 1 A secondary variant (3BL12) reduces voltage drop in the secondary loop, but for the majority of compact low-voltage switchboards, 5 A is the straightforward fit.

600/5 A Accuracy Variant Comparison

Frequently Asked Questions

Can I upgrade from a 3BL21 to a 3BL22 without changing anything else?

Yes. All three 600/5 A variants share identical dimensions (70 × 88.5 × 58 mm), the same 32 mm cable window, and the same pluggable interface for 3NJ6 disconnectors. Swapping a 3BL21 for a 3BL22 requires no mechanical or wiring changes — just unplug the old CT and plug in the new one.

Does this CT need an external power supply?

No. Like all passive current transformers, the 3BL22 is self-powered by the magnetic field of the primary conductor. It requires no external power and generates its own secondary current proportional to the primary load current.

What happens if the secondary circuit is left open while the primary is energised?

Never operate a current transformer with an open secondary while primary current flows — this can generate dangerous voltage peaks at the secondary terminals and damage the CT. Always ensure the secondary is connected to a burden (meter or relay input) or short-circuited before energising the primary circuit.