The Siemens 3NJ6900-4HF12 is a bimetal analogue panel ammeter for the SENTRON 3NJ6 series of in-line switch disconnectors with fuses. It provides thermal-response current indication on a 200 A scale via a 200/1 A current transformer input, with Class 1 accuracy, 1 VA burden, and 2-fold overload capacity.

Bimetal at the Main Feeder Level

A 200 A main feeder is typically the aggregation point for all downstream loads — motors, lighting, HVAC, office equipment, and process machinery. The load profile is inherently variable, with diverse inrush characteristics summing at the incomer. The bimetal ammeter naturally filters this composite variability, showing the operator the sustained current level that determines cable and busbar heating — which is what matters for thermal protection and capacity planning. The 1 A secondary preserves accuracy over the longer CT-to-instrument distances common in main switchrooms.

Decision Parameters