The Siemens 3NJ6900-4HE21 is a moving-iron analogue panel ammeter for the SENTRON 3NJ6 series of in-line switch disconnectors with fuses. It provides true RMS current indication on a 150 A scale via a 150/5 A current transformer input — the 5 A secondary being the universal standard for industrial panel instruments and the default choice for brownfield installations where existing CTs are already 5 A secondary.

Moving Iron RMS for 150 A Circuits

At 150 A, circuits often serve significant non-linear loads — VSD-controlled motors, UPS systems, and industrial power supplies. The moving-iron mechanism reads true RMS current, ensuring the operator sees the actual heating current rather than an averaged approximation. For a 150 A feeder with a mix of linear and non-linear loads, this measurement integrity matters for both operational decisions and preventative maintenance.

Decision Parameters