Nel Nord America invece il NEMA (National Electrical Manufacturers Association) definisce 4 principali Classi o profili di intervento della protezione termica: 5, 10, 20 & 30. Rispettivamente la curva a tempo inverso classe 5, 10, 20 & 30 interviene in 5, 10, 20 & 30 secondi con un valore di corrente pari a 6 volte la taratura impostata della protezione.
Class 5 is usually used for motors requiring extremely fast tripping.
Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.
Class 20 is usually sufficient for general purpose applications.
Class 30 is usually required for high inertial loads to help prevent nuisance tripping
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