Re: Thermal O/L Relay setting

Spir Georges GHALI

electricalexpert65 said:

Ali said:

what is the approperiate setting for O/L shall it be 115% of the FLA or of the actual current of the motor or the same as FLA of the motor

some one says 115% of FLA and others says 100% FLA and other says 125%FLA

i heared many things so i'm confusing

Not more than the rated full load current of the motor.

Dear Ali ;


First, we should the numonal current of the motor ” In ” from the name plate, them we adjust the O/L relay eactly as the ” In ” value, because :

For each thermal relay we have a Tripping Curve, by which we can find that there's no any tripping order if the motor current is equal to In, but if it's more than In the thernal relay needs some time ” accordignly the the tripping curve ” to stop the motor. that means if you adjust the thermal relay at 115% of In, your motor will run on overload for a time bigger than the time if adjusted on 100% of In, so, if the motor can accept it you can do it, but normally we don't do it. In some application, we need to run the motor with an overload ( we should know it ) for some time without tripping ( normally smal time ), in this case, we adjust at 100% of In and from the tripping curve we can know the tripping time at this overload, so we can decide if it will be OK or we should re-adjust, but the more important is the protection of the motor.


By the way, I want to draw your attention that there's many Tripping curves accordingly of the class of thermal relay ” Class 5, 10, 20 ” and for the Electronic Thermal Relays there's more, so, accordingly to the kind of application ” Standard  or Severe ” we do the choice of the kind of tripping curve.