The Siemens 3NJ6900-4HD12 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 100 A scale via a 100/1 A current transformer input. The bimetal mechanism with 2-fold overload capacity makes it suited for 100 A circuits subject to intermittent high-current conditions — such as compressor starts, pump priming, or transformer inrush.

100 A Bimetal: When Larger Feeders Need Thermal Response

At the 100 A level, circuits are more likely to serve aggregated loads with diverse starting characteristics: a 100 A feeder might supply a group of HVAC units that cycle on and off throughout the day. The bimetal ammeter smooths out these load variations, giving the operator a clear indication of the sustained current level rather than a needle that jumps with every compressor start. The 1 A secondary further benefits larger installations where the instrument may be located at a distance from the CT.

Decision Parameters