Khi thit k t in ng c, th r le nhit bo v qu ti nhit l khng th thiu c. Tuy nhin, khi chn mi ngi vn hay bn khon lm sao chn cho ph hp bo m tt nht khi ng c lm vic v ct ti khi qu ti nhit.i vi r le nhit cng nh contactor, ta phi tnh ton c dng lm vic nh mc ca ng c.Khi chn r le nhit, quan tm chnh l cc thng s sau:- Dng lm vic- Dng sn phm ph hp vi contactor (mi loi r le nhit tng thch vi mt dng contactor tng ng, nh sn xut c khuyn co la chn ngay trn catalogue sn phm)Tng t nh tnh ton dng cho chn contactor, ta tnh ton dng nh mc, sau chn nh sau:- Idm = Itt x 2- Iccb = Idm x 2- Ict = (1,2-1,5)IdmTa tnh trong v d c th nh sau:C ti ng c 3 pha, 380V, 3kW, tnh ton dng nh mc theo cng thc sau:Itt = P/(1.73 x 380 x 0,85) y h s cosphi l 0,85.ta c Itt = 3000/(1,73x380x0,85)=5,4ADng r le nhit ta chn vi h s khi ng t 1,2-1,4 ln Idm, ta chn dng r le nhit l:Idm = 1,4xItt = 1,4x5,4=7,6A.Vy dng ca r le nhit ta chn l 8A. Cc r le nhit thng c di chnh dng, t dng lm vic, ta c th chn di dng d ra c th iu chnh c khi s dng thc ti